07.55
Prasetyo
Types of maize
Maize, the American Indian word for corn, means literally
"that which sustains life". It is, after wheat and
rice, the most important cereal grain in the world, providing
nutrients for humans and animals and serving as a basic raw
material for the production of starch, oil and protein, alcoholic
beverages, food sweeteners and, more recently, fuel. The green
plant, made into silage, has been used with much success in the
dairy and beef industries. After harvest of the grain, the dried
leaves and upper part, including the flowers, are still used
today to provide relatively good forage for ruminant animals
owned by many small farmers in developing countries. The erect
stalks, which in some varieties are strong, have been used as
long-lasting fences and walls.
Botanically, maize (Zea mays) belongs to the grass family
(Gramineae) and is a tall annual plant with an extensive fibrous
root system. It is a cross pollinating species, with the female
(ear) and male (tassel) flowers in separate places on the plant.
The grain develops in the ears, or cobs, often one on each stalk;
each ear has about 300 to 1 000 kernels, weighing between 190 and
300 g per I 000 kernels, in a variable number of rows (12 to 16).
Weight depends on genetic, environmental and cultural practices.
Grain makes up about 42 percent of the dry weight of the plant.
The kernels are often white or yellow in colour, although black,
red and a mixture of colours are also found. There are a number
of grain types, distinguished by differences in the chemical
compounds deposited or stored in the kernel.
Special crops grown primarily for food include sweet corn and
popcorn, although dent, starchy or floury and flint maize are
also widely used as food. Flint maize is also used as feed.
Immature ordinary corn on the cob either boiled or roasted is
widely consumed. Floury maize is a grain with a soft endosperm
much used as food in Mexico, Guatemala and the Andean countries.
The dent type of maize has a vitreous horny endosperm at the
sides and back of the kernel, while the central core is soft.
Flint kernels have a thick, hard and vitreous endosperm
surrounding a small, granular, starchy centre.
Origin of maize
The cultivation of maize or Indian corn most probably
originated in Central America, particularly in Mexico, from
whence it spread northward to Canada and southward to Argentina.
The oldest maize, about 7 000 years old, was found by
archaeologists in Teotihuacan, a valley near Puebla in Mexico,
but it is possible that there were other secondary centres of
origin in the Americas. Maize was an essential item in Mayan and
Aztec civilizations and had an important role in their religious
beliefs, festivities and nutrition. They claimed that flesh and
blood were made from maize. The survival of the oldest maize and
its distribution depended on humans who harvested the seed for
the following planting. At the end of the fifteenth century,
after the discovery of the American continent by Christopher
Columbus, maize was introduced into Europe through Spain. It then
spread through the warmer climates of the Mediterranean and later
to northern Europe. Mangelsdorf and Reeves (1939) have shown that
maize is grown in every suitable agricultural region of the world
and that a crop of maize is being harvested somewhere around the
globe every month of the year. Maize grows from latitude 58° in
Canada and the former Union of Soviet Socialist Republics to
latitude 40° in the Southern Hemisphere. Maize crops are
harvested in regions below sea-level in the Caspian Plain and at
altitudes of more than 4 000 m in the Peruvian Andes.
In spite of its great diversity of form, all main types of
maize known today were apparently already being produced by the
native populations when the American continent was discovered.
All maize is classified as Zea mays. Furthermore, evidence from
botany, genetics and cytology has pointed to a common origin for
every existing type of maize. Most researchers believe that maize
developed from teosinte, Euchlaena mexicana Schrod, an annual
crop that is possibly its closest relative. Others, however,
believe that maize originated in a wild maize that is now
extinct. The closeness of teosinte to maize is suggested by the
fact that both have ten chromosomes and are homologous or
partially homologous.
Introgression between teosinte and maize has taken place in
the past and still does today in areas of Mexico and Guatemala
where teosinte grows among the maize crop. Galinat (1977)
indicated that of the various hypotheses on the origin of maize,
essentially two alternatives remain viable: first, that
present-day teosinte is the wild ancestor of maize and/or that a
primitive teosinte is the common wild ancestor of both maize and
teosinte or, second, that an extinct form of pod maize was the
ancestor of maize, with teosinte being a mutant form of this pod
maize.
In any case, most of the modern varieties of maize have been
derived from materials developed in the southern United States of
America, Mexico and Central and South America.
The maize plant
The maize plant may be defined as a metabolic system whose end
product is mainly starch deposited in specialized organs, the
maize kernels.
The development of the plant may be divided into two
physiological stages. In the first or the vegetative stage,
different tissues develop and differentiate until the flower
structures appear. The vegetative stage is made up of two cycles.
In the first cycle the first leaves are formed and development is
upward. Dry matter production in this cycle is slow. It ends with
the tissue differentiation of the reproductive organs. In the
second cycle the leaves and reproductive organs develop. This
cycle ends with the emission of the stigmas.
The second stage, also known as the reproductive stage, begins
with the fertilization of the female structures, which will
develop into ears and grains. The initial phase of this stage is
characterized by an increase in the weight of leaves and other
flower parts. During the second phase, the weight of the kernels
rapidly increases (Tanaka and Yamaguchi, 1972).
The plant develops morphological characteristics and
differences in the vegetative and reproductive stages as
evolutionary consequences of natural selection and domestication.
Some genotypes have adapted to specific ecological zones and so
have developed such barriers as day-length sensitivity and
temperature sensitivity, which limit their adaptability to
specific areas of latitude and altitude. Thus improvement
programmes must be conducted within the areas where the improved
varieties are to be grown. This does not mean, however, that
specific genetic characteristics can be attained by backcrossing.
The morphology or architecture of the plant has also suffered
evolutionary pressures that have resulted in great variability in
the number, length and width of leaves, plant height, positions
of ears, number of ears per plant, maturation cycles, grain types
and number of rows of grain, among many other characteristics.
This variability is of great value in improving the
productivity of the plant and specific organic components of the
grain. The main yield components include the number and weight of
grains. These yield components are determined by quantitative
genetic effects that can be selected relatively easily. The
number of grains depends on the ear and is determined by the
number of rows and the number of kernels per row. The size and
shape of the kernel determine its weight in the presence of other
constant factors such as grain texture and grain density. The
ratio of grain weight to total plant weight for most maize lines
is about 0.52. From 100 kg of cobs, about 18 kg of grain is
obtained. One ha of maize yields about 1.55 tonnes of stalk
residue. In field-dried maize plants from three locations in
Guatemala, plant dry weight varied from 220 to 314 g. This weight
comprised 1.8 percent dried flowers, 14.7 to 27.8 percent stalks,
7.4 to 15.9 percent leaves, 11.7 to 13 percent husks, and 9.7 to
11.5 percent cobs. The field-dried grain represented 30 to 55.9
percent of the whole plant dry weight. These data show the
significant yield of plant residues that are often left in the
field. The distribution may change, however, since it is accepted
that about half of the dry matter is grain and the other half is
made up of plant residues excluding roots (Barber, 1979).
Structure of the maize kernel
Maize kernels develop through accumulation of the products of
photosynthesis, root absorption and metabolism of the maize plant
on the female inflorescence called the ear. This structure may
hold from 300 to 1 000 single kernels depending on the number of
rows, diameter and length of the cob. Kernel weight may be quite
variable, ranging from about 19 to 40 g per 100 kernels. During
harvest the ears of maize are removed from the maize plant either
by hand or mechanically. The husks covering the ear are first
stripped off, then the kernels are separated by hand or, more
often, by machine.
TABLE 1 - Weight distribution of main parts of the
kernel
| Structure |
Percent
weight distribution
|
| Pericarp |
5-6
|
| Aleurone |
2-3
|
| Endosperm |
80-85
|
| Germ |
10-12
|
The maize kernel is known botanically as a caryopsis; a single
grain contains the seed coat and the seed, as shown in Figure 1.
The figure also shows the four major physical structures of the
kernel: the pericarp, hull or bran; the germ or embryo; the
endosperm; and the tip cap (dead tissue found where the kernel
joins the cob). The gross anatomy and the microscopic structure
of these anatomical components were well described by Wolf et al.
(1952) and by Wolf, Khoo and Seckinger (1969). They also studied
the structure of the improved opaque-2 maize and found
differences between its endosperm and that of common maize. The
protein matrix was thinner and there were fewer and smaller
protein bodies, since there is a restriction in zein synthesis in
opaque-2 maize. Robutti, Hoseny and Deyoe (1974) and Robutti,
Hoseny and Wasson (1974) reported on the protein distribution,
amino acid content and endosperm structure of opaque-2 maize.
The weight distribution of the different parts of the maize
kernel is shown in Table 1. The endosperm, the lqrgest structure,
provides about 83 percent of the kernel weight, while the germ
averages 11 percent and the pericarp 5 percent. The remainder is
the tip cap, a conical structure that together with the pedicel
attaches the kernel to the ear of maize. Table 2 shows the
distribution of weight and nitrogen among the anatomical parts of
common and selected kernel varieties, such as high-oil and
high-protein maize and three quality protein maize (QPM)
selections (Bressani and Mertz, 1958). The main difference in the
high-oil variety is the size of the germ, which is about three
times as large as the germ from common maize with a reduction in
endosperm weight. Germ of the high-protein varieties is larger
than that of common maize but about half the size of high-oil
varieties. There are also differences in the weight of the
seed-coats. Table 2 also shows some data for teosinte, the
closest relative to maize. The seed weight is much lower than
that of maize seed, and the endosperm weighs about half that of
maize. The three QPM selections are similar to maize in weight
per seed and in weight of the seed-coat, the endosperm and the
germ. Similar data have been reported by other authors. Table 3
summarizes data for two common varieties and one opaque-2 maize
(Landry and Moureaux, 1980). The two common samples have the same
general characteristics as those reported above; the opaque-2
sample, however, has a larger germ providing more nitrogen than
the QPM selections in Table 2. With respect to the germ, the
increase of weight and of nitrogen amounts in absolute as well as
relative terms is consistent with other results (Watson, 1987).
TABLE 2 - Distribution of weight and nitrogen among
parts of the kernel
| Maize sample | Weight of 20 seeds (g) |
Weight distribution (%)
|
Total N (%) |
Nitrogen distribution (%)
|
||||
| Seedcoata | Endosperm | Germ | Seedcoat | Endosperm | Germ | |||
| US 4251 | 5.62 | 6.3 | 86.3 | 7.4 | 1.31 | 3.3 | 81.2 | 15.5 |
| US high oil (HO) | 5.72 | 6.4 | 71.2 | 22.4 | 1.99 | 2.4 | 68.4 | 29.2 |
| US high protein (H5) | 4.32 | 6.9 | 82.7 | 10.4 | 2.24 | 2.2 | 83.2 | 14.6 |
| US high protein (HP) | 4.97 | 7.4 | 78.9 | 13.7 | 2.14 | 2.7 | 78.2 | 19.1 |
| US normal-Sh1 PT | 4.38 | 6.7 | 79.6 | 13.7 | 2.14 | 2.7 | 78.2 | 19.1 |
| US normal mutant-Sh1 PT | 2.50 | 10.7 | 70.6 | 18.7 | 2.21 | 6.1 | 64.6 | 29.3 |
| Tiquisate (TGY)(Guat.) | 8.24 | 4.9 | 83.9 | 11.2 | 1.37 | 2.8 | 75.2 | 22.0 |
| San Sebastian (SSD)(Guat.) | 8.24 | 4.9 | 83.9 | 11.2 | 1.37 | 2.8 | 75.2 | 22.0 |
| Guatemalan 142-48 | 6.91 | 6.9 | 82.1 | 11.0 | 1.83 | 2.6 | 81.0 | 16.4 |
| Guatemalan Cuyuta | 5.95 | 5.7 | 82.5 | 11.8 | 1.28 | 2.9 | 72.4 | 24.7 |
| Guatemalan teosinte | 1.56 | 55.6b | 44.4 | - | 1.81c | 8.2 | 91.8d | - |
| Nutricta QPM | 5.91 | 5.7 | 82.7 | 11.6 | 1.42 | 1.7 | 72.8 | 25.5 |
| QPM yellow | 6.49 | 5.9 | 81.6 | 12.5 | 1.48 | 2.4 | 73.4 | 24.2 |
| QPM white | 5.31 | 5.9 | 82.4 | 1.6 | 1.36 | 1.4 | 72.8 | 25.7 |
aPericarp plus tip cap
bIncludes the seed-coat (1.3%) and the hull (54.3%)
cThe hull contained 0.26% nitrogen; dehulled teosinte contained 3.81 % nitrogen
dIncludes the germ
Source: Bressani and Mertz, 1958
bIncludes the seed-coat (1.3%) and the hull (54.3%)
cThe hull contained 0.26% nitrogen; dehulled teosinte contained 3.81 % nitrogen
dIncludes the germ
Source: Bressani and Mertz, 1958
TABLE 3 - Weight and nitrogen distribution of parts of
common and opaque-2 maize kernels
| Part of kernel |
Dry matter (%)
|
Nitrogen (%)
|
||||
| Common | Common | Opaque-2 | Common | Common | Opaque-2 | |
| Germ | 13.5 | 8.1 | 35 | 20.1 | 14.9 | 35.1 |
| Endosperm | 80.0 | 84.0 | 61 | 76.5 | 80.5 | 60.7 |
| Seed coat | 6.5 | 7.9 | 4 | 3.4 | 4.6 | 4.2 |
Source: Landry and Moureaux, 1980
World maize production increased from 1979-1981 to 1987, as
shown by continent in Table 4. The land area planted with maize
increased from 105 million ha in 1961 to about 127 million ha in
1987. Although part of the increase resulted from additional land
area planted, significant increases in production resulted from
genetic improvement and more efficient technological field
practices and fertilizer applications, as well as from the
introduction of new, more highly reproductive varieties.
TABLE 4 - World maize production
| Region and year | Area harvested (1 000 ha) | Yield (kg/ha) | Production (1 000 MT) |
| Africa | |||
| 1979-81 | 18 193 | 1 554 | 28 268 |
| 1985 | 19 099 | 1 522 | 29 069 |
| 1986 | 19 580 | 1 575 | 30 840 |
| 1987 | 19 512 | 1 395 | 27 225 |
| North and Central America | |||
| 1979-81 | 39 399 | 5 393 | 212 384 |
| 1 985 | 40 915 | 6 092 | 249 258 |
| 1986 | 37 688 | 6 116 | 230 511 |
| 1987 | 35 187 | 5 690 | 200 211 |
| South America | |||
| 1979-81 | 16 751 | 1 928 | 32 369 |
| 1985 | 17 813 | 2 182 | 38 859 |
| 1986 | 18 799 | 2 021 | 38 001 |
| 1987 | 19 413 | 2 143 | 41 595 |
| Asia | |||
| 1979-81 | 36 815 | 2 296 | 84 531 |
| 1985 | 35 246 | 2 628 | 92 629 |
| 1986 | 37 474 | 2 729 | 102 274 |
| 1987 | 37 399 | 2 788 | 104 269 |
| Europe | |||
| 1979-81 | 11 738 | 4 668 | 54 792 |
| 1985 | 11 556 | 5 423 | 62 673 |
| 1986 | 11 539 | 6 207 | 71 621 |
| 1987 | 11 405 | 6 039 | 68 901 |
| Oceania | |||
| 1979-81 | 76 | 4 359 | 332 |
| 1985 | 124 | 3 804 | 471 |
| 1986 | 107 | 4 402 | 471 |
| 1987 | 84 | 4 302 | 363 |
| USSR | |||
| 1979-81 | 3 063 | 2 989 | 9 076 |
| 1985 | 4 482 | 3 214 | 14 406 |
| 1986 | 4 223 | 2 955 | 12 479 |
| 1987 | 4 600 | 3 217 | 14 800 |
| World | |||
| 1979-81 | 126 035 | 3 345 | 421 751 |
| 1985 | 129 235 | 3 771 | 487 367 |
| 1986 | 129 411 | 3 757 | 486 198 |
| 1987 | 127 605 | 3 584 | 457 365 |
Source: FAO, 1988
The developing countries have more area given to maize
cultivation than developed countries, but yield in the latter is
about four times higher. Since 1961, yields per ha in the United
States, for example, have increased significantly, while yields
in Mexico, Guatemala and Nigeria (selected as countries where
maize intake by the human population is high, particularly in the
first two) have increased only slightly. While most of the
production in developing countries is for human consumption, in
the developed world it is mainly for industrial use and animal
feed. The high yields and production in North and Central America
are mainly attributed to the United States, which outproduces
countries such as Mexico where maize is the most important staple
cereal grain. With changing rural-to-urban populations and
lifestyles in developing countries, there is a continuous shift
to the consumption of wheat, which may influence maize
production. There is a slow increase in its use in industry and
as an animal feed, particularly for poultry and other monogastric
animals. A comparison of the available data for wheat, maize and
rice put maize as the second most important cereal grain, after
wheat and before rice. In terms of yield per hectare, however,
maize outyields the other two. The only food crop outyielding
maize in tonnes per hectare is potatoes in their unprocessed
state, though not on an equal moisture basis.
Uses
As indicated in previous sections, maize has three possible
uses: as food, as feed for livestock and as raw material for
industry. As a food, the whole grain, either mature or immature,
may be used; or the maize may be processed by dry milling
techniques to give a relatively large number of intermediary
products, such as maize grits of different particle size, maize
meal, maize flour and flaking grits. These materials in turn have
a great number of applications in a large variety of foods. Maize
grown in subsistence agriculture continues to be used as a basic
food crop. In developed countries more than 60 percent of the
production is used in compounded feeds for poultry, pigs and
ruminant animals. In recent years, even in developing countries
in which maize is a staple food, more of it has been used as an
animal feed ingredient. "High moisture" maize has been
paid much attention recently as an animal feed because of its
lower cost and its capacity to improve efficiency in feed
conversion. The by-products of dry milling include the germ and
the seed-coat. The former is used as a source of edible oil of
high quality. The seed-coat or pericarp is used mainly as a feed,
although in recent years interest has developed in it as a source
of dietary fibre (Earll et al., 1988; Burge and Duensing, 1989).
Wet milling is a process applicable mainly in the industrial use
of maize, although the alkaline cooking process used in
manufacturing tortillas (the thin, flat bread of Mexico and other
Central American countries) is also a wet milling operation that
removes only the pericarp (Bressani, 1990). Wet milling yields
maize starch and by-products such as maize gluten, used as a feed
ingredient. The maize germ processed to produce oil gives as a
by-product maize germ meal, used as an animal feedstuff. Some
attempts have been made to use these by-products for humans in
food mixes and formulations.
Although the technology has been available for a long time,
the increase in fuel oil prices has resulted in much research on
the fermentation of maize to produce alcohol, popular in some
states of North America. Fermentation also provides some
alcoholic beverages.
Finally, maize plant residues also have important uses,
including animal feeds as well as a number of chemicals produced
from the cobs, such as furfural and xylose. These residues are
also important as soil conditioners.
Chapter 2 - Chemical composition and nutritional value of maize
There are significant amounts of data on the chemical
composition of maize. Many studies have been conducted to
understand and evaluate the effects of the genetic make-up of the
relatively large number of available maize varieties on chemical
composition, as well as the effects of environmental factors and
agronomic practices on the chemical constituents and nutritive
value of the kernel and its anatomical parts. Chemical
composition after processing for consumption is an important
aspect of nutritive value (see Chapter 5); it is affected by the
physical structure of the kernel, by genetic and environmental
factors, by processing and by other links in the food chain. In
this chapter, the chemical nature of maize, of both common and
quality protein types, is described as a basis for understanding
the nutritive value of various maize products consumed throughout
the world.
