Cooking Water pH Differences Influence the Physical Properties, Protein and Starch Modifications in Cooked Rice

Abstract

Rice cooked in pH 3 and pH 9 buffers showed a significant increase in adhesiveness, which demonstrates a staling control effect under refrigeration. However, cooking rice in a pH 5 buffer showed an accelerated staling. Samples cooked at pH 3 and pH 9 showed the highest abundance of solid components and proteins that cover the rice grain surface. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis of eluted rice proteins indicated protein degradation by aspartic proteinase under acidic conditions, and protein solubilization by disulphide bond reduction under alkaline conditions. These results suggest that such property changes promote water absorption and starch swelling under acidic and alkaline conditions, contributing to an increased solid content adhering to the grain surface. However, protein solubilization is limited, and both starch swelling and gelatinization are obstructed at pH 5, suggesting that this condition can accelerate staling.