Protein, high lysine tryptophan quality

Soybean proteins are widely used as food additives in European derived societies, primarily in processed foods, and this trend continues to grow annually. This makes soybean proteins a pervasive component of the human diet in industrialized countries. Solvent extracted soybean meal is also widely used as an animal feed additive (ref. 5, for review), because it is an inexpensive source of high quality protein that contains more of essential amino acids lysine and tryptophan than most cereal crops. Combined with corn, the other primary feed grain used in the United States, a ration can be assembled that is adequate in both sulfur amino acid and lysine contents, and provides a high protein diet that is well balanced for poultry and pigs. 

A diet based exclusively on cereals only provides about 50% of the protein requirement for growing infants. The most limiting amino acid in all cereals is lysine (Chapter 17). The second limiting amino acid for maize is tryptophan, whereas it is threonine in the rest of cereals. The high-lysine cereal types of maize, sorghum, and barley possess better nutritional value due to improved protein quality or essential amino acid balance (Anonymous, 1988, Balaravi et al. 1976). 

Dietary protein sources differ widely in their proportions of the EAA. In general, complete proteins (those containing sufficient quantities of EAA) are of animal origin (e.g., meat, milk, and eggs). Plant proteins often lack one or more EAA. For example, gliadin (wheat protein) has insufficient amounts of lysine, and zein (com protein) is low in both lysine and tryptophan. Because plant proteins differ in their amino acid compositions, plant foods can provide a high-quality source of essential amino acids only if they are eaten in appropriate combinations. One such combination includes beans (low in methionine) and cereal grains (low in lysine). 

The protein quality of cereal grains and their products is not adequate to sustain optimum growth, especially for preschool children. In developing countries, the consumption of cereals with legumes (i.e., beans, soybeans, peas, chickpeas, and lentils) greatly improves protein quality and quantity. Legumes contain at least twice as much protein as cereals and complement their amino acid balance because they contain high amounts of lysine and tryptophan. 

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