Determination of glandular structures of thymus praecox subsp. skorpilii species distributed in Ordu province and the effect of different drying methods on some physicochemical properties

This study aims to anatomically examine the secretory structures, determine the amounts of volatile compounds and total phenolics, and investigate the effects of different drying methods (shade drying, freeze drying, hot air drying (50 °C), and microwave drying (360, 600, 900 W)) on the volatile compounds, total phenolic content (TPC), and color parameters of Thymus praecox subsp. skorpilii (Velen.) Jalas, a species distributed in the Ordu region. The plants were collected from relevant localities during their generative periods. Some of the plants were placed in 70% alcohol for anatomical examinations. Thymus (thyme) is a commonly used spice in our country. The selection of the ideal drying method for the studied thyme species is crucial for preserving the plant, and extending its shelf life. Additionally, the high biochemical content of this species enhances its value in alternative medicine, pharmacy, cosmetics industry, and its economic importance in both domestic and international trade. Therefore, researching the secretory structures, volatile compounds, and the most suitable drying method for the species is of great importance. In the species, there are a total of eight types of hairs, including 5 covering hairs and 4 secretory hairs. These hairs can be either unicellular or multicellular. The calyx part of the species has a significantly higher number of covering and secretory hairs compared to other parts. Microwave drying method considerably reduced the drying time compared to other drying methods. The L*, a*, and b* color parameters of the samples dried by freeze-drying and microwave drying were found to be similar to those of fresh sample. However, the hot air and shade drying processes resulted in significantly higher a* values and lower b* values compared to the fresh sample. No significant effect of microwave power levels on the color parameters of the samples was observed. The total phenolic content (TPC) values of the dried samples ranged from 20.68 to 37.31 mg GAE/g dry matter. The freeze-drying method provided the highest TPC value, followed by microwave drying at 900 W. The TPC values decreased with the decreasing microwave power level. A total of 26 volatile organic compounds were identified in the fresh sample, 27 in the freeze-dried sample, and 28 in the samples dried using shade, hot air, and microwave drying methods. The most abundant volatile compounds in the fresh sample were α-pinene (15.63%), 3- octanone (14.76%), and limonene (13.47%). In the dried samples, limonene (19.87– 49.36%), β-caryophyllene (10.58–22.95%), and α-pinene (7.37–18.04%) were the top three predominant compounds