Soy protein concentrate (SPC) was autoclaved at 121°C and 15 p.s.i for 0, 10, 30 min., 2 and 4 hr. The solubility of the control, 10 and 30 min. autoclaved samples was quite low (about 12%) as compared with the 2 and 4 hr. samples (about 27%) in potassium phosphate buffer (pH 7.6, 0.5 ionic strength). When urea and beta-mercaptoethanol were added in increasing concentration to the buffer, the solubility of control, 10 and 30 min. samples increased dramatically up to 60%; while the solubility of 2 and 4 hr. autoclaved samples changed very little remaining at 25 to 40% throughout.

Digestibility of SPC samples was determined by three in vitro methods, all including treatment of SPC with selected proteases, followed by measurement of (1) TCA soluble N production (2) breakdown products via SDS-PAGE and (3) pH drop resulting from enzyme action. In vivo apparent digestibility was determined in a rat feeding study. The digestibility of SPC samples were found to be significantly affected by length of autoclaving. The digestibility of control was higher than autoclaved samples as determined by both in vivo and in vivo assays. The digestibility of 10 and 30 min. autoclaved SPC samples was significantly higher than 2 and 4 hr. autoclaved samples in in vivo assays. While, apparent in vivo digestibility of 10, 30 min. and 2 hr. autoclaved samples was significantly higher than the 4 hr. sample. Correlation coefficients of in vivo digestibility as determined by the TCA soluble N measurement, SDS-PAGE and pH drop method with in vivo apparent digestibility were 0.96, 0.92 and 0.95, respectively.

There was no actual destruction of amino acids except cysteine; 10, 30 min., 2 and 4 hr. SPC samples contained 6, 20, 27 and 39% less cysteine respectively than the SPC control. The chemical score of SPC samples indicated that cysteine became the first limiting amino acid in the 2 hr. sample.

PERs of the 2 and 4 hr. autoclaved SPC samples were significantly less than control, 10 and 30 min. autoclaved samples. While C-PERs of control, 10 min. samples were higher than 30 min., 2 and 4 hr. samples. Decreased PER values of autoclaved SPC samples were likely due to i) decreased protein digestibility and rate of enzymatic hydrolysis, ii) destruction of essential amino acids, and iii) decreased food intake.