CHAPTER 1
CEREALS: RATIONALE FOR FERMENTATION

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INTRODUCTION

The global importance of cereal crops to the human diet and moreover to the written history of man and agriculture cannot be over stated. Cereal grains are the fruit of plants belonging to the grass family (Gramineae). The sustenance provided by cereals is frequently mentioned in the Bible, and they are by many other criterea the most important group of food crops produced in the world. Cereal crops are energy dense, containing 10 000-15 000 kJ/Kg, about 10-20 times more energy than most succulent fruits and vegetables. Nutritionally, they are important sources of dietary protein, carbohydrates, the B complex of vitamins, vitamin E, iron, trace minerals, and fiber. It has been estimated that global cereal consumption directly provides about 50 percent of protein and energy necessary for the human diet, with cereals providing an additional 25 percent of protein and energy via livestock intermediaries. Some cereals, notably wheat, contain proteins that form gluten, which is essential for making leavened bread. Although dried cereal grains constitute living cells that respire, when kept in an appropriate environment, whole grains can be stored for many years. In 1996, world cereal production amounted to more than two billion metric tons (Figure1). Major cereal crops produced worldwide include wheat, rice, maize and barley (Figure 2). Other major cereal crops produced include sorghum, oats, millet and rye. Asia, America, and Europe produce more than 80 percent of the world’s cereal grains. Wheat, rice, sorghum, and millet are produced in large quantities in Asia; corn and sorghum are principal crops in America, and barley, oats and rye are major crops in the former USSR and Europe (Chaven and Kadam, 1989).

Cereals have a variety of uses as food. Only two cereals, wheat and rye, are suited to the preparation of leavened bread. The most general usage of cereals is in cooking, either directly in the form of grain, flour, starch, or as semolina, etc. Another common usage of cereals is in the preparation of alcoholic drinks such as whiskey and beer (barley; sorghum), vodka (wheat), American bourbon (rye), Japanese sake (rice), etc. A variety of unique, indigenous fermented foods, other than leavened breads and alcoholic beverages, are also produced in regions of the world that rely mainly on plant sources of protein and calories. In developed countries, that obtain most of their protein from animal products, cereals are increasingly used as animal feed. More than 70 percent of the cereal crop produced in developed countries is fed to livestock; whereas, in developing countries, 68-98 percent of the cereal crop is used for human consumption (Betschart 1982; Chaven and Kadam, 1989).

Global cereal production from 1961-1996. Data from FAO, Rome

Figure 1 – Global cereal production from 1961-1996. Data from FAO, Rome

 

Global production of major cereal crops

Figure 2 – Global production of major cereal crops from 1961-1996. Data from FAO, Rome

HISTORY

The origin of farming practice appears to be located in the "Fertile Crescent", a wide belt of Southeast Asia, which includes Southern Turkey, Palestine, Lebanon and North Iraq. Abundant rainfall occured in the highlands of this area where there still exists a wide variety of wild cereals. Triticum dicoccoides (wheat) and Hordeum spontaneum (barley) were collected by local dwellers. There is evidence that the people of Uadi el-Natuf Tell of Southeast Asia were the first grain cultivators at about 7 800 B.C (Table 1). By 5 000 BC, wild animals became rare forming only 5 percent of the diet, while cereals and farmed animals provided a sizeable part of human food (Furon, 1958). Early wild wheat (Triticum) and barley (Hordeum) species were diploid, carried few seeds, and abscissed from the plant at maturation, thus making harvest difficult. Polyploid plants can originate in nature but have little chance for self-propagation without cultivation (Raven et al. 1986). The cultivation and irrigation of cereals allowed the expansion of polyploid grains. The polyploid grains exhibit less genetic variation, since each gene is represented in several copies, causing more genetic uniformity and considerable increase in stability and yield. The first stable lines of polyploid cereals were identified as early as 6 000 BC. On the other hand, genetic variability in diploid wild types was essential in order to develop plants adapted to different environmental conditions and geographic areas (Feldman and Sears, 1981). Triticum turgide dicoccoides was crossed with Triticum fanschii to give Triticum aestvum, the progenitor of actual wheat. T. aestivum has 42 chromosomes compared to the 14 chromosomes of T. monococcum. Today there are more than 20 000 cultivars of T. aestivum over the world. In Roman times, T. dicoccoides (spelt) was used for breadmaking and T. vulgaris (siligo) was used for soups because of it’s low gluten content (Simoon, 1982). Wheat and barley were initially cultivated in Southeast Asia, whereas rice was cultivated in Asia, maize in America, and sorghum and millet in Africa (Table 1). Over the last 200 years active programmes in genetic selection and manipulation have changed the character of the original Triticacee from few grains and low gluten to abundant grains rich in gluten forming proteins. Triticale (genus Trticosecale) is a relatively new cereal that was developed by crossing wheat and rye in order to combine the tolerance of rye for poor soil and climatic conditions with the superior technological characteristics of wheat (Bushuk and Larter, 1980).

