FACTORS AFFECTING CYSTINE REACTIVITY IN PROTEOLYTIC DIGESTS OF PHASEOLUS-VULGARIS

Raw/dehulled and cooked/dehulled white and brown beans (Phaseolus vulgaris) were tested for in vitro protein digestibility, extent of proteolysis, and cysteine/cystine chemical reactivity. Characterization of the proteolytic digests was carried out by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and ultrafiltration of peptides. The increase in protein digestibility following cooking treatment of both varieties was found to be related to a higher extent of proteolysis, as monitored by SDS-PAGE and recovery of low molecular weight peptides (< 30 000) after ultrafiltration of the digests. Dehulling, when it preceded cooking, slightly increased the degree of hydrolysis of the bean proteins. A very low amount of free SH groups was found in the proteolytic digests. The maximum percentage of reactive cystine (cystine susceptible to reduction), detected in the digests after reduction by dithioerythritol, never exceeded 75% of the total cysteine content. Cooking/dehulling had no effect on the cystine reactivity of the white bean (about 65%), while a decrease to 49% was detected in the cooked brown bean and reversed by dehulling. Cystine in all samples was found totally susceptible to reduction after denaturation by urea. The results suggest that the stability of S-S-containing proteins and the presence of tannins are relevant factors for the low cystine reactivity, especially in cooked beans.