Optimization of microwave and infrared drying process of nettle leaves using design of experiments

Optimization of microwave and infrared drying processes of nettle leaves was investigated using dependent variables of drying time and phenolic composition of nettle extracts. The response surface method was selected for design and optimization of the processes. Experiments were performed at different microwave power levels (180, 360, 540, 720 and 900 W) and infrared lamp distances to samples (4, 5.5, 7, 8.5 and 10 cm). The results showed that the drying time (t) decreased and the amount of total phenolic contents (TPC) of nettle extracts increased with an increase in microwave power and also increasing infrared lamp distance to the sample increased the drying time of the samples and decreased the amount of total phenolic contents of nettle extracts. Based on response surface and desirability functions, the optimum conditions for nettle leaves drying were: microwave power 900 W and infrared lamp distance 4 cm. At this point, t and TPC were obtained as 3.014 min and 38.81 mg/g for microwave dryer and 7.73 min and 36.45 mg/g for infrared dryer, respectively.