The influence of continuous and periodic microwave drying on rosemary: drying and temperature kinetics

Abstract

Continuous microwave is a heavily studied drying method known for its effectiveness and efficiency, however, it leads to overheating in most cases. The primary objective of the present research is conducting and evaluating a comparative study of continuous and periodic microwave drying on rosemary for various power levels (6, 9 and 12 W/g) and different pulse ratios to overcome the overheating challenge. The evaluation and assessment were based on drying and temperature kinetics. Drying kinetic study revealed that periodic and continuous microwave drying at 12 W/g had the least drying duration of 12.5 and 11 mins, respectively. Likewise, both processes had the highest drying rates of 0.364 and 0.461 kg H2O/ kg dry basis min. The temperature kinetic study showed that the periodic microwave drying (71.4°C) resulted in a lower maximum sample temperature than continuous microwave drying (79.2°C). The periodic microwave drying with higher pulse ratios had a more even heating throughout the drying process than lower pulse ratios. Thereby, periodic microwave drying at 12 W/g and the highest pulse ratio was deemed to be the most suitable drying process for rosemary. The four thin layer models, namely Page, Modified Page, Midilli & Kucuk and Modified Midilli & others, were the most suitable to describe the drying kinetics of rosemary.