Modeling sorption behavior and process kinetics of lemon balm (Melissa officinalis L.) for optimization of drying with regard to quality and energy requirement

Drying not only represents a standard postharvest technology for preservation of medicinal plants but also a fundamental processing step for the development of herbal medicinal products. The overall objective of this research was to find the optimal drying temperature for lemon balm (Melissa officinalis L.) with respect to drug quality and energy requirement. Experiments with a range of air temperatures were carried out using a high precision laboratory dryer in order to study relevant quality changes of herbs during convective drying via mathematical modeling. Several analytical techniques (CIELAB Colorimetry, Spectrophotometry, High Performance Liquid Chromatography, Scanning Electron Microscope and Gas Chromatography) were applied to evaluate color, rosmarinic acid, microstructure, essential oil content and composition. Innovative methods were used for the determination of moisture sorption isotherms at temperatures typically found in handling of medicinal plants. As the dried drug commonly consists of leaves and residual stems, the sorption isotherms of each plant organ were separately established. The influence of alternative drying methods (vacuum drying and freeze drying) on drug quality and moisture sorption characteristics of lemon balm was also investigated. Through analysis of the isosteric heat of sorption, the minimum energy required for drying by different methods was estimated under specific conditions. Recommendations for the optimal drying conditions of lemon balm were provided and new possibilities in the field of herb drying were discussed.