The viscosity of starch pastes is affected by the size of starch granules and the number of glucose chains leaching out from starch granules. Microscopic photographs of the starch granules, the pH of which was adjusted by adding citric acid before gelatinization, are lined on the left in Fig. 2.
The granule sizes and shapes before heating for all samples with adjusted pH were similar to those of the control (Fig. 2, top left). There were no differences among all samples (data not shown).
The heated cornstarch granules were swollen and partly ruptured, and then glucose chains were leached out from starch granules. The granule sizes of the samples at pH 6.0 , 5.0, and 4.0 with the pH adjusted before gelatinization were almost the same as those of the control. However, the number of glucose chains at pH 5.0 and 4.0 (blue region in Fig. 2) (web version only) was apparently more than that of the control. Therefore, there are many entanglements of glucose chains as compared with those of the control, and these networks of glucose chains lead to higher viscosities. Thus, the viscosity of starch samples, where the pH is between 5.5 and 3.6, was greater than that of the control. Many small granules were observed for the sample with pH
3.5 as compared with the control and the samples at pH 6.0 ,