Biochemistry

2.7. BIOLOGICAL VALUE OF PROTEINS Proteins are essential nutrients. The ability of a protein to fulfill a nutritional function characterizes its biological value. The efficiency of human consumption of protein substances is determined by two main factors: the balance of the content of essential amino acids in the protein and its digestibility. If the need for one of the essential amino acids is not satisfied, then the use of others is limited, and, therefore, the value of the protein as a whole is reduced. An essential amino acid, which is in a minimal amount in a protein, is called a limiting amino acid, since it reduces the biological value of this protein to the greatest extent. Enrichment of food protein products with missing amino acids is used in human diets in exceptional cases. However, the addition of synthetic amino acids to animal feed is common. Similarly, feed mixtures for poultry, pigs, and cows are being prepared all over the world. The enrichment of feed with the main limiting acids - methionine and lysine makes it possible to more economically use feed mixtures. Supplements of these amino acids are known to improve animal protein utilization by about 20%. Typically, the biological value of a protein is expressed in relative terms. It represents the ratio of the studied parameter of a given protein to a similar parameter of the “ ideal" protein. As the latter, milk casein, egg protein, a mixture of muscle proteins that are easily digested and contain essential amino acids in ratios close to the reference are used. The biological value of this protein in comparison with the reference shows how it is able to satisfy the body's needs for amino acids. To evaluate a protein or food product, data are needed on the content of individual amino acids in it, i.e., the amino acid composition. A significant part of plant proteins is close to animals in their amino acid composition and biological value. However, the seed proteins of most varieties of cereals are deficient in two (rice, oats), and more often in three and four (wheat, com, etc.), essential amino acids. The main limiting amino acid of cereal protein is lysine. The limiting amino acids of cereal proteins are different for seeds of different crops: for wheat, rice and rye, for example, threonine, for maize for tryptophan, etc. Bean proteins have a better balance of the essential amino acids contained in them compared to cereal proteins. In animal proteins, a deficiency of essential amino acids is poorly expressed. Some of them (proteins of milk, meat, By-products) are characterized by a lack of sulfur-containing amino acids. In general, animal proteins are more characterized by the excess content of a number of essential amino acids in comparison with the needs of the body. In the diet of most of the world's population there is a certain deficiency of three essential amino acids: lysine, tryptophan and methionine. The different 68