Modelling of dehydration–rehydration of instant rice in combined microwave-hot air drying

• Instant rice was subjected to convective hot air, microwave and combined microwave-hot air dehydration.
• Dehydration and rehydration kinetics of instant rice were influenced by drying method, microwave power and air temperature.
• Predictive models were developed to describe dehydration and rehydration kinetics.
• Optimal drying conditions with respect to processing time and quality were obtained.

The drying operation is one of the critical steps in the preparation of instant rice. Drying methods and conditions play important roles in achieving the desired quality. In this study, instant rice was subjected to convective hot air, microwave and combined microwave-hot air dehydration. Three air temperature (70 °C, 80 °C, 90 °C) and three microwave power (210 W, 300 W, 560 W) settings were investigated to find the drying kinetics, rehydration kinetics and colour change. The results showed that combined microwave-hot air drying decreased the drying time required when compared to drying with either hot air or microwave energy alone. Predictive models were developed to describe dehydration and rehydration kinetics. Dehydration rate, rehydration rate and total colour change of rehydrated product generally increased with microwave level and air temperature. Combination drying with MW = 300 W and T = 80 °C was optimal in terms of drying time, rehydration time and colour.