Peppermint Drying Performance of Contact Dryer in Terms of Product Quality, Energy Consumption, and Drying Duration

Drying is a commonly used postharvest operation for medicinal and aromatic plants. Their high initial moisture contents, requirement of moderate drying temperatures, and quick deterioration of their quality attributes make their drying processes energy intensive and time consuming. These properties may also cause the dried product to be of heterogeneous quality. A contact dryer that transferred energy to drying plants mainly by heat conduction was developed and tested by mixing or not mixing batches of 15 kg of chopped peppermint plants. The contact dryer had three main operational units: a drying table, a mobile mixing/aeration car, and a control panel. The contact dryer was operated with one of four drying programs. All programs affected the completion duration of drying, essential oil content, and dried product color differently. The shortest drying time (15 h) was obtained using the drying program of gradually increased water temperature from 55-60 to 75-80 degrees C in 6 h and mixing/aeration. However, mixing and aeration changed the product color slightly more and partially increased essential oil loss. These drawbacks can be alleviated by selecting the appropriate duration of mixing and aeration. The menthol and menthone percentages of fresh peppermint essential oil ranged from 31.02 to 34.02% (average value: 32.52%) and 23.23 to 26.47% (average value: 24.85%), respectively. The menthol and menthone percentages of dried peppermint essential oil ranged from 22.74 to 42.07% and from 8.95 to 21.76%, respectively. The significant variations in the essential oil composition of dried peppermint leaves within replications were possibly caused by the variations associated with the age and cutting order of fresh peppermint plants at harvest.