Chemical and molecular properties of processed soy proteins

Abstract

Selected chemical and molecular properties of two soy flours, two soy isolates, three textured soy products, one soy concentrate and raw and autoclaved soy meal were determined. Product protein content ranged from 33.1% to 86.7% and reported PER's ranged from 1.1 to 2.3. Hydrophobic and disulfide interactions were found to occur in the proteins during processing, as well as other interactions causing loss of protein solubility. Solubility decreased greatly with increased heat treatment for untoasted flour, toasted flour and textured soy, although the reported PER's of the products were identical. Solubility increased linearly with decreased PER of acid or alkali processed soy concentrates and isolates. Extractable textured soy proteins were found to be of smaller molecular weight than those of raw soy meal when analyzed by gel filtration and electrophoretic techniques. Textured soy had a major protein at 27,000 molecular weight, and gel patterns were similar to those of soy meal which was autoclaved for 20 minutes. Total sulfhydryl content (sulfhydryl plus reduced disulfides) of textured soy equaled that of soy meal, suggesting little destruction or oxidation of sulfur amino acids during hydrolysis. Time after extraction was found to affect the measured sulfhydryl and disulfide content. All processed soy products were found to contain cysteic acid. Soy isolates contained the highest amount of cysteic acid, and one isolate also contained methionine sulfoxide. None of the products were found to contain methionine sulfone. Length of time of acid hydrolysis was found to affect the measured essential amino acid content of processed soy flours, concentrate and isolates. Methionine and threonine values decreased with time in all products. Release patterns for other essential amino acids varied with each soy product. Amino acids which increased in measured value with increased length of hydrolysis time were analyzed by curvilinear regression. Differences between maximum measured value and 24 hour hydrolysis time value most often occurred in soy isolates. Standard 18 or 24 hour hydrolysis times were found inadequate for accurate measurement of essential amino acids in processed soy proteins.