Effect of Drying Methods on the Quality of the Essential Oil of Spearmint Leaves (<i>Mentha spicata</i>L.)

Abstract

Drying is one of the primary processes involved in the manufacture of herbs, which themselves come in the form of stems, leaves, and roots. The quality of the final product depends on the procedure used and drying method. This article presents the results pertaining to the drying behavior of spearmint in both hot air drying and freeze drying conditions. Conventionally, herbs are dried at high temperatures (40–45°C), which results in a deterioration of product quality. The characteristics of vacuum freeze–dried herbs were studied, and the quality of the freeze-dried products was assessed. It was found that drying time and essential oil content were strongly influenced by chamber pressure. Higher chamber pressure tended to lengthen the drying time but preserved the major volatile compounds of spearmint in the final product. The quality of the freeze-dried product was assessed as being lower than that of the raw material but higher than that of a convectively dried product. Four different mathematical models were fitted to the drying data. A water absorption test confirmed that the rehydration ratios of vacuum freeze–dried mint leaves were higher than those that were convectively dried.