Effect of hot-water-soluble components on the rheological properties of rice starch

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 degrees 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 amy lopectin and intermediate fraction were solu bilized at 70-80 degrees C. The size of the solubilized amylose molecules a nd the ratio of Fl to FII increased as temperature rised. The effects of ho t-water-soluble components on the rheological properties of rice starch wer e evaluated by dynamic rheometry. For clarifying the contributions to starc h gelation from starch granules and hot-water-soluble components, suspensio ns of modified waxy rice starch (Taichung waxy 70, TCW70) with the addition of 2% hot-water-soluble components were used as samples. The addition of h ot-water-soluble components retarded the swelling of the starch granules an d the gelation was dominated by starch granules during heating. However, th e hot-water-soluble components increased the storage modulus (G') and decre ased tan delta of 10% modified waxy rice starch gel during cooling and agin g at 25 degrees C fur 3.5 h. The influence of the hot-water-soluble fractio ns prepared at high temperature on the rheological properties was more prof ound than that of the fractions prepared at low temperature. The increments of G' by adding 80 degrees C fractions were: TCS17 > KSS7 congruent to TNu 67 during aging. However, the differences among the 90 degrees C fractions of the different varieties were: TCS17 congruent to KSS7 > TNu67. Therefore , it is concluded that the fine structure of the hut-water-soluble componen ts will affect the starch rheological properties, and hot-water-soluble com ponents: with high molecular weight promote the starch retrogradation more effectively than those with low molecular weight.