Sulfide liberation from raw soybean protein.

Abstract

Numerous reports have been concerned with the factors or properties that cause an inferior nutritive value of raw soybean meal as compared to properly cooked meal. These reports have been reviewed recently(1). Among such factors have been anti-enzymes, pancreatic hypertrophy, amino acid deficiency or unavailability, toxic proteins, and indigestible protein fractions, but the effects or even the existence of some are still matters of disagreement. Proteins which are known to be highly indigestible, i.e., the keratins, have been shown to be improved in digestibility by steam cooking under pressure. A comprehensive report (2) has reviewed the literature in this field and presented specific data in respect to feather protein. It was shown that the principal changes on treatment with steam were loss of cystine, corresponding appearance of lanthionine and increased susceptibility to enzymatic hydrolysis. The conversion of cystine in part to lanthionine, which involves breakage of disulfide cross links between amino acid chains and loss of one-half the sulfur, was most rapid during the first 30 minutes of processing at 30 1b steam pressure. At the same time, amino nitrogen liberated by rupture of peptide bonds increased very slightly, if at all. Some data (3) obtained during steam processing of hoof and horn meal are in agreement with the later studies on feather protein. Volatile sulfur, particularly H2S, was detected in the exhaust gases. Neartly two-thirds of the original cystine was lost, while the apparent digestibility to pepsin and HCI improved from an original 13 to 73%. The loss of sulfur from proteins during cooking has been known for a long time. Recently, the cooking of cod muscle has been shown to result in a loss of cystine(4). We investigated the possibility that such changes could take place in other protein sources such as the soybean.