EFFECT OF SODIUM SULFITE, SODIUM BISULFITE, CYSTEINE, AND PH ON PROTEIN SOLUBILITY AND SODIUM DODECYL SULFATE-POLYACRYLAMIDE GEL ELECTROPHORESIS OF SOYBEAN MILK BASE

摘要

Protein modification is often used to improve the functional properties of proteins in many different food products. The purpose of this study was to investigate the effect of modification of soy proteins with sodium sulfite, sodium bisulfite, and cysteine on the protein solubility of a soy milk base, as determined by protein dispersibility index (PDI). An increase in the PDI was observed at 6-8 mmol/L concentration of each reagent; the most effective was sodium bisulfite, followed by sodium sulfite and cysteine. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) showed that chemical treatments resulted in an increase in the intensity of several bands with molecular weights of 16 000-20 000, 32 000-42 000, 60 000, and 90 000, which correspond to the polypeptides of the two major proteins of soybean, i.e., 11S (glycinin) and 7S (beta-conglycinin) globulins. These data suggest that the chemical treatment has cleaved the disulfide bonds which link the polypeptides, thereby increasing the PDI. pH has a significant effect on PDI; the highest PDI was obtained at pH 9, and the intensity of several protein bands in SDS-PAGE increased with increase in pH from 6 to 9. Freezing soy milk base at -18 degrees C for 1-4 weeks resulted in a drastic decline in the PDI. [References: 30]