Table 1. Estimates of origin and early cultivation of cereals1

CEREAL

TIME

LOCATION

Wheat

7 000 BC

Near East

Barley

7 000 BC

Near East

Rice

4 500 BC

Asia

Maize

4 500 B.C

Central America

Millet

4 000 BC.

Africa

Sorghum

4 000 BC

Africa

Rye

400 BC

Europe

Oats

100 AD

Europe

Triticale

1930 A.D.

USSR, Europe

1 Source-McGee, 1984

CEREALS

Wheat

One of the oldest of all cutivated plants. Today, there are more than 50000 cultivars of wheat in existence and as a result wheat can be grown in a relatively wide range of climatic conditions. Growing best in temperate climates, it is susceptible to disease in warm, humid regions and cannot be grown as far from the equator as can rye and oats. Wheat was brought to America early in the seventeenth century where it came to prominence in the Great Plains by 1855 (McGee 1984). Different types of wheat are classified based on planting season and endosperm composition. Wheat holds a special place amongst the cereals because upon mixing wheat flour with water, an elastic matrix called "gluten" required for the production of leavened breads is formed. "Hard wheats" tend to contain relatively high levels of starch and relatively low levels of protein, while the reverse is true for "soft wheats". High protein flours are best suited for pastas and breads, while flour from soft wheats is excellent for cakes and pastries, etc.

Rice

The second most abundant cereal crop originated in the Indian subcontinent and Africa. Today, 90 percent of the world rice crop is grown in Asia (FAO,1996). Alexander the Great is credited with introducing rice to Europe around 300 BC. Growing rice requires more water than other cereal crops, although rice is a highly productive crop. There are several thousand rice cultivars which may differ in color, aroma and grain size. The main commercial distinction between rice types is the grain size, i.e. long, medium and short. Long grain rice, also called "Indian", tends to separate relatively easily on cooking and is dry and flakey. Short grain rice, also called "Japanese" is sticky, moist and firm when cooked. Unlike wheat, rice is most often consumed as grain rather than as a flour. Different grades of milling include brown rice (hull removed), unpolished rice (hull, bran and most of germ removed), and polished rice (aleurone layer removed from unpolished rice). Since polishing removes most of the lipid, the latter product is relatively stable during storage. The discovery that rice bran can alleviate beriberi led to the discovery of the vitamin thiamine. The traditional technique of parboiling rice in India and Pakistan (also called "converted rice") prior to milling improves the nutritional quality of the grain by allowing the B vitamins in the bran and germ to diffuse into the endosperm.

Wild rice is native to the Great Lakes region of North America where it was originally harvested from the wild by native Indians. Although wild rice is now cultivated, it is expensive and accounts for less than 1 percent of the American rice market. The rice is first fermented to develop a nutty flavor and to ease hulling.

Maize

Corn was originally cultivated in Central America and became the staple of the Incas of Peru, the Mayas and Aztecs of Mexico and early cliff dwellers of the American Southwest. Columbus brought corn back to Europe where it became a popular crop in the south. Different types of maize are classified on the basis of their protein content and the hardness of the kernel. These include pop, flint, flour, Indian and sweet corns. Much of the niacin in corn is in a bound form and this led to pellagra in areas where corn became the food staple. It was not until the late 1920s that pellagra was identified as a vitamin deficiency. The traditional practice by early American natives, of boiling corn in 5 percent lime or ashes, releases bound niacin making it available as a nutrient.

Millet and Sorghum

Millet and sorghum are often grouped together because their growing conditions, processing and uses are similar. Millets are native to Africa or Asia and have been cultivated for more than 6 000 years. Millets grow well in arid regions with poor soils and are valued for their relatively high protein content among the cereals. Sorghum originated in East Africa and today is an important food crop in Africa, Asia, India and China where it is made into porridge, unleavened bread ("roti" in India) and beer. Whole grain sorghum flour has a relatively short shelf-life and production of low-fat sorghum flour requires removal of about 20 percent of the grain weight by abrasion (Perten, 1983). In North America, millet and sorghum are used primarily as livestock feed.