Chemical composition of parts of the kernel
There are important differences in the chemical composition of
the main parts of the maize kernel as shown in Table 5. The
seed-coat or pericarp is characterized by a high crude fibre
content of about 87 percent, which is constituted mainly of
hemicellulose (67 percent), cellulose (23 percent) and lignin
(0.1 percent) (Burge and Duensing, 1989). On the other hand, the
endosperm contains a high level of starch (87.6 percent) and
protein levels of about 8 percent. Crude fat content in the
endosperm is relatively low. Finally, the germ is characterized
by a high crude fat content, averaging about 33 percent. The germ
also contains a relatively high level of protein (18.4 percent)
and minerals. Some information is available on the chemical
composition of the aleurone layer (see Figure 1), which is
relatively high in protein content (about 19 percent) as well as
in crude fibre. Tables 2 and 3 provide some additional details on
nitrogen distribution in the maize kernel. The endosperm
contributes the largest amount, followed by the germ, with only
small amounts from the seed-coat. About 92 percent of the protein
in teosinte comes from the endosperm. Protein in the maize kernel
has been reported on by a number of researchers (e.g. Bressani
and Mertz, 1958).
TABLE 5 - Proximate chemical composition of main parts
of maize kernels (%)
| Chemical component | Pericarp | Endosperm | Germ |
| Protein | 3.7 | 8.0 | 18 4 |
| Ether extract | 1.0 | 0.8 | 33.2 |
| Crude fibre | 86.7 | 2.7 | 8.8 |
| Ash | 0.8 | 0.3 | 10.5 |
| Starch | 7.3 | 87.6 | 8.3 |
| Sugar | 0.34 | 0.62 | 10.8 |
Source: Watson, 1987
From the data shown in Tables 2 and 3 it is evident that the
carbohydrate and protein contents of maize kernels depend to a
very large extent on the endosperm, and crude fat and to a lesser
extent protein and minerals on the germ. Crude fibre in the
kernel comes mainly from the seed-coat. The weight distribution
among parts of the maize kernel and their particular chemical
composition and nutritive value are of great importance when
maize is processed for consumption. In this regard there are two
important matters from the nutritive point of view. Germ oil
provides relatively high levels of fatty acids (Bressani et al.,
1990; Weber, 1987). Where there are high intakes of maize, as in
certain populations, those who consume the degermed grain will
obtain less fatty acids than those who eat processed whole maize.
This difference is probably equally important with respect to
protein, since the amino acid content of germ proteins is quite
different from that of endosperm protein. This is indicated in
Table 6, in which essential amino acids are expressed as mg
percent by weight and as mg per g N. As Table 2 shows, the
endosperm represents between 70 and 86 percent of the kernel
weight and the germ between 7 and 22 percent. It follows that, in
considering the whole kernel, the essential amino acid content is
a reflection of the amino acid content in the protein of the
endosperm, in spite of the fact that the amino acid pattern in
the germ protein is higher and better balanced. Germ proteins
nevertheless contribute a relatively high amount of certain amino
acids, although not enough to provide a higher quality of protein
in the whole kernel. The germ provides some lysine and
tryptophan, the two limiting essential amino acids in maize
protein. Endosperm proteins are low in lysine and tryptophan, as
is the whole grain protein (see Table 6, in which the FAO/ WHO
essential amino acid pattern is also shown). The deficiencies in
lysine tryptophan and isoleucine have been well demonstrated by
numerous animal studies (Howe, Janson and Gilfillan, 1965) as
well as by a few studies on humans (Bressani, 1971).
TABLE 6 - Essential amino acid content of germ protein
and endosperm protein
| Amino acid | Endosperma | Germb | FAD/WHO pattern | ||
| mg % | mg/g N | mg % | mg/g N | ||
| Tryptophan | 48 | 38 | 144 | 62 | 60 |
| Threonine | 315 | 249 | 622 | 268 | 250 |
| Isoleucine | 365 | 289 | 578 | 249 | 250 |
| Leucine | 1 024 | 810 | 1 030 | 444 | 440 |
| Lysine | 228 | 180 | 791 | 341 | 340 |
| Total sulphur amino acids | 249 | 197 | 362 | 156 | 220 |
| Phenylaianine | 359 | 284 | 483 | 208 | 380 |
| Tyrosine | 483 | 382 | 343 | 148 | 380 |
| Valine | 403 | 319 | 789 | 340 | 310 |
a1.16 percent N
b2.32 percent N
Source: Orr and Watt, 1957
b2.32 percent N
Source: Orr and Watt, 1957
TABLE 7 - Net protein of whole grain, germ and
endosperm of Guatemalan maize varietiesa
| Sample | Yellow | Azotea | Cuarenteño | Opaque-2 |
| Whole grain | 42.5 | 44.3 | 65.4 | 81.4 |
| Germ | 65.7 | 80.4 | 90.6 | 85.0 |
| Endosperm | 40.9 | 42.0 | 46.4 | 77.0 |
aExpressed as percentage of case in (100%)
Source: Poey et al., 1979
Source: Poey et al., 1979
The superior quality of germ protein to endosperm protein in
various samples of maize is shown in Table 7, which compares the
quality of the two parts as percentages of the reference protein,
casein in this case. The maize varieties include three of common
maize and one of quality protein maize (QPM). In all cases the
quality of germ proteins is much higher than that of endosperm
proteins and is obviously superior to the quality of whole kernel
protein. Endosperm protein quality is lower than that of the
whole kernel because of the higher contribution of germ protein.
The data also show less difference in the quality of germ and
endosperm proteins in the QPM variety. Furthermore, the QPM
endosperm and whole grain quality are significantly superior to
the endosperm and whole grain quality of the other samples. These
data, again, are important in regard to how maize is processed
for consumption and in its impact on the nutritional status of
people. They also clearly show that the quality of QPM is better
than that of common maize. The higher quality of QPM endosperm is
also of significance for populations that consume maize without
the germ.
Information on the gross chemical composition of maize is
abundant. The variability of each major nutrient component is
great. Table 8 summarizes data on various types of maize taken
from several publications. The variability observed is both
genetic and environmental. It may influence the weight
distribution and individual chemical composition of the
endosperm, germ and hull of the kernels.
TABLE 8 - Gross chemical composition of different
types of maize (%)
| Maize type | Moisture | Ash | Protein | Crude fibre | Ether extract | Carbohydrate |
| Salpor | 12.2 | 1.2 | 5.8 | 0.8 | 4.1 | 75.9 |
| Crystalline | 10.5 | 1.7 | 10.3 | 2.2 | 5.0 | 70.3 |
| Floury | 9.6 | 1.7 | 10.7 | 2.2 | 5.4 | 70.4 |
| Starchy | 11.2 | 2.9 | 9.1 | 1.8 | 2 2 | 72 8 |
| Sweet | 9 5 | 1 5 | 12.9 | 2.9 | 3.9 | 69.3 |
| Pop | 10.4 | 1.7 | 13.7 | 2.5 | 5.7 | 66.0 |
| Black | 12.3 | 1.2 | 5.2 | 1.0 | 4.4 | 75.9 |
Source: Cortez and Wild-Altamirano, 1972
Starch
The major chemical component of the maize kernel is starch,
which provides up to 72 to 73 percent of the kernel weight. Other
carbohydrates are simple sugars present as glucose, sucrose and
fructose in amounts that vary from 1 to 3 percent of the kernel.
The starch in maize is made up of two glucose polymers: amylose,
an essentially linear molecule, and amylopectin, a branched form.
The composition of maize starch is genetically controlled. In
common maize, with either the dent or flint type of endosperm,
amylose makes up 25 to 30 percent of the starch and amylopectin
makes up 70 to 75 percent. Waxy maize contains a starch that is
100 percent amylopectin. An endosperm mutant called
amylose-extender (ae) induces an increase in the amylose
proportion of the starch to 50 percent and higher. Other genes,
alone or in combination, may also modify the
amylose-to-amylopectin ratio in maize starch (Boyer and Shannon,
1987).
Protein
After starch, the next largest chemical component of the
kernel is protein. Protein content varies in common varieties
from about 8 to 11 percent of the kernel weight. Most of it is
found in the endosperm. The protein in maize kernels has been
studied extensively. It is made up of at least five different
fractions, according to Landry and Moureaux (1970, 1982). In
their scheme, albumins, globulins and non-protein nitrogen amount
to about 18 percent of total nitrogen, in a distribution of 7
percent, 5 percent and 6 percent, respectively. The prolamine
fraction soluble in 55 percent isopropanol and isopropanol with
mercaptoethanol (ME) contributes 52 percent of the nitrogen in
the kernel. Prolamine 1 or zein 1 soluble in 55 percent
isopropanol is found in the largest concentration, about 42
percent, with 10 percent provided by prolamine 2 or zein 2. An
alkaline solution, pH 10 with 0.6 percent ME, extracts the
glutelin fraction 2, in amounts of about 8 percent, while
glutelin 3 is extracted with the same buffer as above with 0.5
percent sodium dodecyl sulphate in amounts of 17 percent for a
total globulin content of 25 percent of the protein in the
kernel. Usually a small amount, about 5 percent, is residual
nitrogen.
Table 9 summarizes data by Ortega, Villegas and Vasal (1986)
on the protein fractionation of a common maize (Tuxpeño-1) and a
QPM (Blanco Dentado-1). Fractions II and III are zein I and zein
II, of which zein I (Fraction II) is significantly higher in the
Tuxpeño-1 variety than in the QPM. Similar results have been
published by other researchers. Amounts of the alcohol-soluble
proteins are low in immature maize. They increase as the grain
matures. When these fractions were analysed for their amino acid
content, the zein fraction was shown to be very low in lysine
content and lacking in tryptophan. Since these zein fractions
make up more than 50 percent of the kernel protein, it follows
that the protein is also low in these two amino acids. The
albumin, globulin and glutelin fractions, on the other hand,
contain relatively high levels of lysine and tryptophan. Another
important feature of the zein fractions is their very high
content of leucine, an amino acid implicated in isoleucine
deficiency (Patterson et al., 1980).
Quality protein maize differs from common maize in the weight
distribution of the five protein fractions mentioned above, as
shown in Table 9. The extent of the change is variable and
affected by genotype and cultural conditions. It has been found,
however, that the opaque-2 gene reduces the concentration of zein
by some 30 percent. As a result, lysine and tryptophan content is
higher in QPM varieties than in common maize.
TABLE 9 - Protein fraction distribution of Tuxpeño-1
and Blanco Dentado-1 QPM (whole grain)
| Fraction |
Blanco Dentado-1 QPM
|
Tuxpeño-1
|
||
| Protein (mg) | Percent protein | Protein (mg) | Percent total protein | |
| I | 6.65 | 31.5 | 3.21 | 16.0 |
| II | 1.25 | 5.9 | 6.18 | 30.8 |
| III | 1.98 | 9.4 | 2.74 | 13.7 |
| IV | 3.72 | 17.6 | 2.39 | 12.0 |
| V | 5.74 | 27.2 | 4.08 | 20.4 |
| Residue | 1.76 | 8.3 | 1.44 | 7.1 |
Source: Ortega, Villegas and Vasal, 1986
The nutritional quality of maize as a food is determined by
the amino acid make-up of its protein. Representative amino acid
values are shown in Table 10 for both common maize and QPM. To
establish the adequacy of the essential amino acid content the
table also includes the FAD/WHO essential amino acid pattern. In
common maize, deficiencies in lysine and tryptophan are evident
as compared with QPM. An additional important feature is the high
leucine content in common maize and the lower value of this amino
acid in QPM.
Oil and fatty acids
The oil content of the maize kernel comes mainly from the
germ. Oil content is genetically controlled, with values ranging
from 3 to 18 percent. The average fatty acid composition of the
oil in selected varieties from Guatemala is shown in Table 11.
These values differ to some extent; it may be expected that oils
from different varieties have different compositions. Maize oil
has a low level of saturated fatty acids, i.e. on average 11
percent palmitic and 2 percent stearic acid. On the other hand,
it contains relatively high levels of polyunsaturated fatty
acids, mainly linoleic acid with an average value of about 24
percent. Only very small amounts of linoleic and arachidonic
acids have been reported. Furthermore, maize oil is relatively
stable since it contains only small amounts of linoleic acid (0.7
percent) and high levels of natural antioxidants. Maize oil is
highly regarded because of its fatty acid distribution, mainly
oleic and linoleic acids. In this respect, populations that
consume degermed maize benefit less in terms of oil and fatty
acids than populations that consume whole-kernel products.
TABLE 11 - Fatty acid content of Guatemalan maize
varieties and Nutricta QPM (%)
| Maize variety | C16:0 Palmitic | C18:0 Stearic | C18:1 Oleic | C18:2 Linoleic | C18:3 Linolenic |
| QPM Nutricta | 15.71 | 3.12 | 36.45 | 43.83 | 0.42 |
| Azotea | 12.89 | 2.62 | 35.63 | 48.85 | - |
| Xetzoc | 11.75 | 3.54 | 40.07 | 44.65 | - |
| Tropical White | 15.49 | 2.40 | 34.64 | 47.47 | - |
| Santa Apolonia | 11.45 | 3.12 | 38.02 | 47.44 | - |
Source: Bressani et al., 1990
Dietary fibre
After carbohydrates, proteins and fats, dietary fibre is the
chemical component found in the greatest amounts. The complex
carbohydrate content of the maize kernel comes from the pericarp
and the tip cap, although it is also provided by the endosperm
cell walls and to a smaller extent the germ cell walls. The total
soluble and insoluble dietary fibre content of maize kernels is
shown in Table 12. Differences in soluble and insoluble dietary
fibre are small between samples, even though QPM Nutricta has
higher levels of total dietary fibre than common maize, mainly
because of a higher level of insoluble fibre. Table 13 shows
values of fibre expressed as acid and neutral detergent fibre,
hemicellulose and lignin in whole maize. The values shown in the
table are similar to those reported by Sandstead et al. (1978)
and Van Soest, Fadel and Sniffen (1979). Sandstead et al. found
that maize bran was composed of 75 percent hemicellulose, 25
percent cellulose and 0.1 percent lignin on a dry-weight basis.
Dietary fibre content in dehulled kernels would obviously be
lower than that of whole kernels.
TABLE 12 - Soluble and insoluble dietary fibre In
common and quality protein maize (%)
| Maize type |
Dietary fibre
|
||
| Insoluble | Soluble | Total | |
| Highland | 10.94 ± 1.26 | 1.25 ± 0.41 | 12.19 ± 1.30 |
| Lowland | 11.15 ± 1.08 | 1.64 ± 0.73 | 12.80 ± 1.47 |
| QPM Nutricta | 13.77 | 1.14 | 14.91 |
Source: Bressani, Breuner and Ortiz, 1989
TABLE 13 - Neutral and acid detergent fibre,
hemicellulose and lignin in five maize varieties (%)
| Maize No. | Neutral detergent fibre | Acid detergent fibre | Hemicellulose | Lignin | Cellular walls |
| 1 | 8.21 | 3.23 | 4.98 | 0.14 | 9.1 |
| 2 | 10.84 | 2.79 | 8.05 | 0.12 | 10.8 |
| 3 | 9.33 | 3.08 | 6.25 | 0.13 | 12.0 |
| 4 | 11.40 | 2.17 | 9.23 | 0.12 | 13.1 |
| 5 | 14.17 | 2.68 | 11.44 | 0.14 | 14.2 |
| Average | 10.79 ± 2.27 | 2.79 ± 0.44 | 8.00 ± 2.54 | 0.13 ± 0.01 | 11.8 ± 2.0 |
Source: Bressani, Breuner and Ortiz, 1989
Other carbohydrates
When mature, the maize kernel contains carbohydrates other
than starch in small amounts. Total sugars in the kernel range
between I and 3 percent, with sucrose, the major component, found
mostly in the germ. Higher levels of monosaccharides,
disaccharides and trisaccharides are present in maturing kernels.
At 12 days after pollination the sugar content is relatively
high, while starch is low. As the kernel matures, the sugars
decline and starch increases. For example, sugars were found to
have reached a level of 9.4 percent of kernel dry weight in
16-day-old kernels, but the level decreased significantly with
age. Sucrose concentration at 15 to 18 days after pollination was
between 4 and 8 percent of kernel dry weight. These relatively
high levels of reducing sugar and sucrose are possibly the reason
why immature common maize and, even more, sweet maize are so well
liked by people.
Minerals
The concentration of ash in the maize kernel is about 1.3
percent, only slightly lower than the crude fibre content. The
average mineral content of some samples from Guatemala is shown
in Table 14. Environmental factors probably influence the mineral
content. The germ is relatively rich in minerals, with an average
value of 11 percent as compared with less than I percent in the
endosperm. The germ provides about 78 percent of the whole kernel
minerals. The most abundant mineral is phosphorus, found as
phytate of potassium and magnesium. All of the phosphorus is
found in the embryo, with values in common maize of about 0.90
percent and about 0.92 percent in opaque-2 maize. As with most
cereal grains, maize is low in calcium content and also low in
trace minerals.
Fat-soluble vitamins
The maize kernel contains two fat-soluble vitamins: provitamin
A, or carotenoids, and vitamin E. Carotenoids are found mainly in
yellow maize, in amounts that may be genetically controlled,
while white maize has little or no carotenoid content. Most of
the carotenoids are found in the hard endosperm of the kernel and
only small amounts in the germ. The betacarotene content is an
important source of vitamin A, but unfortunately yellow maize is
not consumed by humans as much as white maize. Squibb, Bressani
and Scrimshaw (1957) found beta-carotene to be about 22 percent
of total carotenoids (6.4 to 11.3 µg per gram) in three yellow
maize samples. Cryptoxanthin accounted for 51 percent of total
carotenoids. Vitamin A activity varied from 1.5 to 2.6 µg per
gram. The carotenoids in yellow maize are susceptible to
destruction after storage. Watson (1962) reported values of 4.8
mg per kg in maize at harvest, which decreased to 1.0 mg per kg
after 36 months of storage. The same loss took place with
xanthophylls. Recent studies have shown that the conversion of
beta-carotene to vitamin A is increased by improving the protein
quality of maize.
TABLE 14 - Mineral content of maize (Average of five
samples)
| Mineral | Concentration (mg/100 g) |
| P | 299.6 ± 57.8 |
| K | 324.8 ± 33.9 |
| Ca | 48.3 ± 12.3 |
| Mg | 107.9 ± 9.4 |
| Na | 59.2 ± 4.1 |
| Fe | 4.8 ± 1.9 |
| Cu | 1.3 ± 0.2 |
| Mn | 1.0 ± 0.2 |
| Zn | 4.6 ± 1.2 |
Source: Bressani, Breuner and Ortiz, 1989
The other fat-soluble vitamin, vitamin E, which is subject to
some genetic control, is found mainly in the germ. The source of
vitamin E is four tocopherols, of which alpha-tocopherol is the
most biologically active. Gamma-tocopherol is probably more
active as an antioxidant than alphatocopherol, however.
