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Drying methods and their implication on quality of some medical plants
Drying is the most common method for post-harvest preservation of medicinal plants because it allows for the quick conservation of the medicinal qualities of the plant material. The objective of this study was to evaluate the influence of seven drying methods (natural air, convective, forced flow dryer at ambient temperature, vacuum, infrared, microwave and freeze-drying) on quality attributes (essential oil, total phenolics, ascorbic acid content, colour and antiradical activity) of basil (Ocimum basilicum L.), oregano (Origanum vulgare L.) and thyme (Thymus vulgaris L.). Essential oil content was altered by drying method. The highest essential oil yield from basil was obtained after convective drying (1.43%), whereas the lowest – after microwave drying (0.07%). Higher essential oil yields from thyme were obtained after natural drying (1.38%) and freeze-drying (1.37%); from oregano – after forced flow dryer (0.39%) and vacuum drying (0.37%). The lowest concentration of ascorbic acid was found in microwave and infrared dried plants. Regarding total phenolics content and antiradical activity, the best drying methods for basil were freeze-drying and vacuum; for oregano – freeze-drying, natural and forced flow; for thyme – freeze-drying and forced flow. For external quality it is important that dried herbs would have high L* values and low a*/b* ratio. According to these values the best drying methods for all tested plants were freeze-drying and convective drying. The most suitable drying method for thyme was freeze-drying and for basil – vacuum drying and freeze-drying. In case of oregano, the best drying methods for high essential oil content were forced flow and vacuum drying, whereas freeze-drying was more suitable method in terms of phenolics content and antiradical activity.
Acknowledgements: This work was supported by the grant VP1 – 3.1-ŠMM-10-V-02 – 021, Ministry of Education and Science, ESFA, Lithuania.
Keywords: Drying, basil, oregano, thyme, essential oil, phenolics, color