ENGINEERING FOR RURAL DEVELOPMENT Jelgava, 25.-27.05.2016. 1145 RESEARCH IN SOME MEDICAL PLANT DRYING PROCESS Aivars Aboltins 1 , Pavel Kic 2 1 Latvia University of Agriculture; 2 Czech University of Life Sciences Prague, aivars.aboltins@llu.lv, kic@tf.czu.cz Abstract. The drying process by free convection of six different medical plants (flowers of garden marigold (Calendula officinalis), leaves of lemon balm (Melissa officinalis), origanum (Origanum vulgare), common agrimony (Agrimonia eupatoria), common lavender (Lavandula angustifolia), and common sage (Salvia officinalis)) is compared. The constant and changing drying coefficients are determined using the gravimetric methods of measurement and a special mathematical evaluation methodology. Theoretical and experimental results are compared. The experimental results show that the drying coefficient of a plant at the first 10 hours of drying is changing linearly until it becomes constant. The corresponding linearity is strong, characterized by high coefficients of determination. The changes of speeds of sample drying rate are approximately equal, except lemon balm, which is more than twice greater than the others. Keywords: drying coefficient, free convection, medical plant. Introduction Requirements of worldwide progress in cultivation of medicinal and aromatic herbs, plant drugs, different biological materials and their products require also new theoretical foundations and knowledge about their processing and safety storage. The need for high quality raw is increasing. This phenomenon is proven by number of cultivation results and registration procedures, concerning medicinal and aromatic plant cultivars, recently reported from different countries. The achieved progress in cultivation is obvious, in spite of the fact that different strategies and methods are used country by country [1]. In pharmacy, plant raw materials are important sources of new medicines and their substitutes. Natural medicines of plant origin have a wider therapeutic spectrum, milder action and less frequent side effects compared with synthetic substances. According to the data of the World Health Organization, about 70 000 plant species are currently used for medicinal purposes; about 1000 species are used in the European pharmaceutical industry. Preservation of production is a very important problem to be solved by producers of these products. One of the ways of preservation of products is drying. Medicinal plants can be dried in a number of ways: in the open air (shaded from direct sunlight); placed in thin layers on drying frames, wire-screened rooms or buildings; by direct sunlight; in drying ovens/rooms and solar dryers; by indirect fire; microwave; or infrared devices [2]. When possible, temperature and humidity should be controlled to avoid damage to the active chemical constituents. The method and temperature used for drying may have a considerable impact on the quality of the resulting medicinal plant materials. Chemical changes are the most important in the post-harvest of medicinal plants and can be influenced by drying. Moreover, drying can promote changes in the product appearance (color) and smell, modifying the final quality. Drying research is an outstanding example of a very complex field, where it is necessary to look comprehensively on the simultaneous energy and mass transfer process that takes place within and on the surface of the material. In order to get the full view of the drying process, researches have to incorporate and deal with highly non-linear physical phenomena inside drying agricultural products, non-homogenous distribution of temperature and humidity inside dryers, equipment selection, product final quality. That is the reason why a unique theoretical setting of drying has to be determined through the balance of the heat flow, temperature changes and moisture flow. In order to find the optimal drying regime it is necessary to understand the transport mechanisms which take place within and on the surface of the product. The drying process is characterized by the existence of transport mechanisms such as surface diffusion, pure diffusion, capillary flow, evaporation, thermo-diffusion, etc. Many studies were done to process medical plant drying by small heated air. The researchers investigated the influence of some process parameters (temperature, sample thickness, layer thickness, air flow rate, etc.). The effect of the used airflow and drying air temperature on the drying kinetics was