Endogenous Small Thiol Compounds Induced Disulfide Bond Cleavage and the Improvement of In Vitro Digestibility of Soy Proteins

Digestibility is a key determinant of the nutritional quality of food proteins, while disulfide bonds (SSs) restrict enzymatic hydrolysis. This study isolated and characterized endogenous small thiol compounds (ESTCs) from defatted soy meal and investigated their effects on SS cleavage and soy proteins' in vitro digestibility. The results revealed ESTCs are a <500 Da complex mixture of glutathione, cysteine-cysteine dipeptides, and cysteine-rich peptides. ESTC-induced SS cleavage was profoundly influenced by protein predenaturation, temperature, and pH, while excessive heat or high pH treatments impaired SS cleavage, but urea denaturation enabled the highest degree of SS cleavage (DSC) of 44.5%. Reaction kinetics revealed activation energies of 35.3 and 13.6 kJ/mol for undenatured and urea-denatured proteins, respectively. Compared to samples with 0% DSC, SS-cleaved proteins with DSCs of 7.2%, 22.1%, and 44.5% showed significantly increased hydrolysis degrees. This study proposed a green strategy through molecular modification to improve the nutritional quality of soy proteins.