The Effect of Cystine and Casein Supplements upon the Nutritive Value of the Protein of Raw and Heated Soy Beans

Abstract

In experiments with rats, raw soy beans of both the Illini and Herman varieties were found to contain an inefficient protein as measured by grams of growth per gram of protein eaten. The additions of 0.3% l-cystine or the application of heat, such as autoclaving for 1 hour at 15 pounds pressure or the use of the expeller method of oil extraction, which entails heating the beans at a temperature of 140 to 150°C. for 2½ minutes, practically doubled the nutritive value of the protein. The addition of 0.3% of l-cystine to the heated soy bean diet failed to reveal an improvement which could be considered outside of possible experimental error. Casein supplemented the protein of the raw soy bean and caused an increase in the nutritive value of the protein proportional to the amount added. The addition of 0.3% l-cystine resulted in an increase in nutritive value similar to that obtained when the high level of casein was added. The fact that cystine supplemented the protein of the raw soy bean suggests that cystine or its equivalent may exist in the raw protein of the soy bean in a form which is not available to the animal. Since heating of the soy bean gave its protein a nutritive value practically equal to the nutritive value of the protein of the raw soy bean when supplemented with cystine, it appears that heating the soy bean caused the cystine fraction of the protein to become available.