Effect of microwave and hot-air drying techniques on the color properties and specific energy requirement of madimak plants (Polygonum cognatum meissn.)

Abstract

Polygonum cognatum Meissn. is a wild edible plant and is known as madimak in Turkey. It’s young shoots are cultivated in spring and used as vegetable. The present work evaluates the effect of different drying treatments on the color attributes of madimak plants, which were dried using two different methods: hot- air and microwave-drying. The treatments in air drying were carried out at 60, 70 and 80 °C. Microwave drying were done using four different microwave power levels ranging between 160 and 750 W. Microwave drying of the madimak was faster than hot air drying. Drying time decrease greatly with improved microwave power. Drying processes were completed between 0.058 and 0.308 h depending on the microwave power level, while between 2.583 and 4.166 h in hot-air drying. Microwave drying did not affect the color quality of the samples as much as hot air drying did. Chlorophyll a, chlorophyll b and total chlorophylls contents of microwave dried plants were significantly retained. Both color and chlorophyl attributes indicated that microwave drying for madimak plants is higher suitable than hot-air or ambient temperature drying. It was found the lowest colour change and the highest chlorophyll contents at 750 W microwave powers. In addition, minimum specific energy requirements were found as 44.58 kWh/kg and 107.00 kWh/kg for 80 °C hot air drying and 160 W microwave power level, respectively. The results showed that there was no significant difference of specific energy requirement between hot air drying temperatures while the important difference between microwave power levels.