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

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Drying serves as a crucial postharvest technology for preserving medicinal plants and is essential for developing herbal products. This research aimed to identify the optimal drying temperature for lemon balm (Melissa officinalis L.) concerning drug quality and energy efficiency. Experiments were conducted using a high precision laboratory dryer to observe quality changes during convective drying, supported by mathematical modeling. Various analytical techniques, including CIELAB Colorimetry, Spectrophotometry, High Performance Liquid Chromatography, Scanning Electron Microscopy, and Gas Chromatography, were employed to assess color, rosmarinic acid levels, microstructure, and essential oil content and composition. Innovative methods were also utilized to determine moisture sorption isotherms at typical handling temperatures for medicinal plants. The study separately established sorption isotherms for the dried drug's leaves and residual stems. Additionally, the impact of alternative drying methods, such as vacuum and freeze drying, on the quality and moisture sorption characteristics of lemon balm was examined. By analyzing the isosteric heat of sorption, the minimum energy requirements for various drying methods were estimated. Recommendations for optimal drying conditions were provided, alongside discussions on new opportunities in herb drying.