Water-soluble vitamins
Water-soluble vitamins are found mainly in the aleurone layer
of the maize kernel, followed by the germ and endosperm. This
distribution is important in processing, which, as will be shown
later, induces significant losses of the vitamins. Variable
amounts of thiamine and riboflavin have been reported. The
content is affected by the environment and cultural practices
rather than by genetic make-up. Variability between varieties
has, however, been reported for both vitamins. The water-soluble
vitamin nicotinic acid has attracted much research because of its
association with niacin deficiency or pellagra, which is
prevalent in populations consuming high amounts of maize
(Christianson et al., 1968). As with other vitamins, niacin
content varies among varieties, with average values of about 20
µg per gram. A feature peculiar to niacin is that it is bound
and therefore not available to the animal organism. Some
processing techniques hydrolyze niacin, thereby making it
available. The association of maize intake and pellagra is a
result of the low levels of niacin in the grain, although
experimental evidence has shown that amino acid imbalances, such
as the ratio of leucine to isoleucine, and the availability of
tryptophan are also important (Gopalan and Rao, 1975; Patterson
et al., 1980).
Maize has no vitamin B12, and the mature kernel contains only
small amounts of ascorbic acid, if any. Yen, Jensen and Baker
(1976) reported a content of about 2.69 mg per kg of available
pyridoxine. Other vitamins such as choline, folic acid and
pantothenic acid are found in very low concentrations.
Changes in chemical composition and nutritive value
during grain development
In many countries, immature maize is often used as a food,
either cooked whole as corn on the cob or ground to remove the
seed-coat, with the pulp used to make thick gruels or foods like
tamalitos. The changes in chemical composition that take place
upon maturation are important. All relevant studies have shown a
decrease in nitrogen, crude fibre and ash on a dry-weight basis
and an increase in starch and ether extract (e.g. Ingle, Bietz
and Hageman, 1965). The alcohol-soluble proteins increase rapidly
as the kernel matures, while acid- and alkali-soluble proteins
decrease. During this biochemical process arginine, isoleucine,
leucine and phenylalanine (expressed as mg per g N) increase,
while lysine methionine and tryptophan decrease with maturation.
Gómez-Brenes, Elías and Bressani (1968) further showed a
decrease in protein quality (expressed as protein efficiency
ratio). Thus, immature maize should be promoted during weaning or
for infant nutrition.
Nutritional value of maize
The importance of cereal grains to the nutrition of millions
of people around the world is widely recognized. Because they
make up such a large part of diets in developing countries,
cereal grains cannot be considered only as a source of energy, as
they provide significant amounts of protein as well. It is also
recognized that cereal grains have a low protein concentration
and that protein quality is limited by deficiencies in some
essential amino acids, mainly lysine Much less appreciated,
however, is the fact that some cereal grains contain an excess of
certain essential amino acids that influence the efficiency of
protein utilization. The classic example is maize. Other cereal
grains have the same constraints but less obviously.
A comparison of the nutritional value of maize protein with
the protein quality of eight other cereals is given in Table 15,
expressed as percentages of casein. The protein quality of common
maize is similar to that of the other cereals except rice. Both
opaque-2 maize and the hard-endosperm QPM (Nutricta) have a
protein quality not only higher than that of common maize, but
also significantly higher than that of other cereal grains.
The reasons for the low quality of maize proteins have been
extensively studied by numerous investigators. Among the first
were Mitchell and Smuts (1932) who obtained a definite
improvement in human growth when 8 percent maize protein diets
were supplemented with 0.25 percent lysine These results have
been confirmed over the years by several authors (e.g. Howe,
Janson and Gilfillan, 1965), while others (e.g. Bressani, Elías
and graham, 1968) have shown that the addition of lysine to maize
causes only a small improvement in protein quality. These
differing results may be explained by variations in the lysine
content of maize varieties. Work in this field led to the
discovery by Mertz, Bates and Nelson (1964) of the highlysine
maize called opaque-2.
TABLE 15 - Protein quality of maize and other cereal
grains
| Cereal | Protein quality (% casein) |
| Common maize | 32.1 |
| Opaque-2 maize | 96.8 |
| QPM | 82.1 |
| Rice | 79.3 |
| Wheat | 38.7 |
| Oats | 59.0 |
| Sorghum | 32.5 |
| Barley | 58.0 |
| Pearl millet | 46.4 |
| Finger millet | 35.7 |
| Teff | 56.2 |
| Rye | 64.8 |
Some researchers (Hogan et al., 1955) have reported that
tryptophan rather than lysine is the first limiting amino acid in
maize, which may be true for some varieties with a high lysine
concentration or for maize products modified by some kind of
processing. All researchers have agreed that the simultaneous
addition of both lysine and tryptophan improves the protein
quality of maize significantly; this has been demonstrated in
experimental work with animals.
The improvement in quality obtained after the addition of
lysine and tryptophan has been small in some studies and higher
in others when other amino acids have been added. Apparently, the
limiting amino acid after lysine and tryptophan is isoleucine, as
detected from animal feeding studies (Benson, Harper and
Elvehjem, 1955). Most researchers who reported such findings
indicated that the effect of isoleucine addition resulted from an
excess of leucine which interfered with the absorption and
utilization of isoleucine (Harper, Benton and Elvehjem, 1955;
Benton et al., 1956). It has been reported that high consumption
of leucine along with the protein in maize increases niacin
requirements, and this amino acid could be partly responsible for
pellagra.
When a response to threonine addition has been observed, it
has been attributed to this amino acid's correction of amino acid
imbalances caused by the addition of methionine. A similar role
can be ascribed to added isoleucine resulting in improved
performance. Similarly, the addition of valine, which results in
a decrease in protein quality, could be counteracted by the
addition of either isoleucine or threonine.
In any case, isoleucine seems to be more effective than
threonine, producing more consistent results. A possible
explanation for these findings is that maize is not deficient in
either isoleucine or threonine. However, some samples of maize
may contain larger amounts of leucine, methionine and valine, end
these require the addition of isoleucine and threonine besides
lysine and tryptophan to improve protein quality. In any case,
the addition of 0.30 percent L-lysine and 0.10 percent
L-tryptophan easily increases the protein quality of maize by 150
percent (Bressani, Elías and graham, 1968). Many of the results
of the limiting amino acids in maize protein are influenced by
the level of protein in the maize. As was indicated previously,
protein content in maize is a genetic trait that is affected by
nitrogen fertilization. The observed increase in protein content
is highly correlated with zein, or the alcohol-soluble protein,
which is low in lysine and tryptophan and contains excessive
amounts of leucine. Frey (1951) found a high correlation between
protein content and zein in maize, a finding that has been
confirmed by others. Using different animal species, various
authors have concluded that the protein quality of low-protein
maize is higher than that of high-protein maize when the protein
in the diets used is the same. However, weight for weight,
high-protein maize is slightly higher in quality than low-protein
maize. The levels of dietary protein, then, affect the response
observed upon amino acid supplementation with lysine and
tryptophan in particular but with other amino acids as well, such
as isoleucine and threonine.
The chemical components and nutritive value of maize do not
lose their susceptibility to change when the grain is harvested.
Subsequent links in the food chain, such as storage and
processing, may also cause the nutritional quality of maize to
decrease significantly or, even worse, make it unfit for either
human and animal consumption or industrial use.
Drying
Maize harvesting is highly mechanized in developed countries
of the world, while it is still done manually in developing
countries. The mechanized system removes not only the ear from
the plant but also the grain from the cob, while manual
harvesting requires initial removal of the ear, which is shelled
at a later stage. In both situations, maize is usually harvested
when its moisture content is in the range of 18 to 24 percent.
Damage to the kernel (usually during the shelling operation) is
related to moisture content at harvest; the lower the moisture
content, the less the damage.
Changes in the physical quality of the grain are often a
result of mechanical harvesting, shelling and drying. The first
two processes sometimes result in external damage, such as the
breaking of the pericarp and parts around the germ, facilitating
attack by insects and fungi. Drying, on the other hand, does not
cause marked physical damage. However, if it is carried out too
rapidly and at high temperatures, it will induce the formation of
stress cracks, puffiness and discoloration, which will affect the
efficiency of dry milling and other processes (Paulsen and Hill,
1985).
In tropical countries, drying is sped up by bending down the
upper part of the plant holding the ear, a practice that also
prevents the kernels from becoming soaked when it rains. In
either mechanical or manual harvesting, the shelled kernels
contain too much moisture for safe storage, and they must be
dried to safe moisture levels of about 12 percent at 30°C and
about 14 percent at 10°C (Herum,1987). Storage stability depends
on the relative humidity of the interstitial gases, which is a
function of both moisture content in the kernel and temperature.
Low moisture content and low storage temperatures reduce the
opportunity for deterioration and microbial growth. Aeration
therefore becomes an important operation in maize storage as a
means of keeping down the relative humidity of interstitial
gases.
Significant maize losses have been reported in tropical
countries. Losses of up to 10 percent have been found, not
including those losses caused by fungi, insects or rodents. If
these were included, losses could go up to 30 percent in tropical
humid areas or 10 to 15 percent in temperate areas. Schneider
(1987) reported post-production losses in Honduras of 6.5 to 8.7
percent in the field and of 7.4 to 13.9 percent in storage.
Losses due to fungi (mainly aspergillus and penicillium) are
important for both economic and health reasons because of
aflatoxins and mycotoxins (de Campos, Crespo-Santos and
Olszyna-Marzys, 1980).
In a survey on maize sold in rural markets in Guatemala,
Martinez-Herrera (1968) found considerable contamination by
several fungi. Among these, some Aspergillus species, well known
as aflatoxin producers, were frequently present. There is
evidence that maximum aflatoxin contamination of maize in
Guatemala is during the rainy season. Samples analysed 20 days
after maize was harvested had levels of 130 µg aflatoxin per kg
of total maize. The same samples analysed 60 days later showed a
great increase of up to 1 680 µg per kg. These data as well as
data from several other studies strongly indicate the need to dry
maize before storage. Diverse drying systems and equipment are
available, using various sources of energy including solar energy
(Herum, 1987). A number of factors must be considered such as
temperature and air velocity, rate of drying, drying
efficiencies, kernel quality, air power, fuel source, fixed costs
and management. Drying is an important step in ensuring good
quality grain that is free of fungi and micro-organisms and that
has desirable quality characteristics for marketing and final
use.
Drying Methods
Layer drying. In this method, the
harvested grain is placed in a bin one layer at a time. Each
layer of grain is partially dried, before the next is added, by
forcing air through a perforated floor or through a duct in the
bottom of the bin. To improve efficiency, the partially dried
grain is stirred and mixed with the new layer. An alternative is
to remove the partially dried grain and dry it completely in
batches. One of the problems with this and other methods of
drying is in finding a way to mix low-moisture grain with
high-moisture grain to get the desired equilibrium in the final
product. Spoilage often occurs in this attempt. Sauer and
Burroughs (1980) reported that equilibrium was more than 80
percent complete in 24 hours. Methods have been developed to
detect highmoisture maize in mixtures with artificially dried
maize.
Portable batch dryers. Since drying
installations are costly, few maize producers, particularly small
farmers, can afford to have their own. Portable batch dryers are
useful since they can be moved from farm to farm. These dryers
operate with air heated to 140 to 180°F (60 to 82°C).
Continuous flow dryers. The
principle behind these dryers is the continuous flow of grain
through heated and unheated sections so that it is discharged dry
and cool. The equipment is the central point in grain storage
depots.
Storage
Biotic and non-biotic factors
The efficient conservation of maize, like that of other cereal
grains and food legumes, depends basically on the ecological
conditions of storage; the physical, chemical and biological
characteristics of the grain; the storage period; and the type
and functional characteristics of the storage facility. Two
important categories of factors have been identified. First are
those of biotic origin, which include all elements or living
agents that, under conditions favourable for their development,
will use the grain as a source of nutrients and so induce its
deterioration. These are mainly insects, microorganisms, rodents
and birds. Second are non-biotic factors, which include relative
humidity, temperature and time. The effects of both biotic and
nonbiotic factors are influenced by the physical and biochemical
characteristics of the grain. Changes during storage are
influenced by the low thermal conductivity of the grain, its
water absorption capacity, its structure, its chemical
composition, its rate of respiration and spontaneous heating, the
texture and consistency of the pericarp and the method and
conditions of drying.
Nutrient losses have been reported in maize stored under
unfavourable conditions. Quackenbush (1963) showed carotene
losses in maize stored under different temperature and moisture
conditions. In other studies common and QPM maize were stored in
different types of containers with and without chemicals. After
six months samples were examined for damage by insects and fungi
and for changes in protein quality. In both types there was some
damage to the unprotected maize but not to that stored with
chemicals. Protein quality was not affected (Bressani et al.,
1982). Other changes subsequent to drying and storage included a
decreased solubility of proteins; changes in nutritive value for
pigs; changes in sensory properties (Abramson, Sinka and Mills,
1980); and changes in in vitro digestibility resulting from heat
damage (Onigbinde and Akinyele, 1989).
Although damage caused by insects and birds is of importance,
a great deal of attention has been paid to the problems caused by
micro-organisms, not only because of the losses they induce in
the grain, but more importantly, because of the toxic effects of
their metabolic by-products on human and animal health.
Studies on the nutritional effects of insect infestation of
maize are not readily available. Daniel et al. (1977) and Rajan
et al. (1975) have reported losses in threonine and in protein
quality of maize infested with Sitophilus oryzae. In the first
study, protein efficiency ratio (PER) decreased after three
months from an initial value of 1.30 to 0.91. In the second
study, threonine decreased from 3.5 to 2.9 g per 16 g N and PER
decreased from I .49 to I .16. These researchers also reported
that the damaged maize was less efficient in complementing food
legumes.
Also of nutritional significance was an increase in uric acid
from 3.5 to 90.6 mg per 100 g after three months. Thiamine losses
were detected as well.
Bressani et al. (1982) evaluated five chemicals and three
types of containers for their effectiveness in protecting QPM's
nutritional quality against insect damage. About 38 percent of
the untreated grain (control) was damaged by insects. This did
not, however, affect its protein quality.
Several research studies have identified an association
between insect damage and toxin contamination (e.g. Fennellet
al., 1978; Perez, Tuite and Baker, 1982).
Christensen (1967) measured selected changes in United States
No. 2 maize stored for two years with moisture contents of 14.5
and 15.2 percent and at temperatures of 12, 20 and 25°C. Changes
in condition were evaluated by appearance, fungal invasion,
germination percentage and final fat acidity value. Samples
stored at 25°C deteriorated rapidly at both levels of moisture
content. The samples with 15.2 percent moisture changed slightly
after six months at 12°C but appreciably after two years. The
maize stored with 14.5 percent moisture content retained its
original condition when kept at 12°C for the twoyear period and
changed only slightly in 18 months at 20°C. However, large
variability in the insect-fungi interaction was observed. Some
maize-growing regions have experienced extensive insect damage to
maturing ears with no occurrence of aflatoxin, while other areas
with equivalent insect damage have exhibited relatively broad
incidences of the toxin in kernels at harvest.
Many studies have been conducted to assess the nutritional
value of mouldy maize. Although some increase in B-vitamin
content has been reported, possibly as a result of the
metabolites of the micro-organisms, the damage to animal health
far exceeds any beneficial change in chemical composition.
Several researchers have studied the impairment in nutritive
value of mould-damaged maize. For example, Martínez et al.
(1970a) found significant negative effects in poultry and
laboratory rats fed mouldy maize. It is difficult, however, to
decide whether these effects were caused by fungi-produced toxins
or by a loss in nutrients in the substrate because of their
utilization by the micro-organisms.
Christensen and Sauer (1982) reviewed the effects of fungal
invasion on cereal grains. They found that it reduced both the
quality and grade of the grains through loss of dry matter,
discoloration, heating, cooking, mushiness and contamination by
mycotoxins. Microbial indices of fungal invasion and seed
deterioration include visible damage, seed infection, number of
fungal propagules, evolved carbon dioxide and decrease in seed
germination and ergosterol content.
Inhibition of atlatoxin contamination
Two ways of preserving maize from being destroyed by aflatoxin
contamination have been under investigation. One is to inhibit
growth of Aspergillus flavus or Aspergillus parasiticus and the
other is to remove the aflatoxins after they have been produced
by the Aspergillus infection. Most researchers have concentrated
on the inhibition of fungal growth, and some chemicals have
already been found effective in storage conditions. This,
however, does not solve the problem of field contamination by
moulds, since the airborne spores of the organisms are readily
available in the environment. The spores can germinate on the cob
and infect the inner tissues under optimum temperature and
moisture conditions. Therefore, other researchers have pursued
the possibility of detoxification.
Roasting has been shown to be effective in reducing aflatoxin
levels, depending on the initial level of the toxin as well as on
roasting temperatures (Conway and Anderson, 1978). Higher
temperatures may cause up to 77 percent aflatoxin destruction;
however, it is well known that heat also destroys the nutritive
value of the material. Tempering aflatoxincontaminated maize with
aqua ammonia and then roasting it may be a simple and effective
way to decontaminate it. Valuable results using ammonia have been
reported. It is difficult, however, to remove the smell of
ammonia from the treated grain. Other more complex methods have
been tried. For example, Chakrabarti (1981) showed that aflatoxin
levels could be reduced to less than 20 ppb using separate
treatments with 3 percent hydrogen peroxide, 75 percent methanol,
5 percent dimethylamine hydrochloride or 3 percent perchloric
acid. These treatments, however, induced losses in weight and
also in protein and lipids. Other methods include the use of
carbon dioxide plus potassium sorbate and the use of sulphur
oxide.
A process that has received some attention is the use of
calcium hydroxide, a chemical used for lime-cooking of maize
(Bressani, 1990). Studies have shown a significant reduction in
aflatoxin levels, although the extent of reduction is related to
the initial levels. Feeding tests with mouldy maize treated with
calcium hydroxide have shown a partial restoration of its
nutritional value.
Appropriate harvesting and handling can do much to reduce
fungal contamination of maize and can thus prevent the need for
chemical decontamination measures, which not only increase the
cost of the grain but cannot completely restore its original
nutritional value. In this respect, Siriacha et al. (1989) found
that if shelled grain was immediately sun-dried the chance of
contamination was reduced as compared with that of undried maize
shelled mechanically or by hand. Shelling encourages fungal
contamination as it causes damage to the kernel base, which is
rough compared with the rest of the grain. Corn on the cob, even
with its high levels of moisture, resists fungal contamination
relatively well.
Classification of grain quality
To facilitate marketing and to identify the best uses for the
various types of maize produced throughout the world, measures of
grain quality have been identified, although they may not be
accepted by all maize-producing countries. In the United States
maize is classified into five different grades, based on several
factors. Minimum test weight is expressed in pounds per bushel,
pounds per cubic foot or kilograms per cubic metre. The higher
the test weight the higher the grade. The maximum permitted
amount of broken maize and foreign material (BCFM) varies from 2
percent for Grade I to 7 percent for Grade 5. There is a
classification for damaged kernels that includes heat-damaged
kernels. Maize is also classified as yellow, white or mixed
maize. Yellow maize must have no more than 5 percent white
kernels and white maize must not have more than 2 percent yellow
grain. The mixed class contains more than 10 percent of the other
grain.
