The assessment of the relationship between structure and functional property of rice starch

Abstract

In order to establish the relationships between structure and properties, the chemical composition, structural properties and functional properties of five Thai rice starches were determined. These starches were classified into 3 groups according to the true amylose content: low amylose content (1-3%, RD6 and a commercial waxy rice starch), medium amylose content (14-15%, Jasmine rice starch) and high amylose content (21-23%, Supanburi 1 and a commercial rice starch). The rice starches with low amylose and medium amylose content had an amylopectin chain ratio (ACR) of 0.25 (S-type amylopectin), while the rice starches with high amylose content had an ACR of 0.19 (L-type amylopectin). There were no significant differences in granule size (average size of 4-6 micrometre) and shape among these rice starches. For the medium and high amylose starch, the weight average molecular weights of amylose were in the range of 1.8 to 2.9 x 10[superscript 5]. The crystallinity (measured by x-ray diffraction) and ratio of short-range molecular order to amorphous (measured by infrared spectroscopy) decreased from 34.62% to 23.00% and from 0.91 to 0.72, respectively, with increasing amylose content from 1.70% to 22.70%. The functional properties, namely swelling power, water solubility index, pasting characteristics, intrinsic viscosity ([eta]), consistency index (k), and storage modulus (G') were related to the true amylose content. An increase in amylose content (from 1.70% to 22.70%) increased water solubility index (linearly from 6.01% to 26.60%), setback viscosity (exponentially from 300 to 2526 mPa.s), and G'(exponentially from 47 to 478 Pa, for 12%, w/w, measured at 1 Hz and 60 ํC). Furthermore, an increase in amylose content decreased the swelling power (linearly from 34.92 to 14.17 g swollen/g dry starch), [eta] (from 192 to 156 ml/g), and k (from 6.32 to 0.29 Pa.s[superscript n], for 5%, w/w, measured at 60 ํC and 50-1000 sec[superscript -1]). However, the breakdown viscosity, onset of pasting temperature and gelatinization temperature related to the ACR values. L-type amylopectin resulted in a lower breakdown viscosity but higher onset of pasting temperature and gelatinization temperature than S-type amylopectin. The onset concentrations (c[subscript 0]), when the granule-granule interactions occur, were 2.5%, 1.5% and 1.10% for the rice starch with high amylose, medium amylose and low amylose content, respectively. For the same amylose content, a higher loss in the short-range molecular order on gelatinisation decreased this onset concentration. In the concentration range from 6% to 15% (w/w), G' was higher than loss modulus (G") at 0.1-10 Hz, 25 ํC and 60 ํC for all rice starches, which indicates strong interactions between the components or phases. The rice starches with high amylose and medium amylose content had rubber-like gel property, while rice starch with a low amylose content was a weak gel. For the rice starches with high amylose content, G{7f2019}of the starch gel increased upon cooling, except for the RD6 starch at the lowest concentration measured (6%), which exhibited only the flow component, all others exhibited the combination of ideal elastic, retard elastic and flow components, which could be represented by the 4-element Burger model. Retrogradation was followed during storage for 100 hours at 25 ํC on gels (30%, w/w) and was found to occur for the high amylose starch only. For the initial stage of retrogradation, G' and the ratio of short-range molecular order to amorphous increased rapidly at a rate of 0.44-0.57 kPa/h and 0.001-0.002 h[superscript 1], respectively. It was suggested that the high propotion of amylopectin chains with DP is less than or equal 10 prevented retrogradation in the low and medium amylose starches.