EFFECT OF DISULFIDE BOND-CONTAINING PROTEIN ON RICE STARCH GELATINIZATION AND PASTING

Flours from short, medium, and long grain rice varieties were analyzed for viscosity, gel consistency, and degree of gelatinization, with an d without treatment with dithiothreitol or 2-mercaptoethanol, to exami ne the effect of endosperm protein containing intermolecular disulfide bonds on pasting characteristics of rice flour. When flour slurries w ere cooked under negligible shear stress and measurements were made un der low-shear conditions, viscosity and gel consistency increased in t he presence of a reducing agent. When protein disulfide bonds are disr upted, rice starch granules apparently swell to a larger size, thereby increasing viscosity. However, viscosity decreased when the reducing agent was added before cooking and only moderate shear stress was appl ied. This indicated that the swollen granules were more fragile in the presence of a reducing agent. Degree of gelatinization was lower in f lours of the long grain nonsticky rices, and it increased following tr eatment with 2-mercaptoethanol. Proteins with disulfide bonds in the r ice flour restrict starch granule swelling during gelatinization and m ake the swollen granules less susceptible to disruption by shear.