Evaluation of the Effect of Different Drying Methods on Antioxidant and Phytochemical Activity of Essential oil of Origanum vulgare L. subsp. gracile

Drying is the most common way to preserve medicinal and aromatic plants and protect their biochemical compounds. The aim of the present study was to investigate the effect of different drying methods on the antioxidant and phytochemical activity of essential oil of aerial parts of Origanum vulgare L. subsp. gracile in 50 percentage of the flowering stage. This study was conducted in a randomized complete block design with four treatments in three replications at Research Farm of faculty of Agriculture, Urmia University (1365 m above sea level) during 2016. The aerial parts of the plant were dried using four types of drying methods, including room temperature (20-23 °C), open air (direct sunlight), free air (shade) and oven temperature (40°C). The obtained essential oils by hydro-distillation  method were analyzed by GC and GC/MS. Total phenol, total flavonoid, antioxidant activity (DPPH), superoxide radical scavenging activity, and nitric oxide radical scavenging activity were measured by Folin–Ciocalteu reagent (FCR), aluminum chloride, 2, 2-diphenyl-1-picrylhydrazyl, Tris-HCl buffer, and Griess Illosvoy reaction respectively. Based on the results, there was significant difference among different drying methods. The highest amount of essential oil, total phenol content, total flavonoid, DPPH radical scavenging activity, and superoxide radical scavenging activity were observed in shade drying method. However, carvacrol and thymol as the main constituents of essential oil had the highest amount of drying in open air under direct sunlight. After carvacrol and thymol, compounds obtained in a shad-drying method including β-mersin, α-terpinene, γ-trypinene and M-simol had the highest amounts. We concluded that essential oil composition and antioxidant activity of oregano were greatly affected by the drying method and among the different methods of drying, shade drying was the best for the oregano plant to preserve its chemical composition.