Microwave drying kinetics, hypericin content, effective moisture diffusivity and activation energy of hypericum perforatum L.

Abstract

St. John's Wort (Hypericum perforatum L.) flowers with 25 g weight and 2.92 moisture on dry basis were dried in microwave oven using 1000, 850, 750, 650, 500, 350, 160 and 90 W microwave output powers and natural drying as a control, until the moisture fell down to 0.07 on dry basis. Drying processes were completed between 2 and 36 min depending on the microwave powers. The values measured were compared with the obtained data using Page's semi-empirical equations. By increasing the microwave output powers, the effective moisture diffusivity values ranged from 2.70 10-09m2s-1 to 1.54 10-10 m2 s-1. Energy consumption remained constant within the power range of 1000-650 W, whereas 500 and 90 W resulted in significant increase in energy consumption. Mean hypericin content values between natural drying and microwave drying with different microwave power varied between 0.241-0.280 %. Maximum hypericin content was observed natural drying (0.280 %), microwave drying of 850 W (0.278 %) and 750 W (0.277 %). The best quality in terms of hypericin content (0.278 %) was obtained in the drying period with 850 W. Microwave power of 850 W for 150 s which were produced the least energy consumption was only 0.06 kWh.