VISCOELASTIC PROPERTIES OF RICE-FLOUR PASTES AND THEIR RELATIONSHIP TO AMYLOSE CONTENT AND RICE QUALITY

The dynamic viscoelastic behavior of nonwaxy rice-flour pastes (15 varieties) was studied using oscillatory and stress-relaxation tests. When cooked to 95 degrees C, the storage (elastic, G'), loss (viscous, G''), and relaxation (G) moduli of the pastes, as well as their relaxation time (T-0.75) increased with rise in paste concentration. The increase was greater in high-amylose equivalent (AE) varieties than in low-AE varieties. In pastes of high concentration (13%), G', G'', G, and T-0.75 were highest in type I(high AE) and lowest in type VII (low AE) rice. Upon continued cooking at 95 degrees C for 60 min, G', G, and T-0.75 increased in dilute pastes (7%), but decreased in concentrated (>7%) pastes. Again, these changes were greater in low AE rice. G', G'', and G were highly correlated with total AE, as well as with water-insoluble AE content of rice, but not with the water-soluble AE content. The data suggested that starch granules of high AE rice were relatively rigid, elastic, and strong, while those of low AE rice were soft, inelastic, and weak, and broke down more easily. This difference in starch-granule rigidity was probably the cause of varietal difference in cooked rice texture. Relaxation time, as well as G''/G' (tan delta) data, suggested that concentrated rice pastes were more predominantly elastic than viscous.