The effect of ultrasonic pre-treatment on the yield, chemical composition and biological activity of essential oil in Oliveria decumbens flowers

•Ultrasonic pre-treatment improved the yield of Oliveria decumbens essential oil.•The use of ultrasonic pre-treatment represented a time reduction of extraction.•Ultrasonic pre-treatment increased antioxidant activity of Oliveria decumbens essential oil.•The essential oil obtained via US-HD method had high insecticidal activity. Oliveria decumbens Vent, an annual herb, belonging to the Apiaceae family, traditionally used to cure diseases such as gastrointestinal, infections, diarrhea and fever. In this study, we aimed to investigate the effect of ultrasonic pre-treatment prior to hydrodistillation (US-HD) on extraction of O. decumbens essential oil (EO). Optimization of three independent variables including ultrasonic temperature, ultrasonic duration, and ultrasonic power to obtain EO with high yield and biological activity was performed recruiting Box-Behnken design (BBD) of the Response Surface Methodology (RSM). According to the RSM results, the maximum yield of EO was achieved with the ultrasonic temperature of 38.63°C, duration of 25.61 min, and power of 149.71 W. The results showed that the UA-HD of O. decumbens produced the highest yield of EO (5.82 %) compared with conventional hydrodistillation (HD) method (as the yield is 4.40 %). EOs extracted using UA-HD and HD methods were analyzed hiring GC and GC–MS, and the findings showed that thymol and carvacrol were the main compounds which made up more than 60 % of EOs obtained by both methods. Moreover, the ultrasonic pre-treatment significantly increased the percentage of thymol, while the percentage of carvacrol underwent no change. The antioxidant study showed that the EO extracted by the UA-HD method (IC50 = 29.61 μg/mL) is of higher antioxidant activity than the EO of HD method (IC50 = 141.11 μg/mL). Also, no significant difference was observed with respect to the cytotoxic activities of the EOs extracted using UA-HD and HD methods. Moreover, the EO obtained via US-HD method had significantly high insecticidal activity with LD50 value of 32.69 μg/larva, compared to HD method (LD50 = 63.21 μg/larva). So, US-HD method is obviously an effective method to improve the EO yield, then to reduce the extraction time and energy consumption, and also increase antioxidant and insecticidal activities, comparing to HD method.