Essential oil production, chemical composition, bioactive compounds, and antioxidant activity of Thymus vulgaris as affected by harvesting season and drying conditions

Abstract In this study, the effects of seven distinct drying methods (sun, shade, freeze-drying [FD], oven drying at 40°C [OV40] and 70°C [OV70], and microwave 360 W [MW360] and 720 W [MW720]) were assessed in two harvesting seasons (spring and autumn) on the essential oil (EO) content, total phenolic content (TPC), total flavonoid content (TFC), antioxidant activity, and volatile compounds of thyme (Thymus vulgaris L.). The analysis of essential oils facilitated the identification of numerous compounds, 18 of which accounted for approximately 67.84–64.94% of the total thyme EO compounds. Six significant compounds were investigated, including thymol, p-cymene, γ-terpinene, carvacrol, linalool, and borneol. The results demonstrated that the thyme samples harvested in spring possessed higher amounts of γ-terpinene, thymol, carvacrol, EO content, and TFC, while samples harvested in autumn exhibited the highest amounts of borneol, p-cymene, TPC, and higher antioxidant activity. Principal component analysis (PCA) findings revealed that thyme samples harvested in spring, dried in the sun, shade, and FD conditions were rich in EO content, γ-terpinene and samples dried in MW720, OV40, and FD conditions possessed the highest amounts of thymol, carvacrol, and TFC. Drying the samples under MW360, OV40, and FD indicated significantly higher TPC and linalool. Drying the autumn-harvested samples under the sun and shade also exhibited higher linalool and TFC than other methods. Overall, the results conclusively indicated that the harvesting season and drying conditions significantly affected bioactive compounds, antioxidant activity, and EO production and quality.