Determination of drying kinetics of Allium stipitatum under a proposed combined microwave-hot air method and investigation of effect of operating conditions on its color parameters

Allium stipitatum is one of the most important plants native to Iran, which has a special position in food basket of people because of its many healing properties. In this study, effect of a proposed combined microwave-hot air method on kinetics and physical properties of Allium stipitatum was investigated. In the proposed method, after applying microwaves (600, 750 and 900W), samples were subjected to combined microwave-hot air drying process. Results showed that Logarithmic model with values of R2 =0.996 and RMSE=0.002 was the best one to model drying kinetics of Allium stipitatum slices under the proposed method. In addition, drying curves analysis showed that the more microwave power, the less drying time and the more process rate. Moreover, in the combined section, effect of microwave power will be dominant due to higher energy. The optimum operating point to achieve the lowest moisture content and drying time ocuured at primary power of 900W and combined conditions of 450W at 80°C. Increasing microwave power increased effective moisture diffusivity from 0.167×10-8 to 0.283×10-8 m2/s and decreased average activation energy from 17.429 to 2.074 kJ/mol. The optimum operating conditions in terms of appearance quality (colorimetry) were related to drying at microwave power of 900W and combined conditions of 450W and 80°C, which was introduced as proposed optimum point. In general, the proposed combined method compared to microwave method alone, was an efficient method in ordrt to keep the product quality characteristics and its use in drying Allium stipitatum is recommended.