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Significance of Drying Methods in Essential Oil Production
Plant material is the key parameter in essential oil production. Apart from growing conditions, geographical and environmental factors, harvest and post-harvest processes also have significant impact in obtaining high-quality essential oil from medicinal and aromatic plants. In order to gain the maximum accumulated compounds (secondary metabolites), the plant must be harvested at the right time with right methods. Consequently, depending on the plant, the production method and capacity of the distiller, either fresh or dried plants can be used in producing essential oils.
Since high fresh material supply occurs within a short period of time usually exceeding the distillation capacity of the facility, there is crucial need for drying plant materials. Furthermore, since medicinal plants usually contain 75–85% water, removal of water (reducing to 12-15%) from the plant materials is an essential process for inhibiting enzymatic and microbial activities. Drying allows the distiller to store plant materials while keeping their original characteristics without substantial effects on texture, colour, flavour and biochemical profile. Therefore, plants should be dried using a convenient method that reduces humidity without affecting the quality of active components.
Basically, there are three prominent drying methods that stand out;
Besides the above-mentioned, there also exist alternative methods such as solar tunnel drying, microwave assisted drying, freeze drying and fire heat drying, which are usually not preferred in industrial essential oil production due to high investment and operational costs.
Sun drying is one of the most popular drying methods, particularly in the Mediterranean region where sunshine is abundant. Although it is one of the easiest drying methods, plant material is exposed to environmental contamination and harmful sunrays. Moreover, uncertainty of weather conditions may affect the process negatively.
Shade drying is used when exposure to direct sunlight may affect active ingredient levels. It is a slower process than sun drying but the desired chemical compounds, colour and aroma can be preserved, and the plant material has limited exposure to environmental conditions.
Oven drying with different temperatures have been experienced in recent decades for drying of most types of the medicinal and aromatic plants. It is more time efficient than shade and sun drying, but it requires extensive investment in equipment and increases energy costs.
Shade drying is distinguished as the appropriate procedure
Several scientific studies have analysed the effects of different drying methods on essential oil yield and composition and have determined that there are significant qualitative and quantitative differences. Almost all researches have proven that shade drying method provides the highest essential oil quality and yield, followed by oven drying at 40°C, sun drying and oven drying at 60°C.
Apart from a few photoactive substances, most of the chemical compounds that are stored in plant materials are temperature and sun light sensitive. High temperatures and intense solar radiation reduce the quality of essential oils (both in chemical content and colour). Therefore, sun drying and oven drying at high temperatures are the least favourable drying methods in essential oil production.
In summary, essential oil content is affected not only by agricultural practices and distillation methods but also by post-harvest operations. Controlled drying immediately after harvesting reduces essential oil losses and preserves the characteristic components. Shade drying method is distinguished as the appropriate procedure for drying most of the medicinal aromatic plants.