Convective drying of medicinal, aromatic and spice plants: a review

Purpose of review: Convective drying with hot air is the most common way to preserve medicinal and aromatic plants. The choice of the optimal drying air temperature is a central economic and ecological criterion. This review compiles experimental studies in terms of models for drying kinetics and losses of active ingredients during drying at different drying conditions. Main findings: Most experimental studies focused on species with essential oils as active ingredients, revealing increasing essential-oil losses with increasing temperature. However, there are differences in temperature sensitivity between species. To describe drying kinetics in most studies the Page model was used. However, as relative humidity of drying air is commonly neglected in the model, drying kinetics of bulks cannot be described. Directions for future research: Experiments with a range of drying air temperatures have to be performed for further medicinal, aromatic and spice plant species. As common reference for calculating losses of active ingredients the content in fresh material should be analysed. Over-drying has to be prevented in drying experiments and analyses of active ingredients should be repeated after a storage period of several months. Furthermore, bulk models are needed to estimate drying processes that are relevant in practice.