Investigating the phytochemical changes of Thymus vulgaris L. essential oil under the influence of ultrasound waves

Many researches have been conducted to introduce ultrasound as an auxiliary factor to increase the efficiency of extraction of essential oils from medicinal plants. However, few researchers have studied the role of ultrasound waves in stimulating chemical reactions of the essential compounds of plants. This research was conducted with the aim of investigating the effect of ultrasound on the main components of essential oil of Thymus vulgaris L. after extracting the essential oil and without the presence of plant tissue. For this purpose, the aerial parts of the plant were harvested at the flowering stage from the south of Tehran in June 2021 . The essential oil of the plant was extracted using a distillation apparatus (Clevenger) and its compounds were determined and measured by GC and GC/MS. Subsequently, ultrasonic waves were applied to the extracted essential oil by a titanium probe (400 w) with three variable factors of temperature, time, and the ratio of essential oil/ water at three levels. Finally, 27 treated essential oil samples were prepared and their chemical compounds were quantified by GC. The primary essential oil contained 44.37% of thymol, 29.80% of p-cymene and 21.24% of γ-terpinene. The results showed that the application of ultrasound to the essential oil increased the amount of thymol and decreased the amounts of γ-terpinene and p-cymene. By optimizing the conditions (1:0 ratio of thyme essential oil and water, 20˚C and 10 min), the active ingredient thymol was reduced from 44.37% to 70.18%. Ultrasonic waves in the presence of water and oxygen caused the production of hydroproxyl radical (HOO˙) and then showed oxidation reactions to produce thymol from γ-terpinene and p-cymene. The results showed that in addition to helping to increase the efficiency of extraction, ultrasound can cause a change in the chemistry and the ratio of phytochemical compounds in the essential oil by stimulating radical reactions.