Fermented cereals a global perspective. Chapter 1. (original) (raw)

NUTRITIONAL QUALITY OF CEREALS

Cereals, together with oil seeds and legumes, supply a majority of the dietary protein, calories, vitamins, and minerals to the bulk of populations in developing nations (Chaven and Kadam 1989). Some components of cereal nutritive value are summarized in Table 9. The following synopsis of cereal nutrition has been adopted from a review by Chavan and Kadam (1989). Cereal grains are low in total protein compared to legumes and oilseeds. Lysine is the first limiting essential amino acid for man; although rice, oats and barley contain more lysine than other cereals. Corn protein is also limiting in the essential amino acid tryptophan, while other cereals are often limiting in threonine. The annual global yield of essential amino acids from major cereals has been compared to a hypothetical population of 3 billion adults and 2 billion children (Phillips 1997) (Table 10). Accordingly, if all cereals were effectively and fully utilized for human consumption they would more than meet man’s needs for essential amino acids.

Table 9. Comparative nutritive value of cereal grains 1

1 Adapted from Chavan & Kadam (1989)

Table 10. Annual Global Yield of Essential Amino Acids From Major Cereals and Global Human Requirements1

1 Adapted from Phillips (1997). Yield of amio acid from cereals is global production x amino acid profile x digestibility; human requirement is based on hypothetical population of 3 billion adults and 2 billion children; % provided is the calculated global production from wheat, rice, maize and sorghum divided by estimated global requirement by the hypothetical human population.

Barley, sorghum, rye and oat proteins have lower digestibilities (77-88%) than those of rice, maize and wheat (95-100%). The biological value and net protein utilization of cereal proteins is relatively low due to deficiencies in essential amino acids and low protein availability (Chaven and Kadam 1989). The digestible energy of rice is significantly better than that of other cereals (Table 9).

Cereals also provide B-group vitamins and minerals, although refining results in losses of these nutrients (Miller 1996) (Table 11). The endosperm of wheat contains only about 0.3% ash. Phosphorous, potassium, magnesium, calcium and traces of iron and other minerals are found in cereals (Bowers 1992). Barley and wheat provide 50 and 36 mg Ca/100 g respectively. Barley provides 6 mg of iron per 100 g; millet provides 6.8; oats, 4.6 and wheat, 3.1. In contrast, soybeans provide more of these nutrients, i.e., Ca (210 mg/100 g) and Fe (7 mg/100 g) (Haard and Chism 1996). Some grains, notably barley, sorghum, and oats, contain appreciable amounts of crude fiber (Table 2) and are referred to as coarse grains. The nutritive and sensory value of cereal grains and their products are, for the most part, inferior to animal food products. Methods that can be employed to improve the nutritive value of cereals include traditional genetic selection, genetic engineering, amino acid and other nutrient fortification, complementaion with other proteins (notably legumes), milling, heating, germination and fermentation.

Table 11. Influence of milling on the trace mineral content of wheat 1

1 Adapted from Miller (1996)

ANTINUTRIENTS AND TOXIC COMPONENTS IN CEREALS

Cereals and other plant foods may contain significant amounts of toxic or antinutritional substances. In this regard, legumes are a particularly rich source of natural toxicants including protease inhibitors, amylase inhibitors, metal chelates, flatus factors, hemagglutinins, saponins, cyanogens, lathyrogens, tannins, allergens, acetylenic furan and isoflavonoid phytoalexins (Pariza 1996). Most cereals contain appreciable amounts of phytates, enzyme inhibitors, and some cereals like sorghum and millet contain large amounts of polyphenols and tannins (Salunkhe et al. 1990). Some of these substances reduce the nutritional value of foods by interfering with mineral bioavailability, and digestibility of proteins and carbohydrates. Since legumes are often consumed together with cereals, proper processing of cereal-legume mixtures should eliminate these antinutrients before consumption (Chaven and Kadam 1989; Reddy and Pierson 1994). Relatively little is known about the fate of antinutrients and toxicants in traditional fermented foods.

Phytates

Phytic acid is the 1,2,3,4,5,6-hexaphosphate of myoinositol that occurs in discrete regions of cereal grains and accounts for as much as 85% of the total phosphorous content of these grains. Phytate reduces the bioavailability of minerals, and the solubility, functionality and digestibility of proteins and carbohydrates (Reddy et al. 1989). Fermentation of cereals reduces phytate content via the action of phytases that catalyze conversion of phytate to inorganic orthophosphate and a series of myoinositols, lower phosphoric esters of phytate. A 3-phytase appears to be characteristic of microorganisms, while a 6-phytase is found in cereal grains and other plant seeds (Reddy and Pierson 1994).

Tannins

Oligomers of flavan-3-ols and flavan-3,4-diols, called condensed tannins, occur widely in cereals and legumes (Haard and Chism 1996). These compounds are concentrated in the bran fraction of cereals (Salunkhe et al. 1990). Tannin-protein complexes can cause inactivation of digestive enzymes and reduce protein digestibility by interaction of protein substrate with ionizable iron (Salunkhe et al. 1990). The presence of tannins in food can therefore lower feed efficiency, depress growth, decrease iron absorption, damage the mucosal lining of the gastrointestinal tract, alter excretion of cations, and increase excretion of proteins and essential amino acids (Reddy and Pierson 1994). Dehulling, cooking and fermentation reduce the tannin content of cereals and other foods.

Saponins

These sterol or triterpene glycosides occur widely in cereals and legumes (Shiraiwa et al. 1991). Saponins are detected by their hemolytic activity and surface active properties. Although the notion that they are detrimental to human health has been questioned (Reddy and Pierson 1994), they have been reported to cause growth inhibition (Cheeke 1976).

Enzyme Inhibitors

Protease and amylase inhibitors are widely occurent in seed tissues including cereal grains. Trypsin-, chymotrypsin-, subtilisin-inhibitor, and cysteine-protease inhibitors are present in all major rice cultivars grown in California, although the individual inhibitor amounts are quite varaiable and are concentrated in the bran fraction (Izquerdo-Pulido et al. 1994). They are believed to cause growth inhibition by interfering with digestion, causing pancreatic hypertrophy and metabolic disturbance of sulfur amino acid utilization (Reddy and Pierson 1994). Although these inhibitors tend to be heat stable, there are numerous reports that trypsin inhibitor, chymotrypsin inhibitor, and amylase inhibitor levels are reduced during fermentation (Chaven and Kadam 1989; Reddy and Pierson 1994).