PH-INDUCED MODIFICATIONS IN THE THERMAL-STABILITY OF SOYBEAN PROTEIN ISOLATES

pH-induced modifications in the thermal stability of soybean protein i solates were studied by differential scanning calorimetry. The thermal stability of the 11S globulin was higher than that of the 7S globulin and was very sensitive to pH changes. The glycinin denaturation tempe rature decreased by 10 degrees C when the pH was increased from 6 to 1 1, while the 7S globulin denaturation temperature did not change. At p H 11, soybean isolate gave only one endotherm representing both globul ins. 11S globulin underwent conformational changes as the pH increased , which were reflected by lower cooperativity in the denaturation proc ess. 11S globulin also had a higher activation energy for the denatura tion process than did 7S globulin. The activation energy of 11S globul in was higher than that of 7S globulin (in the pH 6-10 range) and show s a maximum kinetic stability at pH 8. The 7S and 11S globulin half-li ves at different treatment temperatures were calculated to pHs between 6 and 11, and thermal treatments causing different degrees of denatur ation were carried out to produce a maximum increase of the surface hy drophobicity (He). For thermally untreated isolates, the increase in p H led to an increase in exposed hydrophobicity. The combination of pH 10-11 and thermal treatments at temperatures of about 65 degrees C led to higher exposure of hydrophobic groups, conditions for which would be most suitable for obtaining isolates with a higher emulsifying capa city. Denaturing thermal treatments at this pH value induced aggregati on along with thus a fall in Ho.