Methionine supplementation of soy milk to correct cystine loss resulting from an alkaline soaking procedure

THE STUDY of the effects of alkaline pH on protein quality have for the most part been limited to corn protein. Bressani et al. (1958) have reported the effects of lime (CaO) and heat during the production of tortillas on the nutritive value of zein. Of the essential amino acids they found decreases in leucine, cystine, histidine and lysine. DeGroot and Slump (1969) studied the effects of severe alkaline treatment on isolated soy protein and found significant changes in the amino acid levels and the net protein utilization as well as the formation of the amino acid derivative lysinoalanine. More severe treatment caused a decrease in digestibility. In an earlier paper Badenhop and Hackler (1970) reported some of the effects of soaking soybeans in a sodium hydroxide solution as a pretreatment step in the preparation of soy milk. The results of that study indicated that an alkaline treatment with 0.05N NaOH produced a soy milk with a pH of 7.4 which had a significantly better flavor. A further increase in pH resulted in a poorer flavor, however. Other effects of alkaline pretreatment included an increase in available niacin, reduced trypsin inhibitor activity, reduced cystine content and decreased nutritive value as indicated by protein efficiency ratio measurements. The present investigation was undertaken to better define the effects of an alkaline soaking treatment on the nutritional value of the protein.