Drying of medicinal plants

Abstract

Drying is the most common method of medicinal plant preservation and, due to high investment and energy costs, drying is also a large expense in medicinal plant production. Drug quality and consequently earnings are significantly influenced by the drying regime. Conventionally, low drying temperatures between 30 and 50°C are recommended to protect sensitive active ingredients, but the decelerated drying process causes a low capacity of drying installations. Therefore, the objective of research in medicinal plant drying is to find the optimum drying temperature for various medicinal plant species in terms of quality and drying costs. Investigations on Salvia officinalis are presented as an example. Optimal drying temperature was 50°C, because quality reduction due to discoloration occurred at higher temperatures. Compared to drying at 45°C, drying time was reduced by 60% and energy consumption was reduced by 35%. However, comparisons of different species revealed that no general recommendations can be made, but that each species has to be investigated individually. Influence of drying on microbial count of raw drugs is discussed, because this represents a major problem for medicinal plant growers. Furthermore, different types of dryers are described, suitable for specific plant organs like leaves, flowers, roots or seeds. Special emphasis is laid on the choice of drying temperature, because of its strong influence on economic parameters, such as drying capacity, energy requirement and drug quality. Energy saving measures are discussed, including the cutting of plant material and the recycling of exhaust air in drying. Finally, future prospects of medicinal plant drying are drawn