Effects of Microwave Thawing Conditions on the Physicochemical Characteristics of Frozen Rice

The effects of freezing and thawing rates on the physicochemical properties of rice were investigated to improve the quality of frozen rice as a home meal replacement product. Cooked rice was frozen by an individual quickfreezing (IQF) system at -45◦C or by a conventional freezing (CF) system at -24◦C. The frozen rice was then thawed using a microwave oven with multiple output powers of 200, 600 and 1000W. The quality of rice was evaluated by the water content, color and the textural properties of hardness and adhesiveness. In addition, the internal microstructure of the rice grain was observed by scanning electron microscope. Results showed that rice thawed by 600 W of microwaves contained lower water content and had harder textures than those treated with 200 or 1000 W. Lightness and redness of rice was increased by the freezing-thawing process, whereas yellowness decreased. The total color difference of individual quick-frozen rice was lower than conventional frozen rice. Microstructure imaging showed the thawed rice contained a crack in the core of the grain. Moreover, the large pore may occur with the 1000W microwave. High freezing and thawing rates seemed to improve the quality of frozen rice. However, using more than 600W of microwaves leads to the hardening of texture or porous structures in the rice.