Swelling power and solubility index
Fig. 1a and b shows that the swelling power and solubility indexes of both starch pastes with and without acetic acid increased with the increase of the temperature, particularly from >60°C. In contrast to starch paste without acetic acid, that one with acetic acid swelled signiWcantly more at above 60°C (Fig. 1a), and had higher solubility at above 80°C (Fig. 1b).
Swelling behavior of starch is mainly due to swelling of amylopectin. Fig. 1a shows that the amount of water absorbed in starch granules by heating was more in acetic acid solution than in distilled water. It was suggested that amylopectin molecule absorbed more water with the addition of acetic acid resulting in the increase of swelling.
When dilute starch suspension is heated at 90°C, almost all the amylose is eluted. As shown in Fig. 1b, 44% of starch was eluted to the acetic acid solution at 90°C, compared to 9% of starch eluted to distilled water. Our previous paper reported that the amylose content of Nipponbare was 16.5%. Therefore, it was just conceivable that more amylopectin was eluted to the acetic acid solution, besides amylose. In the previous paper, it was indicated that the water absorption above 60°C was accelerated in the rice grains cooked with acetic acid. Furthermore, by the addition of acetic acid, the amounts of solids extracted from rice grains during cooking were increased. We found these eVects of acetic acid on the rice starch, as well as the rice grains.