Influence of Drying Techniques on Drying Characteristics and Quality of Mint Leaves

(Gamal R. Gamea, Ali Mufarreh Al-Amri and Emad A. Almuhanna)

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Abstract

The method of conservation by drying is an important one, especially when used with medicinal and aromatic plants such as peppermint. There are many changes to colour and active substances, which affect the quality of the product. Three methods were used for drying mint leaves: oven drying (40, 50, 60 and 70 °C), microwave drying (180, 360, 540, 720 and 900 W) and solar drying with three levels of loading (0.5, 1.25 and 5 kg/m2). The drying characteristics and quality aspects of the dried mint leaves were studied. The shortest drying time to achieve the equilibrium moisture content (0.35 g, 0.38 g w/g d. m.) was five minutes by microwave drying at 900 and 720 W, with a loading level of 0.5 kg/m2, while the longest time taken to achieve equilibrium moisture content (0.72 g w/g d. m.) was 20 hours at 40 °C. Meanwhile, the solar drying method took eight hours to achieve the final moisture content (0.68 g w/g d. m.) at both 1.25 and 0.5 kg/m2 loading levels. The results showed that drying the mint leaves at a temperature of 50 oC, with a loading level of 1.25 kg/m2, was the best way to obtain the largest amount of essential oil. The Midili-Kucuk model adequately described oven and solar drying behaviours, and the logarithmic model adequately described microwave drying behaviour. In conclusion, solar drying (with its low cost) and oven drying (at 50 oC and loading level of 1.25 kg/m2) could both be effective methods of drying mint leaves, since they retain much of the essential oil. However, based on the results of this study, microwave drying is not an advisable method of obtaining aromatic oils from medicinal and aromatic plants.

KEYWORDS
Drying modelling, mint leaves, microwave, oven drying, solar drying

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