Parboiling stage and drying method affect the physicochemical properties and in vitro digestibility of starch-based rice flour.

This study investigated the effects of different parboiling stages (soaking, steaming and drying) and drying methods (oven drying, freeze-oven drying and sun drying) on the physicochemical properties and in vitro digestibility of Japonica (JR) and Indica (IR) rice. As parboiling process progressed, peak viscosity of rice flour decreased, while gelatinization temperatures increased. Differential Scanning Calorimetry (DSC) thermograms revealed two starch polymorphs in native and soaked rice flour: native (residual) starch and amylose-lipid complexes. Retrograded amylopectin emerged as a third form in steamed rice. Steaming resulted in further decrease in residual starch compared to soaking alone, indicating greater starch gelatinization. Soaking induced contrasting changes in gel texture, with increased hardness in JR and decreased hardness in IR, further intensified by steaming. Soaked rice flour exhibited the lowest rapidly digestible starch content (70.2 % in JR and 64.8 % in IR) due to the increase in non-starch components and amylose leaching. No significant differences were observed between oven-dried and sun-dried rice. Nonetheless, freezing prior to oven drying increased retrograded amylopectin but slightly reduced resistant starch. Parboiling had similar effect on physicochemical properties of brown and white rice. No significant difference was found regarding in vitro digestibility between parboiled brown rice and brown rice flour.