Although the moisture content of maize, an important part of
its chemical composition, is not considered a quality factor, it
has much influence on composition, quality changes during storage
and processing and economics. High-moisture maize with a soft
texture is easily damaged in storage, while maize with low levels
of moisture becomes brittle. The most commonly accepted moisture
level for marketing purposes is 15.5 percent. Density of maize -
weight per unit volume - is important in storage and
transportation since it establishes the size of container for
either purpose. Moisture content and density or test weight are
related; the higher the moisture level the lower the specific
density test weight. This characteristic of maize is also
important for milling.
Another important quality characteristic of maize is its
hardness, since this influences grinding power requirements, dust
formation, nutritional properties, processing for food products
and the yield of products from dry and wet milling operations.
Hardness of maize is genetically controlled, but it can be
modified by both cultural practices and post-harvest handling
conditions. Many investigators have proposed methodologies for
measuring hardness for a number of different applications
(Pomeranz et al., 1984, 1985, 1986). Maize varieties with a horny
endosperm such as flint and popcorn types, have hard kernels,
while starchy and opaque maize varieties are soft. Some flint
types are intermediate.
Finally, freedom of the kernel from fungi is recognized as a
quality characteristic.
Chapter 4 - Post-harvest technology: processing
Forms of maize consumption
Maize is consumed in many forms in different parts of the
world, from maize grits, polenta and corn bread to popcorn and
products such as maize flakes (Rooney and Serna-Saldivar, 1987).
The grain is fermented to give ogi in Nigeria (Oke, 1967) and
other countries in Africa (Hesseltine, 1979) and is decorticated,
degermed and precooked to be made into arepas in Colombia and
Venezuela (Instituto de Investigaciones Tecnológicas, 1971;
Rodriguez, 1972).
In Egypt a maize flat bread, aish merahra, is widely produced.
Maize flour is used to make a soft dough spiced with 5 percent
ground fenugreek seeds, which is believed to increase the protein
content, improve digestibility and extend the storage life of the
bread. The dough is fermented all night with a sourdough starter.
In the morning the dough is shaped into small, soft, round
loaves, which are left for 30 minutes to "prove".
Before baking the loaves are made into wide, flat discs. Aish
merahra keeps fresh for seven to ten days if it is stored in
airtight containers. A similar product called markouk is eaten in
Lebanon.
Maize is also widely used to make beer. In Benin, for example,
malt is obtained by germinating the grain for about five days.
The malt is then exposed to the sun to stop germination. The
grains are lightly crushed in a mortar or on a grinding stone.
The malt is cooked and the extract is strained off, cooled and
allowed to stand. After three days of fermentation it is ready to
be drunk as beer (FAO, 1989).
The lime-cooking process for maize is particular to Mexico and
Central America (Bressani, 1990), although today the technology
has been exported to other countries such as the United States. A
dough prepared from limecooked maize is the main ingredient for
many popular dishes such as atole, a beverage with a great
variety of flavours, and tamalitos, made by wrapping the dough in
maize husks and steam-cooking it for 20 to 30 minutes to
gelatinize the starch. This form is usually prepared with young
chipilín leaves (Crotalaria longirostrata), the flowers of
loroco (Fernaldia pandurata) or cooked beans mixed with the
dough, thus improving the nutritional quality of the product and
its flavour (Bressani, 1983). The dough is also used for tamales,
a more complex preparation because of the number of ingredients
it contains, in most cases with chicken or pork meat added to the
gelatinized dough. It is also used to provide support for
enchiladas, tacos (folded tortillas containing meat, etc.) and
pupusas, the latter made with fresh cheese placed between two
layers of dough and baked like tortillas. When the dough is fried
and flavoured, it yields foods such as chips and chilaquiles. If
the dough is allowed to ferment for two days, wrapped in banana
or plantain leaves, it provides a food named pozol from which a
number of drinks can be made. It has been claimed that this
preparation is of high nutritional quality.
There are many ways to convert maize into interesting and
acceptable forms which, if presented in attractive and easily
prepared products, could to some extent counteract the trend
toward greater consumption of wheat derived foods in arepa- and
tortilla-eating countries and elsewhere.
Lime-cooking in rural areas
A number of researchers have described how maize is cooked in
rural areas of countries where tortillas are eaten. Illescas
(1943) first described the process as carried out in Mexico. It
involves the addition of one part whole maize to two parts of
approximately I percent lime solution. The mixture is heated to
80°C for 20 to 45 minutes and then allowed to stand overnight.
The following day the cooking liquor is decanted and the maize,
now referred to as nixtamal, is washed two or three times with
water to remove the seed-coats, the tip caps, excess lime and any
impurities in the grain. The addition of lime at the cooking and
steeping stages helps to remove the seed-coats.
The by-products are either thrown away or fed to pigs.
Originally, the maize was converted into dough by grinding it a
number of times with a flat stone until the coarse particles were
fine enough. Today the initial grinding is done with a meat
grinder or disc mills and the dough is then refined with the
stone. A portion of about 50 g of the dough is patted flat and
cooked on both sides on a hot iron or clay plate.
In Guatemala a similar process (described by Bressani, Paz y
Paz and Scrimshaw, 1958) uses either white or yellow maize, but
the lime concentration varies from 0.17 to 0.58 percent based on
the weight of maize, with a grain-to-water ratio of 1:1.2, and
the maize cooking time varies from 46 to 67 minutes at a
temperature of 94°C. The rest of the process is essentially the
same, except that the dough is prepared with a disc mill and is
cooked for about 5 minutes at a temperature of about 170°C at
the edges and 212°C in the centre.
Tamalitos, for which the dough is steamed, are softer and keep
longer. For recently harvested maize less lime is used and
cooking time is decreased; the procedure is modified conversely
when the grain is old and dry. The dry matter losses are about 15
percent, but they can vary between 8.9 and 21.3 percent.
Industrial lime-cooking
Factors such as the migration of people from rural to urban
areas created a demand for ready-cooked or pre-cooked tortillas.
Equipment for processing raw maize into lime-treated maize and
then into a dough and tortillas was developed and industrial
production of tortilla flour began in Mexico and other countries.
Mechanized production in Mexico became important soon after the
Second World War. Two types of industry are found in urban areas.
One is the small family-owned home tortilla industry, where the
process is as described above but with larger and mechanical
equipment used to supply a larger market. This development became
possible through the introduction of rotary mills and the
tortilla maker designed by Romero in 1908. This equipment was
later replaced by a more efficient type in which the dough is
passed through a rotating metal drum where it is cut into
tortilla shapes. These fall onto a moving belt or continuous
cooking griddle, dropping into a receptacle at the end of the
belt. This small industry may use whole maize, in which case the
dough is cooked in large receptacles, or it may start with
industrial tortilla flour.
The second type of industry is the large industrial conversion
of maize into an instant precooked tortilla flour. The process
has been described by various workers (e.g. Deschamps, 1985). It
is based for all practical purposes on the traditional method
used in rural areas. More recently, the process of producing the
flour has been expanded to produce tortillas.
Maize is bought after the buyer has inspected its quality and
sampled it. Batches of maize with a high percentage of defective
grains are rejected. Those that are accepted are paid for
according to the defects found in the raw material. Maize is also
selected according to its moisture content, since very high
moisture will result in storage problems. During the cleaning
stage, all impurities such as dirt, cobs and leaves are removed.
The cleaned maize is sent to silos and warehouses for storage.
From there it is conveyed to treatment units for lime-cooking.
There it is converted into nixtamal, using either a batch or a
continuous process. After cooking and steeping, the lime-treated
maize is washed with pressurized water or by spraying. It is
ground into a dough (masa) which is then transferred to a dryer
and made into a rough flour. This flour, consisting of particles
of all sizes, is forced through a sifter where the coarse
particles are separated from the fine ones. The coarse particles
are returned to the mill for regrinding and the fine ones, which
constitute the final product, are sent to the packing units. Here
the flour is packed into lined paper bags.
One complete unit must have equipment for lime treatment,
milling, drying and sifting and a daily production capacity of 30
to 80 tonnes of flour. These figures are the minimum and the
maximum; to increase its production capacity, a commercial
enterprise must install several parallel units. The use of such
units seems to be more a tradition than a technical necessity,
since it would be perfectly feasible to design plants with a
capacity lower than 30 tonnes or higher than 80 tonnes per day.
Plants that are very large or very small are apparently not
considered viable.
The industrial yield of alkali-cooked maize flour fluctuates
between 86 and 95 percent depending on the type of maize, the
quality of the whole kernels and the lime-treatment conditions.
Industrial yields have been reported to be higher than those at
the rural and semi-industrial levels, possibly because of the
quality of the grain processed.
Tortilla flour is a fine, dry, white or yellowish powder with
the characteristic odour of maize dough. This flour when mixed
with water gives a suitable dough for the preparation of
tortillas, tamales, atoles (thick gruels) and other foods. All
maize flours made in Mexico must conform to the specifications of
the government's Department of Standards and Regulations.
When the flour has a moisture content of 10 to 12 percent it
is stable against microbial contamination. If the moisture
content is over 12 percent it is easily attacked by moulds and
yeast. The problem of bacterial attack is almost nonexistent
since the minimum of moisture required for bacterial growth is so
high that flour with this moisture content would already be
transformed into masa. Another matter related to the stability of
flour is rancidity, which is normally not a problem unless the
flour is packed at high temperatures. The minimum time required
for the flour to spoil in Mexico is four to six months during the
winter and three months during the summer. Nevertheless, it is
usually sold to the consumer within 15 days of being sold to
retailers and wholesalers, while its shelf-life is one month
(Delvalle, 1972).
Tortillas made from lime-treated maize flour can be made at
home or in factories. Such flour has been a great advantage for
households and for factories both large and small, although its
use in rural areas is not widespread.
In Guatemala, about 3 000 metric tons of maize are produced
yearly for tortilla flour production. This amount is
significantly lower than that in Mexico; the population is
smaller and there are few small tortilla factories. About 90
percent of the production is sold in urban areas and 75 percent
goes into tortilla making. Other countries where lime-treated
maize flour is produced are Costa Rica and the United States. In
Costa Rica tortilla consumption per person is about 25.6 kg per
annum. Approximately 62 percent of the production is commercial,
30.6 percent is home-made from commercial flour and 7.4 percent
is home-made from grain.
Modifications of lime-cooking
The traditional method of cooking maize with lime to make
tortillas at the rural level is both time-consuming (about 14 to
15 hours) and hard work. The cooking and soaking operations take
up 70 to 80 percent of the time, which in a sense may be
acceptable to the rural housewife. Nevertheless, the availability
of an instant tortilla flour offers many advantages such as
convenience, less labour and lower use of energy, for a safe,
stable and nutritious product. At the industrial or commercial
level, grinding and dehydration are large factors in the cost.
Lime-cooked maize contains about 56 percent moisture, which must
be decreased to 10 to 12 percent in the flour. Therefore, any
method that would decrease both time and cost and still yield
acceptable tortillas would be advantageous.
Efforts in this respect have been made by a number of workers.
Bressani, Castillo and Guzmán (1962) evaluated a process based
on pressure cooking at 5 and 15 lb pressure per square inch (0.35
and 1.05 kg per cm²) under dry and moist conditions for 15, 30
and 60 minutes, without the use of lime. None of the treatments
had any effect on chemical composition and true protein
digestibility, but all reduced the solubility of the nitrogen.
Pressure cooking at 15 lb per square inch (1.05 kg per cm²)
under dry conditions reduced the nutritional quality of the
product, particularly when carried out for 60 minutes. The
pressure cooking method without lime did not reduce crude fibre
content, which is one of the particular effects of lime, and the
calcium content was significantly lower than in dry dough (masa)
prepared by the traditional method.
Khan et al. (1982) conducted a comparative study of three
lime-cooking methods: the traditional way, a commercial method
and a laboratory pressurecooking procedure. For each process
maize was undercooked, optimally cooked and overcooked to measure
some of the physical and chemical changes that might occur.
Although the traditional method caused the greatest loss of dry
matter from the grain, it gave the best tortillas in terms of
texture, colour and acceptability. The pressure-cooking procedure
yielded a sticky dough and undesirable tortillas. The commercial
method was the least desirable. This study allowed the authors to
propose a method to evaluate the completeness of cooking.
Bedolla et al. (1983) tested various methods of cooking maize
and sorghum as well as mixtures of the two grains. The methods
tested included the traditional one, steam cooking as tested by
Khan et al. (1982) and a method using a reflux (condensing)
system. They found that the methods of cooking affected the total
dry matter lost during processing into tortillas.
Variation of cooking conditions can result in lower processing
times. For example, Norad et al. (1986) found that a 40 percent
reduction in cooking time could be achieved by pre-soaking the
grain before alkali cooking. In these studies dry matter losses,
water uptake, calcium content and enzyme susceptible starch
increased, whereas amylograph maximum viscosity decreased in both
presoaked and raw maize upon cooking. The decrease in viscosity
and increase in the other parameters was faster in the pre-soaked
maize.
Dry-heat processes have also been studied. Johnson, Rooney and
Khan (1980) tested the micronizing process to produce sorghum and
maize flours. Micronizing is a dry-heat process using gas-fired
infrared generators. Rapid internal heating takes place, cooking
the product from the inside out. The authors used this process to
produce tortilla flour, claiming that it would be quicker and
more economical than the traditional method.
Molina, Letona and Bressani (1977) tested production of
instant tortilla flour by drum drying at the pilot plant level.
Maize flour was mixed with water at a ratio of 3: I with 0.3
percent lime added on the basis of maize weight. After mixing,
the dough was passed through a double-drum dryer heated with
steam at 15, 20 or 25 lb per square inch (1.05, 1.40 or 1.75 kg
per cm2), 93, 99 and 104°C surface temperature and 2, 3 or 4
rpm. The process produced an instant tortilla flour with
physico-chemical and organoleptic characteristics identical to
those of the reference sample prepared by the traditional method
but different from those of a commercial product.
Extrusion cooking has also been evaluated as an additional
technology for producing tortilla flour. Bazua, Guerra and
Sterner (1979), using a Wenger X-5 extruder, processed ground
maize mixed with various lime concentrations (0.1 to 1.0
percent). The dough and tortillas made by extrusion were compared
with those made by the traditional process for their organoleptic
properties as well as lysine tryptophan and protein content. No
appreciable differences were noted at comparable use levels of
calcium hydroxide. Both the traditional process and the extrusion
modification induced losses of tryptophan related to the amount
of lime added. With a 0.2 percent addition 8 percent of the
tryptophan was lost, while with 1 percent lime more than 25
percent was lost. Some lysine losses were also observed. The
organoleptic results suggested that it is possible to make
culturally acceptable tortillas using extrusion as an alternative
to the lime-heat treatment.
Maize for tortillas
Grain quality is a concept now growing in importance in
breeding programmes aimed at increasing acceptance of genetically
improved seeds by farmers as well as by consumers and food
processors. The grain quality characteristics include yield,
technological properties and, when possible, nutritional elements
as well. Technological properties include stability during
storage, efficiency of conversion into products under given
processing conditions and acceptability to the consumer. The
technological aspect of maize quality for tortilla preparation is
of little importance to small farmers in the least developed
countries, who seldom use seed other than that kept from harvest
to harvest. Furthermore, the rural housewife knows how to adjust
cooking conditions to the type of maize she will process for
consumption. But maize is now being converted into a tortilla
flour using industrial processes, where the grain being used may
be of different varieties from various producers and different
environments. It may have a variety of structures or may have
been poorly handled after harvest, factors which influence the
yield and physico-chemical and organoleptic as well as culinary
properties of the product. This would appear to be of growing
importance in countries such as the United States where the maize
tortilla is becoming a very popular food.
That physical characteristics of maize are important became
clear some time ago, when Bressani, Paz y Paz and Scrimshaw
(1958) showed that the yield of dry matter in the form of
dried-maize dough or flour was affected by the maize cultivar. In
their rural home studies dry matter losses from white maize
averaged 17.2 percent with a variability of 9.5 to 21.3 percent.
Dry matter losses from yellow maize averaged 14.1 percent, with a
range from 8.9 to 16.7 percent.
Cortez and Wild-Altamirano (1972) conducted a series of
measurements on 18 cultivars of maize produced in Mexico. These
included kernel weight, colour and lime-cooking time using a
standard cooking procedure with 1.5 percent lime at 80°C and a
steeping time of 12 hours. Cooking efficiency and time were
measured by the ease with which the seed coat could be removed.
Evaluations of the cooked maize included measurement of the
volume of I kg of maize, the yield of dough from I kg of grain
and the moisture content of the dough. The dough was further
evaluated by measuring its strength and water absorption. The
dehydrated dough was then ground to 60-mesh size and evaluated
for moisture, colour, specific volume and other physical
characteristics using a mixograph. The tortillas made from the
dough of each maize sample were further evaluated for
extensibility, volume, plasticity, softness and roughness of the
surface.
From this extensive study, the authors reached several
conclusions. Maize varieties or cultivars of higher weight per
volume, a harder endosperm, more moisture and a high protein
content produced the best tortillas. Two cultivars of popcorn
maize were among the best types for tortillas. The Swanson
mixograph was useful in establishing differences in maize types.
The time required to cook the samples ranged from 30 to 75
minutes, and dry matter losses ranged from 10 to 34 percent.
Rooney and Serna-Saldivar (1987) found that maize with hard or
corneous endosperm required a longer cooking time. Bedolla and
Rooney (1984) stated that the texture of the dough was affected
by the endosperm texture and type, drying, storage and soundness
of the maize kernel. MartínezHerrera and Lachance (1979)
established a relationship between kernel hardness and the time
needed for cooking. They reported that within a maize variety,
higher calcium hydroxide concentration slightly decreased cooking
time. Furthermore, knowing the initial hardness of a variety made
it possible to predict the time required to cook it. Khan et al.
(1982) and Bedolla and Rooney (1982) measured a parameter termed
nixtamal shear force (NSF), an indication of kernel hardness. The
measurement was related to both cooking time and processing
method. These authors showed that the NSF measurement could
reveal small differences in maize with similar endosperm texture
and could be used to predict optimum cooking time.
Dry matter losses resulting from lime-cooking constitute a
good index of maize quality for tortilla preparation. Jackson et
al. (1988) reported that greater losses resulted from
stress-cracked and broken kernels than from sound kernels.
Therefore they concluded that any system for assessing maize for
alkaline cooking should include measures of broken kernels, the
potential for breakage and ease of pericarp removal. Specific
studies on the effects of drying and storage on quality of maize
for tortilla making are not readily available. Bressani et al.
(1982) reported on QPM storage as related to tortilla quality.
The Nutricta QPM variety was stored under a number of field or
rural conditions. Containers made of cloth not treated with
insecticides allowed insect infestation and therefore higher
dry-matter losses during cooking; but the protein quality was not
affected.
Possibly the most interesting feature of the process of
converting maize into tortillas is the use of an alkaline medium,
and particularly calcium hydroxide. The most obvious effect of
adding lime is the facilitation of seed coat removal during
cooking and steeping. According to Trejo-González, Feria-Morales
and Wild-Altamirano (1982), added lime maintains an alkaline pH,
which is needed to hydrolyse the hemicelluloses of the pericarp.
Lime uptake by the kernel follows that of water, but the rate is
lower than that of water. Norad et al. (1986) showed that soaking
the kernels before cooking led to a higher calcium content in the
grain. Calcium content of masa was affected by lime levels and
also by cooking-steeping temperatures. Several other authors
(e.g. Pflugfelder, Rooney and Waniska, 1988a) have shown in one
way or another that lime uptake during alkaline cooking is
affected by physical and chemical characteristics of maize dough.
