FOOD. Science. Technology. Quality

Food. SCIENCE. Technology. Quality

Food. Science. TECHNOLOGY. Quality

Food. Science. Technology. QUALITY




Correlation between the physical property, eating quality and the molecular structure of rice-starchy systems


Investigations on the physicochemical property and molecular structure of starches in Taiwan are reviewed in relation to the eating quality of cooked rice. In addition to some conventional indices (i.e. Brabender viscoamylographic indices, gel consistency, and sensory properties), dynamic rheological parameters are also involved to clarify the importance of molecular properties of starch on the eating quality. The samples discussed were isolated from 9 indica, 9 japonica and 4 waxy varieties. The average number degree of polymerization (DPn) of their amylopectin molecules are in the order of japonica ≥ waxy ≥ indica; while the average chain length (CL), and average exterior chain length (ECL) are indica ≥ waxy ≥ japonica. Indica amylopectins, especially from the starches of high amylose contents (AC, > 26%), carry a greater proportion of long chains than the other two varieties. As to amylose, the DPn and CL values of high-molecular-weight subfractions are somewhat higher for indica amylose than for the japonica. Generally, Brabender viscoamylographic indices of rice flours are well correlated with the apparent AC, gel consistency (GC), and sensory cohesiveness as well as stickiness of cooked milled rice. But the flours with 0-21% AC show similar pasting and soft-gel properties. Dynamic rheological measurements suggest more precisely that different type of starches give their individual rheological patterns during gelatinization and retrogradation, primarily depending on AC and the molecular structure of amylopectin, rather than amylose. Although the AC is commonly regarded as the determinant of eating quality of cooked rice, the molecular and granular structures of starches still give potentially important influences on the physical properties of starchy systems including cooked rice or rice paste.