Changes in physico-chemical and rheological properties of rice during flaking

Hydration, solubility, pasting and rheological properties of roasted-parboiled rice and flakes were examined in comparison to raw rice. There was an increase in the hydration capacity, swelling power and solubility during roasting-parboiling of rice as a result of gelatinization of its starch. Flaking caused further increase due to damage of starch by application of mechanical force. The changes were higher in flakes from roller-flaker as compared to those from edge-runner. Pasting characteristics of flour slurries in Rapid Visco Analyzer showed a typical profile for pregelatinized products viz. a higher initial viscosity but a lower peak viscosity for roasted-parboiled rice and flaked rice than for raw rice. The viscosity curves and flow curves of the products over a wide range of shear rate indicated a non-Newtonian, pseudoplastic behaviour for all the samples. All samples showed typical hysteresis loop in their viscosity curves indicating their thixotropic nature. Flakes from roller-flaker exhibited lower viscosity but more thixotrophy indicating higher starch breakdown in them than in edge-runner flakes, which seemed to have more of heat damaged starch.