Type and concentration of acid on solubility and molecular size of acid–methanol-treated rice starches differing in amylose content

Rice starches differing in amylose content were treated in methanol with acid at 45 °C for 1 h, effects of acid type and concentration on the solubility and molecular size of starch were investigated. Significant differences were found in solubility and weight-average degree of polymerization (DPw) among starches treated with different acids at the equal normality. Starch treated with HCl had lower DPw than treated with H2SO4 or HNO3. The hydrogen-ion concentration in methanol was negatively correlated (p < 0.001) with the DPw of starch despite of type and concentration of acid used. The increment in degradation extent of waxy rice starch with increasing hydrogen-ion concentration in methanol was lower than that of non-waxy rice starch. Results indicate that the distinct deionization extent of acids in methanol influences the degradation of starch, and that the hydrogen-ion concentration can be used to monitor the degradation of rice starch during acid-treatment in methanol.