Correlations Between Physicochemical Properties, Molecular Structure Of Rice Starch And Rice Eating Quality

Evaluation of rice is mainly composed of rice milling quality, appearance quality, cooking and eating quality, and nutrition quality, among which cooking and eating quality is one of the most important indices. At present, some studies have been carried out on correlations between rice nutrition factors and rice eating quality, but the mechanism about formation of rice flavor is still unknown. Starch, the major components of rice endosperm, accounts for 77～78% of the polished rice with moisture content of 14%. Research of the relationship between rice flavor and starch component is necessary in improving rice qualities. In the present study,20 varieties of rice, including Japonica and Indica, were selected. Measurement were carried out to determine cooking and eating quality, apparent amylose content, cooked rice elongation, RVA characteristic curve, starch components and molecular structure. Data were collected to explore the influences of physicochemical properties, cooking property and starch molecular structure on rice flavor. The results were as follows:1. Eating qualities of different rice varietiesReferring to GB/T 15682-2008, cooked rice fragrance, appearance quality, palatability, taste and texture of cool rice were evaluated. There were remarkable differences among values of eating quality, which varied from 97.8 to 35.0 with a gap of 62.8. The average value of Japonica rice was 73.4, which was much higher than that of indica rice (39.5).2. Apparent amylose contents of different rice varietiesReferring to GB/T 15683-1995, apparent amylose contents were measured. The maximum value was 26.3% while the minimum value was 14.1%, with a gap of 12.2%. The variation ranges of apparent amylose content for Japonica rice and Indica rice were 14.1-18.5% and 20.5-26.3%, with average values of 16.7% and 22.5%., respectively. Apparent amylose content showed a significant negative correlation with eating quality (r=-0.841**). 3. Starch components and molecular structures of different rice varietiesIsolation and quantitative analysis of rice starch components were carried out by column chromatography, and distributions of chain length were measured for different starch components. The results showed that all contents of different starch components correlated with eating quality more or less, i.e. rice with better eating quality usually contained more amylopectin, less intermediate fraction and amylose. The molecular structure of starch was crucial to eating quality of rice, whereas all the contents of long chain in amylopectin, intermediate fraction, amylose and total starch showed significant negative correlations with eating quality (r=-0.945**,-0.681**,-0.623** and -0.863**, respectively).4. Cooked rice elongations of different rice varietiesElongation of rice is the characteristics of extending and stretching when cooked. It is a general index of cooking quality, appearance quality and eating quality. The maximum and the minimum elongation proportion in the present study were 57.6 and 22.8%, respectively. The variation range of Japonica varieties was 57.6-23.1% whereas all elongation proportions of Indica varieties were below 25%. Elongation showed a significant positive correlation with eating quality of rice (r=0.808**).5. Analysis of RVA characteristic curve of different rice varietiesA Super 3 Rapid Viscosity Analyzer (RVA) was employed to measure viscosity of rice starch. The result indicated that parameters of starch RVA had significant correlations with eating quality, among which HPV, CPV, SBV and CSV showed significant negative correlations (r=-0.731**,-0.812**,-0.839** and -0.784**, respectively), while BDV showed a significant positive correlation (r=0.796**).The present study indicates that eating quality of rice is determined by contents of long chain and ratio of three starch components. Content of long chain in amylopectin is crucial to eating quality(r=-0.945**), while those in intermediate fraction and amylose also influence eating quality. Content of apparent amylose, an important index of rice quality, still plays a guiding role in evaluation of rice quality.