Effect of drying methods on the chemical composition and colour of peppermint (Mentha × piperita L.) leaves

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The study was done with the two peppermint (Mentha × piperita L.) cultivars – ‘Krasnodarskaja’ and ‘Peppermint’, whose leaves were dried using different methods. Investigation of fresh and dried herbs was carried out according to the biochemistry and technology methods. Essential oil was extracted using the hydro distillation method, the amount of chlorophyll a and b was measured spectrophotometrically according to the Vernon method. The highest content of essential oil (0.77%) and chlorophyll (1.69%) was found in the cv. ‘Peppermint’. Chlorophyll a to b ratio was different in the fresh peppermint leaves: in the cv. ‘Peppermint’ it was 1.35 and in the cv. ‘Krasnodarskaja’ – 1.44. Peppermint leaves were dried using active ventilation, convection, infrared, vacuum, microwave, and sublimation methods. The quality of dried herbs depended on the properties of plants and drying techniques. The highest content of essential oil (0.64–0.68% of dry mass) was found in the variously dried peppermint leaves of the cv. ‘Krasnodarskaja’. The lowest content of essential oil (0.08% and 0.065% of dry mass) was determined in microwave dried herbs. The highest content of chlorophyll was found in the lyophilized peppermint leaves (715.0 mg 100 g-1 of dry mass) of the cv. ‘Peppermint’. Regardless of the drying method, significant differences between the ratio of chlorophyll a to b were observed in the dried herbs of the cv. ‘Krasnodarskaja’. The fresh and dried peppermint samples of the cv. ‘Peppermint’ had the lowest brightness L* value (from 22.61 to 35.24) and the lowest yellowness b* values (from 9.35 to 17.00). The biggest changes in greenness a* value were in microwave dried peppermint leaves.