Barley

In the United States, barley is mostly used for feed, brewing and alcohol production with only about 2 percent used for human food. Barley flour is produced by abrasion dehulling, followed by milling of the "pearled" barley. Shellenberger (1980), reviewed the uses of barley flour and grits in products such as soups, dressings, and baby foods.

Oats

About 95 percent of the world oat crop is used for livestock feed (McGee, 1984). However, oat consumption as human food has recently increased to 19 percent in the United States (Bowers, 1992), perhaps due to the reported health benefits of the soluble fibre of oats. Oats thrive in a moist, cool climate and became an important crop in Northern Europe at the beginning of the seventeenth century. Oats have a relatively minor status among cereals because they are more difficult to process and are unstable due to their high lipid content and lipase activity. Reports of the possible blood cholesterol lowering effect of oat bran have increased the popularity of its use for human food in developed countries.

Rye

Rye appears to have originated in central Asia around 4 000 BC as a weed contaminating barley and wheat and became domesticated around the Baltic Sea in 400 BC where it grows well in a cool, moist climate with poor soil (McGee, 1984). It was traditionally a popular grain for bread making in Northern and Eastern Europe. Rye flour has a relatively low gluten content compared to wheat flour, but contains a unique class of carbohydrates (pentosans) that facilitate bread making. The milling of rye yields a flour that is classified based on color or ash content (Rozsa, 1976).

BOTANICAL STRUCTURE OF CEREALS

All cereals belong to the taxonomic family known as the Gramineae. Other globally important crops in the Gramineae family include sugar cane and bamboo. Botanically speaking, cereal grains are a type of "dry" fruit called a caryopse (Figure 3). A botanical fruit is defined as the ripened ovary or the ovary and adjoining parts. The flesh of the fruit may originate from the floral receptacle, from carpillary tissue, or from extrafloral structures, such as bracts. The caryopse fruit structure differs from that of other fruits (e.g., fleshy fruits) in that a thin, dry fruit wall is fused together with the seed coat. Cereals are sometimes thought of as seeds by the layman since the bulk of their tissue is the true seed; i.e. the part resulting from sexual reproduction of the plant. Seeds are produced within the fruit which serves to protect and aid their dispersal for propagation in a variety of ways for different plants. The seed is the result of sexual reproduction of the plant when the flower is fertilized and is the organ of propagation. Kernel structure is important with respect to minimizing damage during grain harvest, drying, handling, storage, milling, germination and in enhancing nutritional value (Pomeranz and Bechtel 1978). Selection of cultivars with a large embryo or without hulls for example, will improve the protein content if the entire grain is consumed.

The seed portion of cereals consists of numerous components (Fig. 3) which basically include three parts: a seed coat or testa (bran), storage organ or nutritive reserve for the seed (endosperm), and a miniature plant or germ. The fruit tissue consists of a layer of epidermis and several thin inner layers a few cells thick. The aleurone layer which is just below the seed coat, is only a few cells thick, but is rich in oil, minerals, protein and vitamins. Starch and protein are located in the endosperm which represents the bulk of the grain and is sometimes the only part of the cereal consumed. Starch is housed in the form of subcellular structures called granules that are embedded in a matrix of protein. The developing endosperm contains protein bodies which become a continous phase as the grain matures. There is generally a gradient of more protein and less starch per cell from the outer to the inner region of the endosperm. The diameter, shape, size distribution and other characteristics of starch granules vary with different cereals. Starch granules range in size from 3-8 Ám in rice; 2-30 Ám in corn, and 2-55 Ám in wheat. Reserve proteins in the endosperm are in the form of smaller "protein bodies" that range in size from 2-6 Ám that become disordered and adhere to the starch granules in the mature grain of species like wheat.

Diagrammatic illustrations of cereal grains (caryopis fruit)
A. Rice;
Diagrammatic illustrations of cereal grains (caryopis fruit)
B. Wheat,
Diagrammatic illustrations of cereal grains (caryopis fruit)
C. Maize,
Diagrammatic illustrations of cereal grains (caryopis fruit)
D. Barley,
Diagrammatic illustrations of cereal grains (caryopis fruit)
E. Oat.
Diagrammatic illustrations of cereal grains (caryopis fruit)

Figure 3 -Diagrammatic illustrations of cereal grains (caryopis fruit).
An example of a true seed crop (soybean) is shown for comparison. From Haard & Chism (1996)

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