Martínez-Herrera and Lachance (1979) found that higher
calcium hydroxide concentrations slightly decreased cooking time,
but the differences were not statistically significant. These
authors also reported an interaction between maize variety and
calcium hydroxide concentration. However, the coefficient of
variation was high (29.1 percent); this was attributed to
inherent variability in the kernels of the different varieties.
Bedolla and Rooney (1982) reported that increases in cooking
time, cooking temperature, lime concentration and steeping time
produced lower viscoamylograph peak viscosities at both 95 and
50°C, which was interpreted to mean a greater degree of starch
gelatinization. Trejo-Gonzalez, Feria-Morales and Wild-Altamirano
(1982) showed that calcium was fixed or was bound in some way to
the starch of the maize kernel. Other effects included greater
solid losses with increasing amounts of lime; changes in colour,
aroma and flavour; and a delay in the development of acidity,
which extends shelf-life. If added in exceedingly large amounts,
lime affects organoleptic properties of the food; this effect is
often observed when maize has been stored for a long time.
Ogi and other fermented maize products
Acid porridges prepared from cereals are eaten in many parts
of the world, particularly in developing countries, where they
may form part of the basic diet. Some examples of acid porridges
include pozol in Mexico and Guatemala, ogi in Nigeria, uji in
Kenya and kenkey in Ghana. These porridges are usually made from
fermented raw or heat-treated maize, although sorghum and millet
are often used.
Ogi manufacture
The traditional process of making ogi has a number of slight
variations described by several authors. Ogi is traditionally
prepared in batches on a small scale two or three times a week,
depending on demand. The clean grain is steeped in water for one
to three days to soften. Once soft, it is ground with a grinding
stone, pounded in a mortar or ground with a power mill. The bran
is sieved and washed away from the endosperm with plenty of
water. Part of the germ is also separated in this operation. The
filtrate is allowed to ferment for 24 to 72 hours to produce a
slurry which when boiled gives the ogi porridge. Ogi is usually
marketed as a wet cake wrapped in leaves, or it may be diluted to
8 to 10 percent solids in water and boiled into a pap or cooked
to a stiff gel.
Akinrele (1970) reported that the souring of the maize took
place spontaneously without the addition of inoculants or
enzymes. He identified the organisms involved in this unaided
fermentation and investigated their effects on the nutritive
value of the food. He identified the moulds as Epholosporium,
Fusarium, Aspergillus and Penicillium species and the aerobic
bacteria as Corynebacterium and Aerobacter species, while the
main lactic acid bacterium he found was Lactobacillus plantarum.
There were also yeasts: Candida mycoderma, Saccharomyces
cerevisiae and Rhodotorula sp.
Although ogi is supposed to have an improved B-vitamin
content, the results observed are quite variable, at least for
thiamine, riboflavin and niacin. Banigo and Muller (1972)
identified the carboxylic acids of ogi fermentation. They found
11 acids, with lactic, acetic and butyric acids being the most
important.
The ogi-making process is quite complex, and the porridge can
also be prepared from sorghum, rice, millet and maize. Therefore,
laboratory procedures have been developed to learn more about the
process and introduce changes to convert the grains to food more
efficiently. These have been described by Akingbala, Rooney and
Faubion (1981) and Akingbala et al. (1987), whose studies have
been useful also in evaluating varieties of cereal grains for
their efficiency in making ogi. The authors also reported on the
yields of ogi from whole maize kernels (79.1 percent) and dry
milled flour (79.8 percent).
The commercial manufacture of ogi does not differ
substantially from the traditional method. Modifications have
been introduced, such as the dry milling of maize into a fine
meal or flour and subsequent inoculation of the flour-water
mixture with a culture of lactobacilli and yeast. In view of the
importance of ogi in the Nigerian diet, large-scale production is
indicated. The material could be dried and packaged in polythene
bags for a good shelf life. There is some problem in achieving a
controlled fermentation with pure cultures. Some modifications
include spray-drying the slurry or drum drying.
Other fermented maize products
Ogi has a number of other names such as akamu or ekogbona,
agidi and eko tutu. These, with the Kenyan uji and Ghanaian koko,
are substantially the same preparation with changes in the grain
used or some modification of the basic process. For the Mexican
pozol, maize is processed with lime as for tortillas. The
nixtamal, or cooked maize without the seed-coat, is ground to a
coarse dough which is shaped into balls by hand. The balls are
then wrapped in banana leaves to avoid drying and are allowed to
ferment for two to three days, or more if necessary. The
micro-organisms involved are many.
Arepas
Another major food made from maize, used daily in Colombia and
Venezuela, is arepa. Mosqueda Suarez (1954) and Cuevas, Figueroa
and Racca (1985) described the traditional preparation method as
practiced in Venezuela. De Buckle et al. (1972) defined the
Colombian arepa as roasted maize bread without yeast, round in
shape, prepared from maize that has been degermed. Whole maize is
dehulled and degermed using a wooden bowl called a pilon and a
double-headed wooden mallet. The moistened maize is pounded until
the hulls and part of the germ are released from the endosperm.
The hulls and germs are removed by adding water to the mixture
containing the endosperm. The endosperm is cooked and then
stone-milled to prepare a dough. Small portions of this dough are
made into balls, then pressed into flat discs which are cooked
rapidly on both sides.
The traditional method of preparing arepas has been
substantially modified by the introduction of precooked maize
flour, which reduces the time from 7 to 12 hours to 30 minutes
(Cuevas, Figueroa and Racca, 1985). There are two stages in the
industrial process. The first is the preparation of maize grits
by cleaning, dehulling and degerming the maize; the second is the
processing of the grits to produce precooked flour. Efforts have
been made to modify the process even further by extrusion
cooking.
Other maize preparations
In Latin America there are many maize-based foods besides
tortillas and arepas. Some of these are drinks like colados,
pinol and macho, basically suspensions of cooked maize flour.
These three products have a very low protein quality. The
production of humitas, a tamale-like food consumed in Bolivia and
Chile, was described by Camacho, Bañados and Fernandez (1989).
Made from immature common or opaque-2 maize to which is added a
number of other ingredients, humitas is produced from precooked
maize flour which resembles the lime-treated masa. Other products
include mote, made from cooked maize and cheese, pupusas, made
from lime-treated maize and cheese, and patasca, which is like a
lime-treated maize kernel. From immature maize a sweet, tasty
atole of high nutritive value is made; Khan and Bressani (1987)
described the process, which consists of grinding the maize in
water followed by filtration and cooking. Immature maize, either
common or opaque2, and sweet maize are also extensively consumed.
Chavez and Obregon (1986) reported on the incorporation of the
opaque-2 gene into sweet maize to provide a food of high
nutritional quality.
Maize has also been used as a substrate for fermented
beverages called chicha. Cox et al. (1987) have reported on the
microflora of these fermented products, which are made by
basically the same process but using a variety of additives.
Milling
The maize kernel is transformed into valuable foods and
industrial products by two processes, dry milling and wet
milling. The first yields grits, meal and flours as primary
products. The second yields starch and valuable derived products.
Dry milling
The dry milling of maize as practiced today has its origins in
the technologies used by the native populations who domesticated
the plant. The best example is the method used to make arepa
flour or hominy grits. The old technology was soon replaced by a
grinding stone or stone mill, followed by the grits mill and
finally by sophisticated tempering-degerming methods. The
products derived are numerous, with their variety depending to a
large extent on particle size. They are classified into flaking
grits, coarse grits, regular grits, corn meal, cones and corn
flour by means of meshes ranging from 3.5 to 60. Their chemical
composition has been well established and their uses are
extensive, including brewing, manufacturing of snack foods and
breakfast cereals and many others.
Wet milling
The largest volume of maize in developed countries such as the
United States is processed by wet milling to yield starch and
other valuable byproducts such as maize gluten meal and feed. The
starch is used as a raw material for a wide range of food and
non-food products. In this process clean maize is soaked in water
under carefully controlled conditions to soften the kernels. This
is followed by milling and separation of the components by
screening, centrifugation and washing to produce starch from the
endosperm, oil from the germ and food products from the residues.
The starch has industrial applications as such and is also used
to produce alcohol and food sweeteners by either acid or
enzymatic hydrolysis. The latter is done with bacterial and
fungal alpha-amylase, glucoamylase, beta-amylase and pullulanase.
Saccharides of various molecular weights are liberated yielding
sweeteners of different functional properties. These include
liquid or crystalline dextrose, high-fructose maize syrups,
regular maize syrups and maltodextrins, which have many
applications in foods.
Chemical changes
The conversion of maize into tortillas involves a process in
which water, heat and calcium hydroxide are used. All three
influence the chemical composition of processed maize, causing
changes in nutrient content. The changes that take place are
caused by both physical losses of the kernel and chemical losses.
The latter may result from destruction of some nutrients and
chemical transformation of others.
The proximate composition of maize and of home-made and
industrially prepared tortillas is shown in Table 16. Changes in
fat and crude fibre content are shown, and in some cases an
increase in ash content. The values for homemade and industrially
produced tortillas are similar for most major chemical components
with the exception of fat, which is higher in industrially
produced tortillas.
Dry matter losses
From studies on maize cooking by rural housewives using their
own traditional method, Bressani, Paz y Paz and Scrimshaw (1958)
reported a loss of solids (17.1 percent for white maize and 15.4
percent for yellow maize) when maize was made into dough. Bedolla
and Rooney (1982) have reported losses of 13.9 and 10 percent
respectively for white and yellow maize using the traditional
process and losses of 7 and 5.7 percent in steam cooking. In
other studies where variations in the processing technique were
evaluated, Khan et al. (1982) found losses of 7 to 9 percent in
commercial processing, 9 to I I percent in pressure cooking and
11 to 13 percent using the traditional method. These workers also
reported that dry matter loss increased as cooking time
increased.
TABLE 16 - Proximate composition of raw maize and
home-made and industrially produced tortillas
| Product | Moisture (%) | Protein (%) | Fat (%) | Ash (%) | Crude fibre (%) | Carbohydrates (%) | Calories per 100 g |
| Maize | |||||||
| White | 15.9 | 8.1 | 4.8 | 1.3 | 1.1 | 70.0 | 356 |
| Yellow | 12.2 | 8.4 | 4.5 | 1.1 | 1.3 | 73.9 | 370 |
| White | 13.8 | 8.3 | - | 1.2 | |||
| Tortillas | |||||||
| While | 47.8 | 5.4 | 1.0 | 0.8 | 0.7 | 44.5 | 204 |
| Yellow | 47.8 | 5.6 | 1.3 | 0.8 | 0.6 | 44.4 | 212 |
| White | 41.9 | 5.8 | - | 0.9 | - | - | - |
| Industrial | 40.5 | 5.8 | 0.9 | 1.1 | 1.4 | 50.3 | 226 |
| Industrial | 44.0 | 5.3 | 3.4 | 1.2 | 0.7 | 42.8 | 215 |
| Industrial | 45.2 | 5.2 | 3.1 | 1.4 | 1.1 | 41.1 | 206 |
Sources: Bressani, Paz y Paz and Scrimshaw, 1958; Cravioto et
al., 1945; Ranhotra, 1985; Saldana and Brown, 1984.
Likewise, the integrity of the maize kernel influences losses.
Jackson et a/. (1988) reported that dry matter losses in the
traditional cooking procedure were higher (10.8 to 12.1 percent)
with broken kernels then with undamaged ones (6.3 to 8.9
percent). Besides the integrity of the kernel and the heating
process used, other factors such as length of steeping influence
dry matter losses. Long steeping caused larger losses than a
short steeping time. Dry matter losses of QPM with a hard
endosperm are similar to those of common maize. Recently,
Bressani et al. (1990) reported losses of 17.1 percent for the
Nutricta QPM variety as compared with 17.6 percent from a white
tropical maize. Sproule e! al. (1988) found a 9.6 percent dry
matter loss from QPM as compared with a 10.4 percent loss in
common maize.
Dry matter losses depend, then, on a number of variables such
as the type of maize (hard or soft endosperm), kernel integrity
(whole or broken kernels), cooking procedure (traditional, steam
cooking, pressure cooking, commercial), the levels of lime used,
cooking time and steeping time, as well as other operations such
as rubbing to eliminate the seed-coat during washing of the
kernels. This process also eliminates other parts of the kernel:
the tip cap and possibly the aleurone layer and small amounts of
germ. Paredes-López and Saharopulus-Paredes (1983) used scanning
electron microscopy to show that the outside surface of
lime-treated maize had important structural deterioration. They
indicated that the aleurone layer was retained as well as some
pericarp layers and that the germ remained attached to the
endosperm. Gómez et al. (1989) noted that important structural
changes took place in maize during "nixtamalization".
The alkali weakened the cell walls, facilitating the removal of
the pericarp. It solubilized the cell wall in the peripheral
endosperm, caused swelling and partial destruction of starch
granules and modified the appearance of the protein bodies. The
dough contained fragments of germ, pericarp, the aleurone and
endosperm, as well as free starch and dissolved lipids. Thus some
of the chemical changes that have been observed can be accounted
for by the chemical compounds present in these three or four
parts of the kernel. The dry matter content has been analysed by
Pflugfelder, Rooney and Waniska (1988a), who reported 64 percent
non-starch polysaccharides (fibre), 20 percent starch and 1.4
percent protein.
Nutrient losses
Studies on the losses of nutrients during the transformation
of maize into tortillas are not abundant, even though significant
changes due to processing do take place (Cravioto et al., 1945;
Bressani, Paz y Paz and Scrimshaw, 1958). Ether-extractable
substances are lost, 33 percent in yellow maize and 43 percent in
white maize. This is difficult to explain, although it could be
partially accounted for by the loss of the pericarp, the aleurone
layer, the tip cap and some of the germ, parts of the kernel
containing ether-extractable substances. Losses in crude fibre
were reported to be about 46 percent in white maize and 31
percent in yellow maize. Lime treatment at 96°C for about 55
minutes hydrolyses the pericarp, which is removed during washing,
pulling the tip cap with it, and this would account to a large
extent for fibre loss. Nitrogen losses amount to about 10 and 5
percent for white and yellow maize, respectively. Again, this may
be partly due to the physical loss of the pericarp and tip cap.
Even though tortillas may have a slightly higher protein content
than the original maize on an equal moisture basis, as has been
reported by various workers, this may be caused by a
concentration effect, since soluble sugars from the kernel are
lost. Ash content increases because of the absorption of lime,
which significantly increases calcium content (Saldana &
Brown, 1984; Ranhotra, 1985). Significant losses take place in
thiamine (52 to 72 percent), riboflavin (28 to 54 percent) and
niacin (28 to 36 percent). In yellow maize 15 to 28 percent of
the carotene was lost (Cravioto et al., 1945; Bressani, Paz y Paz
and Scrimshaw, 1958).
Fat and fatty acids.
Ether-extractable substances of 33 and 43 percent were reported
by Bressani, Paz y Paz and Scrimshaw (1958) from yellow and white
maize respectively, as processed in Guatemalan rural homes.
Pflugfelder, Rooney and Waniska (1988b) found losses of 11.8 to
18.1 percent and suggested that these could be partly due to the
vigorous handling of cooked maize at the industrial plant. Of the
total masa lipid, 25 to 50 percent was free and partially
emulsified. Bedolla et al. (1983) found ether extract values of
5.0, 3.1 and 3.6 percent in raw maize, cooked maize and tortillas
respectively, or about a 28 percent change. This loss has not
been fully explained; however, it may result from the loss of the
seed-coat, the tip cap, the aleurone layer and possibly part of
the germ, and also from ether soluble substances, not necessarily
fat. Even though ether-extractable substances are lost in the
process of converting maize into tortillas, the fatty acid
make-up of the fat does not change in common maize or QPM, as
shown in Table 17. Differences between maize samples, either raw
or processed, are larger than those between raw maize and
tortillas, suggesting that the alkaline cooking method does not
alter the fatty acid make-up of the fat.
TABLE 17 - Fatty acid content of common and quality
protein maize and tortillas (%)
| Product | C16:0 | C18:0 | C18:1 | C18:2 |
| Common maize | 12.89 | 2.92 | 37.08 | 47.10 |
| Opaque-2 maize | 15.71 | 3.12 | 36.45 | 43.83 |
| Common maize tortilla | 13.63 | 2.95 | 37.14 | 45.76 |
| Opaque-2 tortilla | 15.46 | 3.25 | 35.84 | 43.03 |
Source: Bressani et al., 1990
Fibre content. The crude fibre
content of maize - as determined by the Association of Official
Analytical Chemists (AOAC) methodology decreases as the kernel is
converted into tortillas. Various investigators (e.g. Saldana and
Brown, 1984) have explained how and why such a loss takes place.
With newer methodology to determine fibre, Reinhold and Garcia
(1979), using the Van Soest method, reported that the neutral
detergent fibre (NDF) and acid detergent fibre (ADF) in tortillas
(6.60 and 3.75 percent, respectively, on a dry weight basis) were
significantly higher than those found in the dough (an average of
5.97 and 2.98 percent, respectively). No difference was reported
in hemicellulose, with dough containing 3.18 percent and the
tortillas 2.89 percent. Using the same method, Bressani, Breuner
and Ortiz (1989) found 10.8 percent NDF in maize and 9 percent in
tortillas, as well as ADF of 2.79 and 3 percent respectively.
Hemicellulose averaged 8 percent in maize and 6 percent in
tortillas, while the values for lignin were 0.13 and 0.15
percent. These values and others are shown in Table 18. Using the
method of Asp et al. (1983), Acevedo and Bressani (1990) detected
a decrease in insoluble fibre from raw maize (13 percent) to the
dough (6 percent) and an increase in tortillas (7 percent).
Soluble fibre increased from 0.88 percent in raw maize to 1.31
percent in the dough, and further increased to 1.74 percent in
tortillas. Fibre decreases from raw maize to dough are due to the
losses in seed-coat described previously. Increases from dough to
tortillas, however, may be due to the browning reaction, as has
been reported in baked wheat products (Ranhotra and Gelroth,
1988).
TABLE 18 - Dietary fibre in common and quality protein
make and tortillas (%)
| Product | Insoluble dietary fibre | Soluble dietary fibre | Total dietary fibre | Neutral detergent fibre | Acid detergent fibre | Hemicellulose | Lignin |
| Raw common maize | 11.0 | 1.4 | 12.4 | 10.8 | 2.8 | 8.0 | 0.13 |
| Common maize tortilla | 9.5 | 1.4 | 10.9 | 9.0 | 3.0 | 6.0 | 0.15 |
| Raw QPM | 13.8 | 1.1 | 14.9 | - | - | - | - |
| QPM tortilla | 10.3 | 1.9 | 12.2 | - | - | - | - |
| Other tortilla | 3.4 | - | - | 6.6 | 3.7 | 2.9 | - |
| Other tortilla | 4.1 | - | - | - | 3.8-5.0 | - | - |
Sources: Acevedo and Bressani, 1990; Bressani, Breuner and
Ortiz, 1989; Bressani et al., 1990; Krause, 1988; Ranhotra, 1985;
Reinhold and Garcia 1979.
