Cooked old aged Japanese rice and Indica rice are harder and less sticky than newly-harvested Japonica cooked rice. The Japanese generally have a preference for soft and sticky cooked rice. Relative to this, several methods to change rice texture have been investigated. It has been reported that rice vinegaror citrus fruit juice are eVective for increasing the stickiness of cooked rice. Furthermore, acetic acid which is a main component of vinegar and
citric acid which is a main acid of fruit juice have been reported to give the same eVects as rice vinegar and citrus fruit juice.
We reported the eVects of acetic acid added to rice during cooking on the physicochemical properties of cooked rice and proteins. In the Wrst report, the transparency, glossiness, and stickiness of rice cooked with acetic acid were increased and the hardness was decreased depending on molar concentrations of acetic acid ranging from 0.05 to 0.2 M. To investigate the eVect of acetic acid on such textural changes of cooked rice more precisely, we used 0.2M acetic acid, though the concentration of acetic acid is about 0.1 M when we prepare the mixed vinegar of ‘Sushi’, including salt and sugar. In the second paper, we found that when rice was soaked in the 0.2M acetic acid, proteins such as albumin, globulin and glutelin were dissolved more than in the distilled water and glutelin was degraded by aspartic proteinase, suggesting that the dissolution and degradation of proteins might accelerate the absorption, swelling and gelatinization of starch of rice.
In this study, we have focused on the change of starch to elucidate of the mechanism of the textural changes in rice cooked with acetic acid. Here, we have examined the thermal properties for gelatinization and rheological properties of rice starch heated with acetic acid. To investigate the eVect of acetic acid on the rice starch clearly, we used the same 0.2 M acetic acid as in the previous paper.