Thermal behavior of soy protein isolates under different conditions of
temperature, time, pH, protein concentration, and presence of reducin
g agents was studied. Thermal treatments above 85 degrees C showed a d
ecrease in concentration of the AB-11S subunit and of the two protein
species of 20 and 29 kDa, and a gradual increase in the concentration
of the A and B polypeptides of glycinin. None of the thermal treatment
s tested led to modifications of the relative proportions either of th
e high molecular weight aggregates (100-200 kDa) observed in the elect
rophoretic profiles or of the alpha' and alpha subunits of beta-congly
cinin. Increasing the pH to 9 or 10 and increasing the protein isolate
concentration enhanced AB-11S aggregation during the thermal treatmen
t. Either the presence of Na2SO3 or the pH 9-10 favored the beta-beta-
conglycinin/B-glycinin aggregation. This interaction requires an incre
ase of SH groups. Initially the beta-beta-conglycinin/B-glycinin aggre
gates were stabilized by hydrophobic interactions and later by SS bond
s.