Ash. Changes in ash content have not
received much attention from researchers. Most findings, however,
have shown an increase in total ash content from maize to
tortillas, which may be expected because of the lime used for
cooking. Along with this increase in ash there is a significant
increase in calcium content. According to Pflugfelder, Rooney and
Waniska (1988b), calcium content in the dough is influenced by
lime levels, cooking and steeping temperatures and maize
characteristics. The changes in other minerals are variable and
may depend on the purity of the lime used as well as on the type
of grinding equipment. In one study (Bressani, Breuner and Ortiz,
1989; Bressani et al., 1990) the magnesium content increased from
8 to 35 percent from maize to tortilla; there was no change in
sodium and a small decrease in potassium. Iron content also
increased; however, the increases may have resulted from
contamination. Phosphorus content also increases from maize to
tortilla (Table 19). One aspect of nutritional interest is that
the calcium-to-phosphorus ratio, which is about 1:20 in maize,
changes to approximately 1:1 in the tortilla.
TABLE 19 - Mineral content of raw maize and home and
industrial samples of tortillas (mg/100 9)
| Product | P | K | Ca | Mg | Na | Fe | Cu | Mn | Zn |
| Maize | 300 | 325 | 48 | 108 | 54 | 4.8 | 1.3 | 1.0 | 4.6 |
| Home-made tortilla 1 | 309 | 273 | 217 | 123 | 71 | 7.0 | 2.0 | 1.0 | 5.4 |
| Home-made tortilla 2 | - | - | 202 | - | - | 2.7 | 0.3 | - | 3.4 |
| Home-made tortilla 3 | 294 | - | 104 | 72 | - | 3.5 | 1.3 | - | 4.6 |
| Industrial tortilla 1 | 315 | - | 182 | 106 | - | 4.0 | 2.5 | - | 3,2 |
| Industrial tortilla 2 | 240 | 142 | 198 | 60 | 2 | 1.2 | 0.17 | 0.41 | 1.2 |
| Industrial tortilla 3 | 269 | 185 | 205 | 63 | 9 | 1.5 | 0.19 | 0.40 | 1.1 |
Sources: Bressani et al., 1990; Krause, 1988; Ranhotra, 1985;
Vargas, Munoz and Gómez 1986
Carbohydrates. Maize and tortillas
contain significant amounts of soluble carbohydrates, but very
little is known on how they change during alkaline processing.
Starch losses of about 5 percent have been reported; these are
recovered in the solids lost. A decrease in sugar from 2.4
percent in maize to 0.34 percent in tortillas was also found.
Robles, Murray and Paredes-Lopez (1988) found that alkali-cooking
and soaking of maize caused large increases in viscosity and that
cooking time had a significant effect on pasting properties,
although there was no extensive gelatinization of the starch.
Differential scanning calorimetric studies yielded similar
gelatinization endotherms for untreated maize and nixtamal
flours. In the process enzyme-susceptible starch increases as
cooking time lengthens.
Protein and amino acids. Most
researchers report a small increase in N content which is
attributed to a concentration effect. The solubility of all
protein fractions is decreased from raw maize to tortillas, with
an increase in the insoluble fraction.
Bressani and Scrimshaw (1958) extracted the nitrogen from raw
maize and tortillas using water, sodium chloride, 70 percent
alcohol and sodium hydroxide. The solubility of the water, salt
and alcohol protein fractions was significantly lower in
tortillas, with the alcohol-soluble proteins affected most. Only
a small decrease of about 13 percent in the solubility of the
alkali-soluble fraction was detected. Because of this, the
insoluble nitrogen fraction increased from 9.4 percent in maize
to 61.7 percent in tortillas.
Ortega, Villegas and Vasal (1986) observed similar changes in
both common and QPM maize using the Landry-Moureaux (1970)
protein fractionation technique. The solubility of true zeins
decreased 58 percent in the tortillas prepared from common maize
and 52 percent in QPM tortillas. The authors indicated that
hydrophobic interactions may have been involved in the change in
protein solubility observed. Sproule et al. (1988) noted a
decrease in the albumin plus globulin-nitrogen, expressed as
percentage of total nitrogen, from maize to tortillas.
The changes in amino acid content from maize to tortillas are
summarized in Table 20. In vitro enzymatic studies of amino acids
indicated that total nitrogen and alpha-amino nitrogen were
released faster from maize than from tortillas. Values for
alpha-amino nitrogen released, expressed as a percentage of the
total nitrogen release, were higher for tortillas than for raw
maize after 12 hours of hydrolysis with pepsin. The percentage of
alpha-amino N from the total was similar for maize and tortillas
at 60 hours of hydrolysis with trypsin and pancreatic. After 60
hours of hydrolysis with pepsin, trypsin and pancreatin, the
percentage of enzymatically released amino acids with respect to
the acidhydrolysed amino acids suggested a faster release from
tortillas than from maize. This information was recorded up to 36
hours for most of the amino acids except leucine, phenylalanine,
tryptophan and valine, which were released at about the same
rate. At 60 hours of hydrolysis the amino acid concentrations of
the maize and tortilla hydrolysates reached comparable levels,
except for methionine (Bressani and Scrimshaw, 1958). These
authors reported losses of arginine (18.7 percent), histidine
(11.7 percent), lysine (5.3 percent), leucine (21 percent),
cystine (12.5 percent) and small amounts of glutamic acid,
proline and serine.
Sanderson et al. (1978) found small losses of arginine and
cystine from alkaline treatment of common and high-lysine maize.
These same authors found 0.059 and 0.049 g of Iysino-alanine per
100 g protein from common and high-lysine maize respectively, but
none was found in raw maize. In commercial masa, they found 0.020
g Iysino-alanine per 100 g protein, while in tortillas the level
found was 0.081 g per 100 g protein.
TABLE 20 - Amino acid changes during the alkaline
cooking of maize (9/16 g N)
| Amino acid | Maize | Tortilla | Maize | Dough | Tortilla | QPM | Dough |
| Arginine | 5.1 | 4.2 | 5.4 | 4.6 | 5.5 | 8.3 | 7.9 |
| Histidine | 2.7 | 2.4 | 2.9 | 2.8 | 3.5 | 3.9 | 3.8 |
| Isoleucine | 4.2 | 4.5 | 3.7 | 3.8 | 3.5 | 3.4 | 3.3 |
| Leucine | 12.2 | 9.6 | 12.6 | 13.4 | 12.1 | 8.3 | 8.3 |
| Lysine | 3.0 | 2.9 | 3.0 | 2.7 | 2.9 | 5.1 | 5.2 |
| Methionine | 1.9 | 1.9 | 2.8 | 2.9 | 2.3 | 1.9 | 1.9 |
| Cystine | 1.0 | 0.9 | - | - | - | - | - |
| Cysteine | - | - | 2.0 | 1.7 | 1.9 | 2.5 | 2.2 |
| Phenylalanine | 3.7 | 3.8 | 5.0 | 5.2 | 4.7 | 4.3 | 4.2 |
| Tyrosine | 3.8 | 3.8 | 4.5 | 4.6 | 4.4 | 3.8 | 3.7 |
| Threonine | 3.0 | 3.0 | 3.8 | 3.8 | 3.4 | 3.6 | 3.6 |
| Tryptophan | 0.5 | 0.5 | - | - | - | - | - |
| Valine | 4.5 | 4.8 | 4.8 | 5.3 | 4.9 | 5.1 | 5.0 |
| Glutamic acid | 20.3 | 19.0 | 18.8 | 19.5 | 18.9 | 15.4 | 15.7 |
| Aspartic acid | 6.2 | 6.2 | 7.2 | 6.9 | 5.8 | 8.4 | 8.4 |
| Glycine | 4.8 | 4.8 | 4.0 | 4.3 | 3.5 | 4.7 | 4.6 |
| Alanine | 8.8 | 8.8 | 7.7 | 8.1 | 7.6 | 6.1 | 6.1 |
| Serine | 4.5 | 4.2 | 5.0 | 5.0 | 4.7 | 4.4 | 4.5 |
| Proline | 11.0 | 10.1 | 9.2 | 10.7 | 8.7 | 7.0 | 7.6 |
Sources: Bressani and Scrimshaw, 1458; Sanderson el al., 1978
Lunven (1968), using his own amino acid column chromatography
technique, observed significant losses in both lysine and
tryptophan during the alkaline treatment of common maize. Ortega,
Villegas and Vasal (1986) found small losses in tryptophan in
tortillas of both common maize (about 1 1 percent) and QPM (15
percent). On the other hand, they reported minimal losses in
lysine from both types of maize, similar to those previously
noted. Higher losses for both amino acids have recently been
reported by Bressani et al. (1990) from common maize and QPM
(Nutricta) maize converted into tortillas by rural processing.
Ortega, Villegas and Vasal (1986) also indicated that on the
basis of the very small loss of lysine in the alkaline product,
minimal amounts of lysino-alanine were probably present in the
tortillas of common maize and QPM used in their study.
TABLE 21 - Vitamin content of raw maize and tortillas
(mg/100 9)
| Product | Thiamine | Riboflavin | Niacin | Folic acid | Panthothenic acid | Vitamin H. | Carotene | Total carotenoids |
| Raw maize | ||||||||
| White | 0.38 | 0.19 | 2.00 | - | - | - | - | - |
| Yellow | 0.48 | 0.10 | 1.85 | - | - | - | 0.30 | 1.32 |
| White | 0.34 | 0.08 | 1.64 | - | - | - | 0.15 | - |
| Tortillas | ||||||||
| White | 0.10 | 0.04 | 1.01 | - | - | - | - | - |
| Yellow | 0.11 | 0.05 | 1.01 | - | - | - | 0.12 | 0.41 |
| White | 0.19 | 0.08 | 0.96 | - | - | - | 0.06 | - |
| Industrial | 0.13 | 0.08 | 1.11 | - | - | - | - | - |
| Industrial | 0.07 | 0.04 | 1.61 | 0.014 | 0.24 | 0.12 | - | - |
| Industrial | 0.08 | 0.05 | 2.11 | 0.015 | 0.16 | 0.27 | - | - |
Sources: Bressani, Paz y Paz and Scrimshaw, 1958; Cravioto e,
al., 1945; Ranhotra, 1984; Saldana and Brown, 1984.
Vitamins. Losses in thiamine,
riboflavin, niacin and carotene occurred during processing of
maize into tortillas by lime-cooking. A summary of some data is
shown in Table 21. The vitamin that has attracted the attention
of a number of researchers has been niacin because of its
relationship to pellagra. The biological implications of the
lime-cooking process on niacin availability and pellagra is
discussed in the next section. This section discusses the changes
in concentration of niacin resulting from limecooking. Bressani,
Gómez-Brenes and Scrimshaw (1961) reported that the seed-coat of
maize contained 4.2 mg niacin per 100 g, while the germ and
endosperm contained about 2 mg niacin per 100 g. About 79.5
percent of the kernel niacin was provided by the endosperm, and
10 percent each by the germ and seed-coat. After lime-cooking,
the endosperm contributed about 68 percent of the total niacin
and the germ about 5.5 percent. After cooking, 26 percent of the
total was found in the cooking water. The percentage of niacin
extracted in water was 68.5 percent of the total with raw grain
and 76 percent with lime-cooked maize. Furthermore, enzymatic
hydrolysis with pepsin yielded 69 percent of the niacin of all
the samples, and after trypsin and pancreatin hydrolysis niacin
yields were 78 and 100 percent respectively. This information was
interpreted to mean that niacin is slightly more available from
lime-treated maize than from raw maize.
Nutrient availability
Although the lime-cooking process to convert maize into
tortillas induces some important losses in nutrients, the process
also causes important changes in nutrient availability.
Calcium. Because of the use of
calcium hydroxide in converting maize into tortillas, the calcium
content of the product increases significantly, up to about 400
percent. Bioavailability studies conducted by Braham and Bressani
(1966) with animals showed that somewhat less of the calcium was
available in lime-treated maize (85.4 percent) than in skim milk
(97 percent). Calcium bioavailability increased when lime-treated
maize was supplemented with its limiting amino acids, lysine and
tryptophan. Recently, Poneros and Erdman (1988) confirmed the
high bioavailability of calcium from tortillas with or without
the addition of ascorbic acid. As pointed out in a previous
section, the use of calcium hydroxide improves the
calcium-to-phosphorus ratio in tortillas, which possibly favours
the utilization of the calcium ion by the animal. This is an
important finding for populations who do not consume diets high
in this essential mineral. Furthermore, the finding that better
quality in maize protein favours calcium big-utilization is of
nutritional significance and provides an additional reason for
the commercial production of QPM for people who depend on maize
for their nutrition.
Amino acids. Bressani and Scrimshaw
(1958) carried out studies using in vitro enzymatic digestions
with pepsin, trypsin and pancreatic. At the end of the pepsin
digestion, the amount of alpha-amino nitrogen as a percentage of
total digested nitrogen was twice as high from tortilla (43.1
percent) as from maize (21.4 percent) and levels of histidine,
isoleucine, leucine, lysine methionine, phenylalanine, threonine
and tryptophan were higher from the tortilla hydrolysate than
from maize, suggesting a faster release from the proteins. These
authors proposed that the difference in rate of release could
derive from the significant decrease in the solubility of the
prolamine protein fraction in tortillas, as compared with maize.
Serna-Saldivar et al. (1987), however, working with
ileum-cannulated pigs, found that at this level in the intestinal
tract the digestibility of most of the essential amino acids was
somewhat higher from water-cooked maize than from limecooked
maize. Digestibility of the protein decreased slightly, possibly
because of the heat treatment involved (Bressani et al., 1990).
Other researchers have suggested that during maize processing,
hydrophobic interactions, protein denaturation and cross-linking
of proteins are probably responsible for changes in the
solubility of these components, which could affect amino acid
release during enzymatic digestion.
Niacin. The alkaline treatment of
maize has been reported to destroy its pellagragenic factor.
Evidence from a large number of researchers has suggested that
pellagra results from an imbalance of the essential amino acids,
increasing the niacin requirement of the animal. This point has
been extensively debated between those who claim that niacin in
maize is bound and not available to the animal and those who
favour the theory of improved amino acid balance induced by the
alkaline-cooking process, as lime treatment results in release of
the bound niacin. Pearson et al. (1957) have shown that boiling
maize in water has the same effect (that is, it increases niacin
availability). Bressani, Gómez-Brenes and Scrimshaw (1961) found
that in vitro enzymatic digestion liberated all the niacin from
raw maize as from tortillas and reached the conclusion that
differences in amino acid balance rather than in bound niacin
were responsible for the differences between raw and
lime-processed maize in biological activity and pellagragenic
action. Lime treatment of maize improves amino acid balance, as
demonstrated by Cravioto et al. (1952) and Bressani and Scrimshaw
(1958). Other workers have shown that experimental animals grow
better when fed lime-treated rather than raw maize. Using cats
which cannot convert tryptophan into niacin - Braham, Villareal
and Bressani (1962) showed that niacin from raw and lime-treated
maize was utilized to an equal extent, suggesting its
availability is not affected by processing.
Dietary Fibre. It has been shown
above that when maize was processed into tortillas by
lime-cooking, total dietary fibre (TDF) decreased at the dough
stage and increased in the tortilla to levels only slightly below
those found in raw maize. In these studies the levels of TDF in
tortillas averaged 10 percent on a dry weight basis. If a person
consumed about 400 g of tortilla (dry-weight), the TDF intake
would be 40 g, a value significantly higher than the recommended
intake. Even small children can consume relatively large amounts
of dietary fibre, which can affect the availability of iron.
Hazell and Johnson (1989), however, indicated that maize-based
snack foods prepared by extrusion cooking have a higher iron
availability than raw maize. These authors indicated that
refining of raw maize, product formulation, extrusion cooking and
addition of flavourings were responsible to different degrees.
Likewise zinc intake could be affected. The other mineral that
could be affected would be calcium; however, Braham and Bressani
(1966) and Poneros and Erdman (1988) showed that calcium is
relatively well available from tortillas and that its
availability is increased when the protein quality is improved
through addition of the limiting amino acids. An excess of
calcium rather than dietary fibre could be responsible for zinc
availability, as has been indicated in a number of studies.
Protein quality of maize and nutrient bioavailability
Growing rats retained calcium from tortillas better when it
was supplemented with lysine its limiting amino acid, and with a
mixture of amino acids. Protein quality is an important factor in
bioavailability of nutrients from maize and its lime-treated
products. As already stated, niacin availability also improves
when protein quality is improved, and studies with QPM have shown
better utilization of niacin. The same observation has been made
on the utilization of carotene, which is higher in
Iysine-supplemented yellow maize than in the unsupplemented
product.
Changes in quality. Changes in
nutritional value, particularly that of protein, during the
transition from raw maize to tortillas have been studied mainly
in animals. Even though chemical losses in some nutrients take
place upon limecooking of maize, protein quality is slightly but
consistently better in tortillas than in raw maize. Table 22
summarizes the results of various studies where raw maize and the
tortillas made from it have been evaluated. The protein
efficiency ratio of the tortillas is in general somewhat higher
than that of the raw maize, although some studies have reported
otherwise. The difference may be attributed to processing
conditions, particularly the concentration of lime added, which
is lower in rural home cooking than at the industrial level. The
chemically determined amino acid pattern of the tortillas is no
better than the pattern in raw maize. The only explanation is
that the process increases the availability of key amino acids.
This is indicated by the results of feeding studies with young
rats (Bressani, Elías and graham, 1968). Both raw maize and the
lime-cooked dough were supplemented with increasing levels of
lysine alone (from 0 to 0.47 percent of the diet). Maximum PER
for maize was obtained with an addition of 0.31 percent and for
the lime-cooked dough with 0.16 percent. At all levels of
supplemental lysine the dough gave higher PER values than the raw
maize.
Tryptophan supplementation alone was also tested, and in this
case 0.025 percent addition gave the highest PER for maize, with
no response for the dough. The addition of the two amino acids at
the level of 0.41 percent lysine with tryptophan varying from
0.05 to 0.15 percent improved the quality of both materials,
although it was higher for the dough.
TABLE 22 - Protein quality of maize and tortillas
| Type of maize |
Protein
quality (PER)
|
||
| Raw maize | Tortillas | Casein | |
| Common | 1.13 ± 0.26 | 1.27 ± 0.27 | |
| Common | 1.49 ± 0.23 | 1.55 ± 0.23 | 2.88 ± 0.20 |
| QPM (Opaque-2) | 2.79 ± 0.24 | 2.66 ± 0.14 | 2.88 ± 0.20 |
| Common | 1.38 | 1.13 | 2.50 |
| Common Tropical | 0.99 ± 0.25 | 1.41 ± 0.11 | 2.63 ± 0.17 |
| Common Highland Xetzoc | 0.96 ± 0.19 | 1.41 ± 0.20 | 2.63 ± 0.17 |
| Common Highland Azotea | 1.02 ± 0.19 | 1.41 ± 0.17 | 2.63 ± 0.17 |
| Common Highland Sta. Apolonia | 0.71 ± 0.20 | 0.98 ± 0.17 | 2.63 ± 0.17 |
| QPM Nutricta | 1.91 ± 0.23 | 2.12 ± 0.12 | 2.63 ± 0.17 |
| Biological value of common maize | 59.5 | 59.1 | 69.4 |
| Net protein utilization of common maize | 51.2 | 49.4 | 64.5 |
These results were interpreted to mean that the quality of
lime-treated maize is superior to that of raw maize. This
explanation is supported by in vitro studies showing a greater
release of essential amino acids (EAA) from tortillas than from
maize, even though Ortega, Villegas and Vasal (1986) reported in
vitro protein digestibility in maize, dough and tortillas to be
88, 91 and 79 percent respectively. For QPM the respective values
were 82, 80 and 68 percent. Recently, Serna-Saldivar et al.
