RELATIONSHIP BETWEEN THE METHOD OF OBTENTION AND THE STRUCTURAL AND FUNCTIONAL-PROPERTIES OF SOY PROTEIN ISOLATES .2. SURFACE-PROPERTIES

The surface properties of soy protein isolates that had been exposed t o different neutralization conditions (pH 7 or 9), thermal treatments (98 degrees C, 5 and 30 min; 80 and 92 degrees C, 6 and 12 min), reduc tion conditions (0.05% Na2SO3; 0.05% Na2SO3-6 M urea or 2% Na2SO3), an d enrichment on 7S or 11S fractions were studied. These treatments aff ected the 7S and 11S contents, degree of denaturation and aggregation, surface hydrophobicity, content of free sulfhydryl groups, etc. The b ehavior of isolates was evaluated by means of parameters that provide information about the different stages in the emulsion formation and s tabilization processes. It was found that isolates having the best emu lsifying properties were those exposed to a short thermal treatment, a nd the isolates chemically reduced in the presence of Na2SO3 and urea have a high surface hydrophobicity and solubility. Better emulsifying properties were found in isolates at pH 9 than at pH 7, as well as in those enriched in 7S. Electrophoretic analysis of the remaining compon ents in the aqueous phase showed a lower migration to the emulsion int erface of A-11S polypeptide than the rest of protein species (alpha, a lpha', and beta-7S, and B-11S polypeptide). Nondialyzed isolates were better foaming agents than dialyzed isolates. Modifications of either 7S or 11S content had no effect on the foaming properties.