Improving Cooking Properties of Aged Rice Grains

Abstract

The application of both pressurization and enzymatic treatment to aged rice grains was investigated to improve their cooking properties. Pressurization at 100 MPa was favorable in improving the properties. For treatment of the grains with enzymes, actinase was more effective than cellulase and pectolyase. Actinase-treated grains, when cooked, gave a favorable result in terms of the stickiness/hardness ratio and brightness . Stickiness and brightness of cooked rice were developed by removing albumins and globulins from grains . In newly-harvested grains, the proteins were easily removed by extraction with water which contained rice-originating minerals. In aged grains, however, the proteins were already denatured and extracted with difficulty. Proteases hydrolyzed the denatured proteins to be extractable . As a result, starch granules existing on the surface of grains were liberated. Some proteases also hydrolyzed a starch granule-associated protein, and, consequently, made the starch highly gelatinized by heating. The protease treatment of aged grains was effective for two main reasons ; liberation of starch granules and removal of the granule-associated protein . During cooking of rice the liberated starch without the associated proteins gelatinized to give rise to stickiness . The gelatinized starch is considered to form a more isotropic film on the surface of every cooked rice grain to give brightness as well.