(1987) reported on dry matter, gross energy and nitrogen
digestibilities of maize cooked with and without lime. No
differences in dry matter or gross energy digestibility values
were found between the different processing treatments. Cooking
maize with lime, however, reduced nitrogen digestibility from
76.5 to 72.8 percent. These values were measured near the end of
the small intestine in pigs. Values for dry matter, gross energy
and nitrogen digestibility increased when measured over the pigs'
total digestive tract. From nitrogen balance studies, the same
authors reported a retention of intake nitrogen of 45.8 percent
for maize cooked without lime and 41.2 percent for lime-cooked
maize. Retention of absorbed nitrogen was 48.2 percent for the
lime-cooked maize and 52.9 percent for the maize cooked with
water alone. Digestible and metabolizable energy were similar in
maize processed with and without lime. The authors concluded that
the lime-cooking process decreased the nutritive value of maize.
In another study by Serna-Saldivar et al. (1988b), this time
conducted with rats, the authors noted an increase in percentage
of dry matter and gross energy digestibilities from maize to
nixtamal (dough) and to tortillas; however, protein digestibility
decreased. In vitro studies correlated with in vivo values.
graham, Bressani and Guzmán (1966) showed better weight gain in
DurocJersey pigs fed lime-treated maize than in those fed raw
maize, with better feed efficiency. In studies with dogs, lysine
and tryptophan added to lime-cooked maize improved nitrogen
balance to the value obtained with skim milk (Bressani and de
Villareal, 1963; Bressani and Marenco, 1963). It was further
shown that after these two amino acids, isoleucine, threonine,
methionine and valine increased nitrogen retention above values
measured with lysine and tryptophan. Lime-treated maize has also
been evaluated in children (see Chapter 6). Nitrogen balance
results have shown a high response to lysine and tryptophan
addition, which in turn is dependent on the level of protein
intake. At low levels, only lysine improved quality, but as
nitrogen intake increased, the addition of tryptophan with lysine
became important. All studies suggest that in limetreated maize,
lysine is slightly more deficient than tryptophan, and the
contrary seems to be the case for raw maize. Nevertheless, for a
significant improvement in protein nutritional quality of
lime-treated maize, both of these amino acids are required.
Use of QPM. Nutritionally improved
(QPM) maize shows the same changes in protein quality and
bioavailability after lime-cooking and conversion to tortillas as
observed in normal maize. The difference is that QPM tortillas
and products are nutritionally superior to those made from common
maize. They are as acceptable to consumers.
Other effects of lime-cooking
Lysinoalanine formation. In 1969, De
Groot and Slump demonstrated that alkali treatment of proteins
gave rise to peptides such as Iysinoalanine (LAL), lanthionine
and ornithine which had negative effects on animals. They were
not biologically available and had detrimental effects on protein
quality. Consequently, the effect of the alkaline-cooking process
to convert maize into tortillas has received attention from
various researchers. Sternberg, Kim and Schwende (1975) reported
that commercial samples of masa flour, tortillas and taco shells
contained 480,200 and 170 µg LAL per gram. Sanderson et al.
(1978) also found that lanthionine and omithine were formed
during alkaline cooking of maize. These authors found no LAL in
common or in high-lysine raw maize; however, these products
contained 0.059 and 0.049 g percent protein respectively after
alkali treatment. A commercial masa contained 0.020 percent, and
tortillas 0.081 percent protein. These authors also reported
lanthionine and ornithine values in the masa prepared from the
two types of maize. Chu, Pellet and Nawar (1976) reported values
of 133.2 µg of LAL per gram protein when maize was processed
with 4.1 mol per kg of lime for 30 minutes at 170°F (76.6°C).
The use of sodium hydroxide under equal conditions yielded higher
levels of LAL. Since higher levels of LAL were obtained with NaOH
and KOH, the authors suggested that calcium ions may in some way
interfere with the mechanism of LAL formation. It is difficult to
evaluate the significance of LAL formation during tortilla-making
for people who eat relatively large amounts of this food daily.
Since this has been practiced for a long time, the small amounts
may not interfere with nutritive value or cause any pathological
effects. Studies on the effect of lime level on the protein
quality of maize have shown, however, that levels above 0.5
percent of grain weight reduce protein quality. The type of maize
used and its size are of importance in this respect. Softer types
of grains are more affected than hard grains cooked under similar
conditions (Bressani et al., unpublished data).
Mycotoxins and alkaline-cooking of maize.
The presence of mycotoxins in a variety of cereal grains and
other foods and foodstuffs widely recognized, and maize is no
exception. In Central America, where maize is such an important
food, the grain is harvested twice a year in the tropical areas.
One harvest is in August, when rain and temperature conditions
are ideal for the growth of fungi. Martínez et al. (1970b)
reported the presence of six different fungi in maize samples
obtained from different markets throughout Guatemala. The
frequency of Aspergillus versicolor was 57.1 percent; of
Aspergillus wentii, 32.1 percent; of Aspergillus ruber, 26.8
percent; of Aspergillus echinulatus, 25.0 percent; of Aspergillus
flavus, 25.0 percent; and of Chaedosporium spp., 26.8 percent.
Because of the significance of the presence of mycotoxins in
cereal grains, a number of studies have been conducted to assess
the degree of retention of mycotoxins during grain processing.
The effect of calcium hydroxide cooking of maize has received
some attention. Martínez-Herrera (1968) fed infected maize, raw
and alkali-processed, to chickens and rats. The maize was
infected with Fusarium sp., Penicillium spp., Aspergillus niger
and A. flavus. The author found high mortality among birds fed on
the raw infected maize, but none in the group of chickens fed the
same maize processed with calcium hydroxide. In young rats, the
raw and infected grain reduced weight gain and caused some
mortality. Infected grain processed with lime induced no
mortality, however, and weight gain as well as feed efficiency
were like those in the control. Adult rats were also affected by
the infected maize, but not by infected maize processed with
lime. The study did not report levels of mycotoxins before and
after processing.
Martinez (1979) reported on studies of tortilla samples
collected in Mexico City in different seasons. He found that 15
to 20 percent of the samples collected in spring 1978 and in the
rainy season of 1977-1978 contained aflatoxins. Furthermore, he
found that concentrations of aflatoxins B1 varied from 50 to 200
ppb. He also indicated that lime-cooking of maize reduced
aflatoxin concentrations by 50 to 75 percent. Martínez and also
de Campos, Crespo-Santos and Olszyna-Marzys (1980) reported that
lime concentrations of up to 10 percent were no more effective in
reducing aflatoxins than a 2 percent concentration.
Ulloa-Sosa and Schroeder (1969) reported that the
tortilla-making process was not effective in removing aflatoxins
from contaminated maize. Nevertheless, others have obtained
different results. For example, SolorzanoMendizabal (1985) found
that maize inoculated with A. flavus and Aspergillus parasiticus
produced high levels of aflatoxins which were reduced by
lime-cooking, completely in some cases, but most often by up to
80 percent. Lime concentration varied from 0.6 to 8 percent, and
analyses were done on maize, masa, tortillas and cooking waters.
In another study, de Arriola et al. (1987, 1988), using QPM
Nutricta, found that the lime levels at which nixtamal is
normally prepared in Guatemala do not reduce aflatoxin in
contaminated grain sufficiently to make it safe for human
consumption.
Lime levels of 2 percent and above gave high aflatoxin
reduction, but the tortillas were not acceptable. Aflatoxin B1
was reported to be reduced the most. Torreblanca, Bourges and
Morales (1987) found relatively high aflatoxin levels in both
maize and tortillas in a study conducted in Mexico City.
Aflatoxin B1 was found in 72 percent of the maize tortilla
samples tested; furthermore, 24 percent of the samples gave
positive reactions for zearalenone. Carvajal et al. (1987) found
mycotoxins in maize and tortillas in Mexican samples and
indicated that aflatoxins, zearalenone and deoxynivalenol (DON)
were not destroyed by the lime treatment or by temperatures of
110°C.
Price and Jorgensen (1985) found that the alkaline cooking
process reduced aflatoxin levels from 127 µg per kg in raw maize
to 68.6 µg per kg in tortillas. The authors concluded that the
process was poorly effective, since the lower value obtained was
still much above the value established as acceptable (about 20 mg
per kg). These authors found that acidification - as it occurs in
the intestinal tract - increased aflatoxin levels. Abbas et al.
(1988) reported on the effect of 2 percent lime-cooking of maize
on the decomposition of zearalenone and DON. They found
significant reductions, i.e. 58 to 100 percent for zearalenone
and 72 to 82 percent for DON. Furthermore, 15-acetyl-DON was
completely destroyed.
Results obtained by various authors are somewhat conflicting,
since some of them report partial reduction in some mycotoxins
while others note total reduction. In many studies the mycotoxin
levels were relatively high, necessitating stronger processing
conditions in terms of lime concentration and cooking time. The
problem warrants further study. Grain quality is probably the
best means of ensuring the absence of mycotoxins rather than
dependence on lime to reduce them partly or eliminate them in the
final product.
Microbiological aspects of tortillas and tortilla
flour. Studies on the microflora in lime-cooked
maize tortillas are very limited. Capparelli and Mata (1975)
showed that the main contaminants of tortillas as made in the
highlands of Guatemala were coliforms, Bacillus cereus, two
Staphylococcus species and many types of yeasts. When tortillas
are first cooked, bacterial counts are about 103 or fewer
organisms per gram, which is a safe level for consumption. After
they are cooked for about five minutes on a hotplate they are
placed hot in a basket, often covered with a cloth. This captures
the vapour from the tortillas, creating an environment
appropriate for microbial growth. After some ten hours under
these conditions the surfaces of stacked tortillas become slimy
and they are not acceptable for consumption.
Although there are many opportunities in rural areas for
contamination during processing from maize to tortillas, the
factors that possibly contribute the most are the water used
during conversion of cooked maize to dough and the mill used to
grind the cooked maize. Molina, Baten and Bressani (1978)
reported a greater increase in bacteria counts in tortillas
fortified with soybean flour and vitamins than in unfortified
tortillas. In this case the mill used to grind the cooked maize
to make the dough was chlorinated, which helped in lowering the
bacteria count in the soy-supplemented maize. The tortillas made
from it also had a lower bacteria count. The rate of increase in
bacterial number decreased as well. Higher bacteria counts were
reported by Valverde et al. (1983) in the cough and tortillas
made from QPM Nutricta than in those from common maize, showing
the effect of nutritional quality on bacterial growth.
The relatively high moisture content which is responsible for
a very short shelf-life has limited marketing of tortillas.
Nevertheless, there is a demand for them in urban areas, where
they are marketed under refrigerated conditions. A number of
attempts have been made to lengthen their shelf life. Rubio
(1972a, 1972b, 1973, 1974a, 1974b, 1975) patented a number of
methods which included various additives: epichlorohydrin and
polycarboxylic acid and their anhydrides; hydrophilic inorganic
gels; sorbic acid and its salts as well as the methyl, ethyl,
butyl and propyl esters of para-hydroxy benzoic acid; and acetic
and propionic acids. Pelaez and Karel (1980) developed an
intermediatemoisture tortilla with a stable shelf-life. It was
free from microbial growth, including Staphylococcus aureus,
yeasts, moulds and enterotoxin. This was achieved through the use
of glycerol, corn solids DE-42 and salt, as well as the
mycostatic agent potassium sorbate. Protection with appropriate
packaging was claimed for at least 30 days and the appearance,
texture and other characteristics were similar to those of
regular tortillas with a water activity of 0.97. Hickey, Stephens
and Flowers (1982) reported relatively good protection of
tortillas with low levels of sorbates or propionates added to the
dough, and with a spray of sorbate on the surface (both sides)
after cooking on the hot plate. More recently, Islam, Lirio and
Delvalle (1984) claimed that using calcium propionate extended
the shelf-life of tortillas at room temperature to 2 to 5 days;
with dimethyl fumarate shelf-life was 2 to 11 days under the same
storage conditions and using polythene bags. Although advances
have been made in extending shelf-life, it still constitutes a
problem for people who buy food in supermarkets.
Reports on the microbiology of tortilla flour and the
tortillas made from it are not available. Lower total bacteria
counts would be expected, however, because of the process
employed to prepare the flour and use it at home.
Chemical changes
The process of fermenting maize, sorghum, millet or rice to
produce ogi not only removes parts of the maize kernel such as
the seed-coat and the germ, but also involves washing, sieving
and decanting, all of which induce changes in the chemical
composition and nutritive value of the final product. Akinrele
(1970) reported on specific nutrients of a number of ogi samples
produced in different ways: unfermented and fermented with
Aerobacter cloacae, Lactobacillus plantarum and a mixture of the
two bacteria. He compared the values found with those from the
traditionally fermented product. Judging from the ratio of amino
nitrogen to total nitrogen, the author reported that protein was
degraded to a very small amount by any bacterial species. When
compared with the unfermented ogi, A. cloacae appeared to
synthesize more riboflavin and niacin, which did not take place
with L. plantarum. Traditionally produced ogi had more thiamine
and slightly lower values of riboflavin and niacin than that made
with maize and A. cloacae. In any case the changes were small,
and smaller if compared with whole maize, whereas in comparison
with degermed maize, the ogi products contained more riboflavin
and niacin. Akinrele (1970) and Banigo and Muller (1972) reported
on the carboxylic acids in ogi and found lactic acid in greatest
concentration (0.55 percent) followed by acetic acid (0.09
percent) and smaller amounts of butyric acid. The latter
investigators suggested levels of 0.65 percent for lactic acid
and 0.11 percent for acetic acid, responsible for the sour taste,
as goals for flavour evaluations. Banigo, de Man and Duitschaever
(1974) reported on the proximate composition of ogi made from
common whole maize which was uncooked and freeze-dried or cooked
and freeze-dried after fermentation. Changes were relatively
small in all major nutrients, with a slight increase in fibre and
a decrease in ash content when compared with whole maize.
These authors also reported on amino acid content; they found
no differences between maize flour and ogi for all amino acids
including the essential ones. The ogi samples, however, had about
twice the amount of serine and somewhat higher values for
glutamic acid. Adeniji and Potter (1978) reported that ogi
processing did not decrease the protein content of maize, but
total and available lysine were significantly reduced. On the
other hand, tryptophan levels were more stable and in two samples
increased, probably because of fermentation. These authors also
found an increase in neutral detergent fibre and ash but no
change in lignin. Akingbala et al. (1987) found a decrease in
protein, ether extract, ash and crude fibre in ogi as compared
with maize that was processed as a whole grain or dry milled.
Nutritional changes
Nutritional evaluations of ogi and other maize-fermented
products are not readily available. Adeniji and Potter (1978)
found a substantial decrease in protein quality of drum-dried
common maize ogi, which they ascribed to the drying process.
These same authors reported significant losses in lysine Several
authors have more recently tested maize and sorghum and reported
that fermentation improved the nutritional quality of the
product. Akinrele and Bassir (1967) found net protein
utilization, protein efficiency ratio and biological value of ogi
inferior to those values in whole maize, even though some
increase in thiamine and niacin was obtained. It has been
indicated that some of the micro-organisms responsible for ogi
fermentation, such as Enterobacter cloacae and L. plantarum, use
some of the amino acids for growth. This together with the
elimination of the germ from kernels explains the very low
protein quality of ogi and similarly produced maize products.
However, there are some exceptions, such as kenkey and pozol,
both products in which the maize is fermented with the germ.
Although protein quality values are not available for kenkey,
Cravioto et al. (1955) found higher levels of tryptophan and
available lysine which suggested higher protein quality than in
raw maize or lime-treated maize. More recently, Bressani
(unpublished) found the fermented product to be higher in protein
quality than raw maize, but not different in quality from
lime-cooked dough.
Use of QPM
Adeniji and Potter (1978) used high quality protein maize to
make ogi and found similar results to those from common maize,
except that the protein quality was higher (although lower than
that of the original raw maize). Pozol made from QPM has
significantly higher protein quality than raw QPM (Bressani,
unpublished data).
Arepas
Chemical changes
Arepa flour is made in a dry milling process which removes the
pericarp and the germ from maize. Therefore, arepa flour may be
expected to differ from whole maize flour, and this was in fact
reported by Cuevas et al. (1985). The protein, ether extract,
fibre and ash content of arepa flour from both white and yellow
maize were lower than in whole maize. The same is true for
thiamine, riboflavin and niacin as well as for calcium,
phosphorus and iron. These changes evidently result from the
removal of the germ and seed-coat.
Nutritive value
Arepa flour has been subjected to biological assay for protein
quality by Chavez (1972a). He reported a decrease of about 50
percent in protein quality from maize (0.74) to arepa (0.33),
although there was some increase in protein digestibility.
Use of QPM
High protein quality maize has been used to make arepas.
Chavez (1972b) found the process to reduce nitrogen, lysine and
tryptophan, thiamine and niacin and attributed this to germ
removal. Protein quality was also significantly less than in
whole maize, but was nonetheless superior to that of maize and
arepas from normal maize. All products - tortillas, ogi, pozol,
kenkey and arepas made from QPM are of better protein quality and
energy value than the products made from common maize.
Other dry milling products
Chemical changes
The main maize products for food use derived from dry milling
include flaking grits, coarse or fine grits, maize cones and
maize flour. They are products from which the pericarp and germ
have been eliminated and they differ from each other in
granulation, with flaking grits having the largest particle size
and flour the smallest. Basically, their chemical compositions
based on food composition data are very similar.
Nutritive value
The protein quality of these products, as with most dry-milled
maize products, is inferior to that of the original whole grain.
If there are any changes, these come about from the processes
used to turn such products into the different forms in which they
are consumed. For example, the protein digestibility of maize
meal was reported by Wolzak, Bressani and Gómez-Brenes (1981) to
be 86.5 percent and that of corn flakes 72.0 percent. A
significant diminution of protein quality also takes place since
available lysine decreases.
QPM products
Studies on dry milling of QPM, particularly the hard-endosperm
types, are not readily available. Wichser (1966) found yields of
8.8 percent grits from milled QPM, while the yield of grits from
maize hybrids was about 17 percent. The yields of meal and flour
were essentially the same from QPM and hybrid maize. However, the
fat, protein, fibre and ash contents in QPM grits, breakfast
cereal and flour were higher than those in similar products from
hybrid maize.
Not much information on the nutritional value of QPM
dry-milled products is available; however, Wichser (1966) showed
the endosperm of QPM to have a net protein ratio (NPR) of 76
percent of the value of casein (100 percent), while the endosperm
from hybrid maize had an NPR of 47 percent of the value of
casein. These results are very similar to those for maize flour
made for arepa production from QPM and common maize as shown by
Chavez (1972a).
Chapter 6 - Comparison of nutritive value of common maize and quality protein maize
Consumption of maize
Maize in its different processed forms is an important food
for large numbers of people in the developing world, providing
significant amounts of nutrients, in particular calories and
protein. Its nutritional quality is particularly important for
small children. Table 23 shows the consumption of maize as
tortillas or lime-treated maize by children in Guatemala. Amounts
varied from 64 to 120 g per day, providing about 30 percent of
the daily protein intake and close to 40 percent of the daily
energy intake. Garcia and Urrutia (1978) reported an intake of
226 g of tortillas by weaned three-year-old children, providing
about 47 percent of their calories.
