COVALENT BINDING OF METHIONINE AND TRYPTOPHAN TO SOY PROTEIN

Abstract

ABSTRACTThe carbodiimide condensation reaction was used for covalently binding methionine and tryptophan to soybean protein thereby improving its nutritive value. Various conditions of the carbodiimide reaction were analyzed by a fractional factorial design in an attempt to determine the factors affecting the amino acid binding to soy protein hydrolysate (SPH). Of the factors investigated, pH, SPH concentration, carbodiimide concentration, activation time and reaction time were found to significantly affect the methionine binding efficiency, whereas pH, SPH concentration, carbodiimide concentration, amino acid concentration and reaction temperature were found to significantly influence the tryptophan binding efficiency to SPH. Under the best condition found, the methionine and tryptophan contents of methionine‐ and tryptophan‐bound SPH were increased 7.7‐fold and 18.0‐fold, respectively. An in vitro pepsin‐pancreatin digestion test demonstrated that the bound amino acids were readily released. To improve the protein recovery that was only 16–30% when SPH was used, soy protein isolate without pepsin hydrolysis, instead, was used as the starting material. A product with 95–99% protein recovery was obtained and its methionine or tryptophan content was increased 6.3‐fold or 11.3‐fold, respectively. High digestibility was still maintained for these products. Gel filtration chromatography demonstrated that the carbodiimide reaction caused an increase in the molecular weights of soy protein fractions; no selective amino acid binding among the different soy protein fractions during the carbodiimide reaction was observed.