Optimization of Infrared Rotary Roasting Conditions for Immature Rice: Effects on Physicochemical and Cooking Qualities

Abstract

Immature rice is a distinctive cereal product widely consumed in Asian countries due to its natural green color, soft texture, unique flavor, and high nutritional value. However, its fragile structure and pigment sensitivity create significant processing challenges. This study investigates the effects of infrared (IR) roasting temperature (550–650 °C) and time (20–40 min) on the physicochemical, nutritional, and cooked-rice qualities of immature rice (Oryza sativa L., cv. RD6). A two-factor study with three level of factorials was designed and response surface methodology (RSM) was used to evaluate roasting variables and to identify optimal processing conditions (p ≤ 0.05). Increasing roasting severity decreased rice yield, moisture content, water activity, and chlorophyll content, while promoting grain darkening, increasing phenolic content, and enhancing cooked-rice expansion and hardness. Several responses exhibited significant linear and quadratic relationships with roasting conditions, with model coefficients of determination (R2) ranging from 0.676 to 0.829. Multi-response optimization using desirability analysis identified the optimal roasting condition as 650 °C for 20 min, which produced predicted values that closely matched experimental validation (p > 0.05). These results demonstrate that IR roasting provides an effective green-energy processing approach for producing value-added immature rice while maintaining desirable color, nutritional properties, and cooked-rice texture.