Although these findings are not basically bad, adequate
supplementary foods are often not provided or are given only in
insignificant amounts. Food legumes are the most readily
available supplementary food in developing countries; however,
the amounts are generally very small (Flores, Bressani and
Elías, 1973). The average intake of beans per age group for the
six countries in Central America was 7, 12, 21 and 27 g per child
per day at 1, 2, 3 and 4/5 years, respectively. On the basis of
22 percent crude protein in beans, the amounts of protein
provided by this food were 1.5, 2.6, 4.6 and 5.9 g, respectively;
however, amounts of digestible protein on the basis of a true
digestibility of 70 percent were only 1.0,1.8,3.2 and 4.1 g. Thus
beans provided about 14, 18, 22 and 30 percent of the dietary
protein in the total intake from maize and beans. These amounts
and their supplementary effects were very small, particularly for
the one- and two-year-old children.
TABLE 23 - Maize consumption and its contribution to
daily calorie and protein intake of children in a rural area of
Guatemala
| Age (years) | Maize intake (g/day) |
Protein intake
|
Calorie intake
|
||||
| Maize (g/day) | Total (g/day) | Percent of total from maize | Maize (cal/day) | Total (cal/day) | Percent of total from maize | ||
| 1-2 | 64 | 5.4 | 20.0 | 27 | 231 | 699 | 33 |
| 2-3 | 86 | 7.3 | 21.7 | 34 | 310 | 787 | 39 |
| 34 | 120 | 10.2 | 27.9 | 36 | 433 | 981 | 44 |
| 4-5 | 89 | 7.6 | 23.3 | 33 | 321 | 819 | 39 |
Source: M. Flores (cited in Bressani. 1972)
Data for 1979-1981 from FAO (1984) showed that 22 of 145
countries had a maize consumption of more than 100 g per person
per day as indicated in Table 24, which also gives the calories
and protein that maize provides. It should be pointed out,
however, that 1960-1962 figures from FAO food balance sheets
(FAO, 1966) were higher for some countries than the 19791981
figures. The figures confirm the importance of maize as a staple
food in some Latin American countries, particularly Mexico and
Central America, as well as in some African countries. It follows
that if the maize intake is high, maize contributes significant
amounts of calories and protein to the daily intake of people in
these countries.
Table 25 summarizes maize intake, calories per day and protein
per day among the rural and urban populations of the six
countries of Central America. Two general trends are evident. The
first is that maize intake decreases from north to south. The
cereal grain that replaces it is rice. The second trend is that
intake of maize is higher in rural than in urban areas. In at
least three countries maize makes up the greatest proportion of
all the ingested food in the rural sector and is therefore an
important source of nutrients in the diet. The table shows that
maize provides up to 45 and 59 percent of the daily intake of
calories and protein respectively.
TABLE 24 - Maize intake and its calorie and protein
contribution to the daily diet
| Country | Intake (g/person/day) | Calories (per person/day) | Protein (g/person/day) |
| Benin | 160.5 | 481 | 12.7 |
| Botswana | 209.3 | 665 | 17.5 |
| Cape Verde | 334.1 | 1 052 | 28.0 |
| Egypt | 149.7 | 508 | 13.4 |
| El Salvador* | 245.0 | 871 | 23.3 |
| Guatemala | 276.2 | 977 | 15.4 |
| Honduras | 255.9 | 878 | 22.8 |
| Kenya | 286.1 | 808 | 21.3 |
| Lesotho | 315.4 | 1002 | 26.4 |
| Malawi | 468.8 | 1422 | 37.6 |
| Mexico | 328.9 | 1061 | 27.1 |
| Nepal | 116.4 | 379 | 9.4 |
| Nicaragua' | 131.0 | 472 | 11.1 |
| Paraguay | 131.2 | 445 | 11.6 |
| Philippines | 152.1 | 399 | 8.7 |
| Romania | 128.6 | 373 | 8.6 |
| Singapore | 122.2 | 345 | 8.6 |
| South Atrica, Rep. | 314.7 | 961 | 24.6 |
| Swaziland | 381.4 | 1279 | 33.7 |
| Tanzania, United Rep. | 129.1 | 421 | 10.0 |
| Togo | 136.9 | 411 | 10.8 |
| Venezuela | 118.3 | 339 | 7.4 |
| Zambia | 418.6 | 1226 | 31.3 |
| Zimbabwe | 330.9 | 958 | 25.2 |
Sources: FAO, 1984; *FAO, 1966
TABLE 25 - Importance of maize consumption in rural
areas
| Country | Urban maize intake (g/day) | Rural maize intake (g/day) |
Rural calorie intake (per day)
|
Rural protein intake (g/day)
|
||
| From maize | Total | From maize | Total | |||
| Guatemala | 102 | 318 | 1 148 | 1 994 | 27.0 | 60 |
| Et Salvador | 166 | 352 | 1 271 | 2 146 | 29.9 | 68 |
| Honduras | 135 | 225 | 812 | 1 832 | 19.1 | 58 |
| Nicaragua | 56 | 131 | 472 | 1 986 | 11.1 | 64 |
| Costa Rica | 14 | 41 | 148 | 1 894 | 3.5 | 54 |
| Panama | 4 | 4 | 14 | 2 089 | 0.3 | 60 |
Source: INCAP, Guatemala, 1969
Although this information was compiled from dietary surveys
conducted in 1969, figures have not changed significantly in
recent years. For example, in 1976 average consumption in El
Salvador varied from 146 to 321 g per person per day; in Honduras
in 1983 consumption in different regions varied from 1 1 1 to 246
g per person per day; and in Costa Rica in 1986, intake varied
from 14 to 31 g per person per day. Chavez (1973) indicated that
about 45 percent of the national calorie intake is provided by
maize in Mexico. In poor rural areas men may consume about 600 g
of maize and women about 400 g. On this basis the importance of
the nutritional quality of maize is obviously great. Although all
nutrients are of interest, the quality of protein has received
more attention from researchers.
Common maize
Protein quality for children
The protein quality of maize evaluated for children recovering
from protein energy malnutrition has been reported by various
researchers. Table 26 shows the results when lime-cooked maize
was supplemented with maize gluten to obtain a product with
higher protein content and to facilitate higher protein intakes
with lower intakes of solids. The amino acid deficiencies in
maize protein were thus magnified and this facilitated their
detection using the nitrogen balance technique (Scrimshaw et al.,
1958; Bressani et al., 1958, 1963). The results showed decreasing
nitrogen retention as nitrogen intake decreased, which was to be
expected; however, even at a high nitrogen intake of 469 mg per
kg body weight per day, retention was significantly lower than
nitrogen retention from milk given in amounts providing the same
level of protein. Apparent protein digestibility indicated as
nitrogen availability was fairly similar for different nitrogen
intakes, varying from 72 to 78 percent. Table 27 refers to
nitrogen balance studies in children fed water-cooked maize.
Nitrogen retention from maize was significantly lower than from
milk at the same level of protein intake. Protein digestibility
was 80 percent for milk and 75 percent for maize (Viteri,
Martínez and Bressani, 1972). Similar data were obtained with
cooked maize endosperm and whole normal maize (Graham, Placko and
MacLean, 1980), as shown in Table 28. In this case nitrogen
balance was lower for common maize endosperm than for the whole
kernel and lower than the results from the reference protein,
casein. Graham et al. (1980) calculated that in order to match
nitrogen retention from casein, the children would have to obtain
203.9 percent of their energy requirements from maize, which is
obviously impossible.
TABLE 26 - Nitrogen balance in children ted
lime-treated maize as the sole protein source
| Protein intake (g/kg/day) |
Nitrogen (mg/kg/day)
|
% of intake
|
|||
| Intake | Absorbed | Retained | Absorbed | Retained | |
| 3 | 470 | 339 | 9 | 72 | 2 |
| (435 to 479) | (327 to 369) | (-8 to 174) | (61 to 77) | (-2 to 36) | |
| 2 | 331 | 260 | 22 | 78 | 7 |
| (308 to 367) | (207 to 284) | (-41 to 59) | (65 to 82) | (-13 to 17) | |
| 1.5 | 238 | 180 | -11 | 76 | -4 |
| (235 to 241) | (168 to 193) | (-22 to -2) | (70 to 82) | (-9 to -1) | |
Note: Diet consisted of 95% lime-treated maize and 5% maize
gluten
Source: Viteri, Martínez and Bressani, 1972
Source: Viteri, Martínez and Bressani, 1972
TABLE 27 - Nitrogen balance In children fed common
maize and milk
| Food | Intake (g/kg/day) | Protein intake (g/kg/day) | Nitrogen absorbed (mg/kg/day) | Nitrogen retained (mg/kg/day) | % N intake absorbed | % N intake retained |
| Milk | 195 | 1.25 | 157 | 75 | 80 | 38 |
| (175-210) | (114-181) | (40-106) | (61-47) | (22-50) | ||
| Common maize | 192 | 1.25 | 144 | 30 | 75 | 16 |
| (183-198) | (129-157) | (10-59) | (66-20) | (5-30) |
Note: Average values. with dispersion in parentheses
Source: Viteri, Martinez and Bressani, 1972
Source: Viteri, Martinez and Bressani, 1972
TABLE 28 - Nitrogen balance in children fed whole
common maize kernels and maize endosperm flour
| Food fed | Nitrogen absorbed (% of intake) | Nitrogen retained (% of intake) |
| Endosperm | 64.1 ± 11.4 | 15.1 ± 8.9 |
| Casein | 81.8 ± 5.2 | 37.0 ± 14.2 |
| Whole kernel | 73.1 ± 1.9 | 26.8 ± 4.6 |
| Casein | 83.5 ± 2.5 | 39.6 ± 9.1 |
Source: Graham. Placko and MacLean, 1980
As was discussed earlier, germ proteins do contribute
significantly to essential amino acids (EAA), so maize food
products without the germ, including QPM endosperm, are always
lower in protein quality than the whole kernel. Similarly, maize
with a high zein content is of a lower quality than maize with
lower prolamine content, because of a higher relative lysine
deficiency and a higher imbalance of essential amino acids such
as leucine relative to isoleucine.
TABLE 29 - Effects on nitrogen retention of additions
of lysine tryptophan and methionine to lime-treated maize
(nitrogen values in mg/kg/day)
| Diet | Nitrogen intake | Faecal nitrogen | Urinary nitrogen | Nitrogen absorbed | Nitrogen retained |
| Basal (B) | 461 | 117 | 334 | 344 | 10 |
| B + tryptophan | 457 | 115 | 289 | 342 | 53 |
| B + tryptophan + lysine | 464 | 135 | 243 | 329 | 86 |
| B + tryptophan + lysine + methionine | 459 | 135 | 272 | 324 | 52 |
Note: Amino acids used: DL-tryptophan: 0.34%, L-lysine/HCl:
0.56%, DL-methionine: 0.34%
Source: Scrimshaw et al., 1958
Source: Scrimshaw et al., 1958
Amino acid supplementation
It is widely accepted that maize proteins are deficient in
both lysine and tryptophan, as documented from studies with
animals. In tests with children, however, the EAA contents of
lime-treated maize supplemented with 5 percent maize gluten to
raise the protein content (Scrimshaw et al., 1958; Bressani et
al., 1958, 1963) were compared with the amino acid contents of
the 1957 FAO reference protein. This comparison suggested the
following order of amino acid deficiency: tryptophan, lysine
methionine, valine, isoleucine and threonine. It also suggested
the amounts of amino acids needed to reach the reference level.
Representative results from two children fed 3 g of protein per
kg body weight per day are shown in Table 29. There was an
apparent response to the addition of 148 mg DL-tryptophan per g N
which was much improved by the simultaneous addition of
tryptophan and lysine the latter in the amount of 243 mg per g N.
Addition of methionine decreased nitrogen retention.
TABLE 30 - Response to lysine and tryptophan added
alone (nitrogen values in mg/kg/day)
| Diet | Nitrogen intake | Faecal nitrogen | Urinary nitrogen | Nitrogen absorbed | Nitrogen retained |
| Subject No.1 | |||||
| Milk | 586 | 93 | 320 | 393 | 73 |
| Basal (B) | 474 | 185 | 349 | 289 | -60 |
| B + tryptophan | 474 | 108 | 352 | 366 | 14 |
| B | 479 | 111 | 346 | 368 | 22 |
| B + lysine | 482 | 120 | 324 | 362 | 38 |
| Subject No. 2 | |||||
| Milk | 392 | 45 | 295 | 347 | 52 |
| Basal (B) | 320 | 56 | 273 | 264 | -9 |
| B + lysine | 335 | 54 | 257 | 285 | 24 |
| 8 | 346 | 63 | 287 | 283 | -4 |
| B + tryptophan | 337 | 52 | 308 | 285 | -23 |
Note: Levels added to give 75-90 mg tryptophan/g N and 180-270
mg L-lysine HCl/g N
Source: Bressani et al., 1958
Source: Bressani et al., 1958
In other studies, nitrogen balance tests were carried out to
learn about the response previously obtained by tryptophan
addition alone. The results from two subjects (Table 30) clearly
show that tryptophan had no effect on improving protein quality.
The addition of lysine on the contrary, appeared to give a
response, suggesting lysine to be more limiting than tryptophan.
Similar studies were carried out by feeding children 2 g of
protein per kg body weight per day. The results in two children
are summarized in Table 31. Tryptophan addition did not induce a
positive nitrogen retention, but the addition of tryptophan and
lysine with and without isoleucine improved nitrogen balance.
Methionine addition decreased retention of nitrogen, as
previously demonstrated.
TABLE 31 - Effects on nitrogen retention of additions
of lysine tryptophan, isoleucine and methionine to lime-treated
maize (nitrogen values in mg/kg/day)
| Diet | Nitrogen intake | Faecal nitrogen | Urinary nitrogen | Nitrogen absorbed | Nitrogen retained |
| Basal (B) | 320 | 68 | 270 | 252 | -18 |
| B + tryptophan | 320 | 91 | 241 | 229 | ·12 |
| B + tryptophan + lysine | 321 | 105 | 201 | 216 | 15 |
| B + tryptophan + lysine + isoleucine | 321 | 90 | 207 | 231 | 24 |
| B + tryptophan + lysine + isoleucine + methionine | 314 | 84 | 217 | 230 | 13 |
| B | 319 | 98 | 242 | 221 | -21 |
Note: Amino acid levels added: DL-isoleucine 0.45%; other
amino acids added in amounts shown in Table 29
Source: Bressani et al., 1958
Source: Bressani et al., 1958
Nitrogen balance tests were performed with protein intake of
1.5 g per kg body weight per day. The results for one child are
shown in Table 32. Although lysine addition did not induce a
positive balance, it did tend to cause a decrease in nitrogen
losses. The improvement from lysine and tryptophan, with and
without isoleucine, is evident. The addition of methionine, even
at this level of protein intake, decreased the nitrogen balance
as previously indicated for higher intakes of protein.
Because of the consistency of the results, the data obtained
for different protein levels under the various dietary treatments
were pooled. The results are shown in Table 33. There was a
response to tryptophan addition only at the highest level of
dietary protein, but the response to lysine was consistent at all
protein intake levels, suggesting that this amino acid is more
deficient than tryptophan. The response to addition of lysine
alone, however, was small and without much nutritional
significance, which implies the need to add both amino acids at
the same time, as can be done with supplementary foods.
TABLE 32 - Effects of amino acid supplementation of
maize at intakes of 1.5 9 protein per kg body weight per day
(nitrogen values in mg/kg/day)
| Treatment | Nitrogen intake | Faecal nitrogen | Urinary nitrogen | Nitrogen absorbed | Nitrogen retained |
| Basal (B) | 241 | 71 | 187 | 170 | -17 |
| B + lysinea | 239 | 59 | 184 | 180 | -4 |
| B + lysineb | 239 | 48 | 193 | 191 | -2 |
| B + lysine + tryptophan | 239 | 47 | 162 | 192 | -30 |
| B + lysine + tryptophan + isoleucine | 240 | 44 | 150 | 196 | 46 |
| N + lysine + tryptophan + isoleucine + methionine | 240 | 55 | 162 | 185 | 23 |
| B | 235 | 45 | 193 | 190 | -3 |
a0.56% L-lysine HCl
b0.30% L-lysine HCl
Other amino acids added in the amounts shown in Table 29
Source: Bressani et al., 1958
b0.30% L-lysine HCl
Other amino acids added in the amounts shown in Table 29
Source: Bressani et al., 1958
A nitrogen intake level of 239 mg per kg body weight per day
is equivalent to 20 g of maize per kg per day, or about the 200 g
of maize normally ingested by children. Supplementation with
lysine alone would have little effect. When tryptophan is also
added, however, the nitrogen retention is significantly higher
and even surpasses that of milk at the highest level of dietary
protein. The overall conclusion that can be reached from the
results obtained by amino acid supplementation of maize is that
both lysine and tryptophan must be added to obtain a significant
response in protein quality as measured by nitrogen retention. It
also appears that the two amino acids are equally limiting in
spite of the fact that the addition of lysine alone tended to
improve protein quality slightly, while the results from the
addition of tryptophan were inconsistent.
The effect of methionine deserves further comment. It was
attributed to an amino acid imbalance, because maize already
contains enough of this amino acid to meet nutritional
requirements.
TABLE 33 - Nitrogen balance in children fed lime-treated maize
at various levels of protein intake with and without amino acid
supplementation
The results shown in Table 34 indicate that valine also
decreases nitrogen retention and that its effect can be reversed
by the addition of isoleucine and threonine. A more detailed
study in dogs led to the conclusion that there is also a close
interrelationship among all four of these amino acids methionine,
valine, isoleucine and threonine - as supplements to maize
proteins (Bressani, 1962,1963).
It is a point of major importance that children are so
sensitive to small changes in amino acid proportions that they
are readily detectable in a short period by testing the nitrogen
balance. The data presented here emphasize the importance of
establishing a proper balance among the essential amino acids if
a maximum retention of nitrogen is to be obtained. This is the
principle of amino acid supplementation. The results obtained on
the amino acid supplementation of maize confirm data derived from
studies with rats, pigs and other animals. Results of studies on
adult human subjects are shown in the next section.
TABLE 34 - Effect of multiple amino acid
supplementation of maize (nitrogen values in mg/kg/day)
| Diet | Nitrogen intake | Faecal nitrogen | Urinary nitrogen | Nitrogen absorbed | Nitrogen retained |
| Basal (B) | 471 | 117 | 315 | 354 | 39 |
| B + lysine + tryptophan + methionine | 451 | 223 | 244 | 228 | -16 |
| B + lysine + tryptophan + methionine + valine | 454 | 241 | 242 | 213 | -29 |
| B + lysine + tryptophan + methionine + valine + isoleucine | 460 | 128 | 265 | 332 | 67 |
| B + lysine + tryptophan + methionine + valine + isoleucine + threonine | 447 | 190 | 218 | 257 | 39 |
| B + lysine + tryptophan + methioninea + valine + isoleucine + threonine | 450 | 129 | 238 | 321 | 83 |
a0.14% DL-methionine in this diet only, all others
0.34% DL-methionine. DL-valine: 0.90%, DL-threonine: 0.22%.
Other amino acids added in amounts shown in Table 29
Source: Scrimshaw e, al., 1958
Other amino acids added in amounts shown in Table 29
Source: Scrimshaw e, al., 1958
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