Modeling Drying Characteristics of Hawthorn Fruit under Microwave-Convective Conditions

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A laboratory microwave convection dryer was used to dry the hawthorn fruit, applying microwave power in the range of 270–630 W, air temperature in the range of 40–70C and air velocity in the range of 0.4–1.6 m/s. Five empirical drying models for describing time dependence of the moisture ratio change were fitted to experimental data and model parameters in equations were determined by multiple regression analysis. Results showed that the Midilli et al. with R2 = 0.9983, χ2 = 0.0033 and RMSE = 0.0485 had the best performance (among the five models tested) in predicting the moisture. The effective moisture diffusivity, which ranged between 9.29 × 10−10 and 8.81 × 10−9 m2/s, increased with the increase in microwave power and air temperature. The activation energy of samples was found between 12.25 and 27.90 kJ/mol. Maximum shrinkage (64.39%) was achieved at the air temperature of 70C, microwave power of 630 W and air velocity of 0.4 m/s. Practical Applications Hawthorn has been considered as an important traditional herbal medicine due to its disease prevention effects. Advantages of microwave convection drying include short processing time, high mass transfer coefficients, low energy consumption and high quality. As heat and mass transfer and quality changes during drying of hawthorn with microwave-convective method are not described in the literature, it is of importance to gather such data and compare them to find the optimum point of the process. In this study, drying kinetics of hawthorn slices at different air temperatures and microwave powers were studied. Furthermore, simple models for the simulation of drying process, taking into account the fruit shrinkage, were proposed. The results of this study will be helpful in the technological application of microwave-hot air-drying for hawthorn preservation.