Thin layer drying kinetics of pre-gelatinized starch under microwave

Abstract

Abstract Absolutely dried starch is widely used in cooking and other industries. However, the prolonged drying time during falling rate drying period and low energy efficiency limit the application of traditional hot air drying. Microwave energy is the alternative choice considering the ‘volumetric heating’ mechanism. Then, investigations on microwave thin layer drying of starch were conducted by experimental studies and mathematical modeling. Results show that drying time can be reduced significantly with the increase in microwave power density. And there exists an optimal layer thickness, both greater and less than the value will result in a lower drying rate. This phenomenon is completely different from hot air dying and has not been reported in literature before. Explanations are given from the perspective of heat and mass transfer. Data fitting shows that ‘Midilli–Kucuk’ model is the best one to describe drying behavior of starch. An integrated ‘Midilli–Kucuk’ model is also given after considering the effect of operating variables on model parameters. Effective diffusivities vary from 6.08 × 10−7 to 6.10 × 10−5 m2/s, and increase with the increase of microwave power density, decrease with the increase of surface area per unit mass, values are higher when compared with other materials dried under microwave in literature. Finally, nonlinear surface fitting was conducted in order to give a systematic prediction for effective diffusivities.