Effect of Hot-water-soluble Components on the Rheological Properties of Rice Starch

Abstract

The hot-water-soluble components of 2.5% indica (Taichung Sen 17, TCS17 and Kaohsiung Sen 7, KSS7) and japonica (Tainung 67, TNu67) rice starches were prepared by heating from 45 to 70, 80, and 90 °C, respectively and were analyzed by gel-permeation chromatography. Two major peaks (FI and FII) were detected on the chromatogram. The presence of the FI fraction showed that low molecular weight amylopectin and intermediate fraction were solubilized at 70- 80 °C. The size of the solubilized amylose molecules and the ratio of FI to FII increased as temperature rised. The effects of hot-water-soluble components on the rheological properties of rice starch were evaluated by dynamic rheometry. For clarifying the contributions to starch gelation from starch granules and hot-water-soluble components, suspensions of modified waxy rice starch (Taichung waxy 70, TCW70) with the addi-tion of 2% hot-water-soluble components were used as samples. The addition of hot-water-soluble components retarded the swelling of the starch granules and the gelation was dominated by starch granules dur-ing heating. However, the hot-water-soluble components increased the storage modulus (G ') and decreased tan δ of 10% modified waxy rice starch gel during cooling and aging at 25 °C for 3.5 h. The influence of the hot-water-soluble fractions prepared at high temperature on the rheological properties was more profound than that of the fractions prepared at low temperature. The increments of G ' by adding 80 °C fractions were: TCS17 > KSS7 ETNu67 during aging. However, the differences among the 90 °C fractions of the different varieties were: TCS17 EKSS7 > TNu67. Therefore, it is concluded that the fine structure of the hot-water-soluble components will affect the starch rheological properties, and hot-water-soluble components with high molecular weight promote the starch retrogradation more effectively than those with low molecular weight.