Abstract: In the present study, postharvest yellowing rice (PHY) and normal rice flours were evaluated by measuring their rheological, thermal, viscosity and structure properties and the differences in their internal structure were investigated. The results showed that a decrease in total color difference, increase in the content of amylopectin, decrease in the contents of amylopectin, protein and lipid, elevation in gelatinization temperature, and reduction in viscosity and breakdown value were found in PHY as compared to normal rice flour. The rheological properties indicated that the storage modulus and loss modulus of PHY decreased relative to the control, and the molecules were closely arranged, along with enhanced fluidity and shear resistance, and reduced thixotropic loop area, suggesting that PHY had distinct non-Newtonian fluid characteristics. There was no significant difference between the infrared spectra of PHY and normal rice except that the peak intensity of PHY at 1 047 and 1 022 cm-1 was increased when compared with normal rice, which indicated that postharvest yellowing will increase the short-range ordered structure of rice. Exploring the differences in internal structure and rheological properties between PHY and normal rice will be helpful to establish a rice yellowing early warning system and extend the application of postharvest yellowing rice.

Key words: rice, postharvest yellowing, rheological properties, molecular structure, starch