Microwave pretreatment followed by associated microwave-hot air drying of Gundelia tournefortii L.: drying kinetics, energy consumption and quality characteristics

Gundelia tournefortii L.is a well-known medicinal plant. Here, drying characteristics of it were investigated using combined microwave-hot air (CMWHA) drying method which consists of microwave pretreatment (specific powers of 450, 600 and 750 W), called primary shock, followed by associated microwave-hot air drying (180, 300 and 450 Wassisted with 60, 70, and 80 degrees Cinlet air temperature). Drying kinetics of the proposed method was modeled. Thus, experimental moisture ratio data was fitted to 10 thin layer models. It was found that Midilli et al. model had the best fit to show kinetics behavior. According to the results, drying rate was increased by increasing microwave power at a certain temperature. The maximum drying rate was obtained in the combined condition of 450 Wand 80 degrees Cwith a primary shock of 750 W.Effective moisture diffusivity (D-eff) ranged between 1.21 x 10(-6)and 6.19 x 10(-6)(m(2)/s); however it was 8.74 x 10(-7) for hot air drying.Moreover, activation energy was in the range of 12.86 to 15.82 (kJ/mol). Comparison between energy consumptions showed that the lowest value was attributed to combined conditions of 180 W, 80 degrees Cand a primary shock of 600 Wwhich lead to 69% energy saving compared to hot air method. Quality of dried products was evaluated by measurement of colour parameters (L*, a*, andb*), total colour difference (increment E), browning index (BI), Chroma, Hue angle, rehydration, and ratio of shrinkage. Generally, it was found that CMWHA method is the best choice to dryGundelia tournefortii L.in terms of time, mass transfer rate, energy, and quality.