Effects of Severe Alkali Treatment of Proteins on Amino Acid Composition and Nutritive Value

Abstract

Amino acid analyses, protein quality assays, in vitro digestion and absorption tests and feeding studies with rats were conducted to study the effects of alkali treatment on food proteins under varying conditions of pH, temperature and time. Exposure of several high protein products to aqueous alkali at pH 12.2 resulted in the formation of the amino acid derivative lysinoalanine (LAL) which is poorly absorbed. The amount of LAL formed in isolated soy protein (ISP) upon exposure at pH 12.2 increased with rising temperature and a longer exposure period. The presence of LAL in proteins was attended with decreased contents of cystine and lysine, and decreased net protein utilization (NPU) values. More severe treatment of ISP with alkali of pH 12.2 at 60 or 80° also caused a decrease in serine content and in digestibility of the protein. The LAL content of ISP treated under various conditions showed a highly significant negative correlation with NPU values (r = - 0.96). The presence of LAL in proteins was a sensitive criterion of alkali damage. The NPU assays of ISP supplemented with amino acids showed methionine to be the first limiting amino acid and threonine the second. The decreased NPU of alkali-treated ISP could not be completely alleviated by amino acid supplementation, probably as a result of decreased utilization of threonine. Most of the essential amino acids from alkali-treated ISP were released at a relatively slow rate by pepsin-pancreatin digestion. Threonine and methionine were the only essential amino acids in an enzymatic digest of ISP which showed decreased absorption by everted intestinal sacs. Upon feeding rats diets with relatively high levels of proteins treated at pH 12.2 and 40° for 4 hours no clinical or histological abnormalities were observed other than an increased degree of nephrocalcinosis in females which could be prevented by additional dietary calcium.