Process optimization for supercritical carbon dioxide extraction of Origanum vulgare L. essential oil based on the yield, carvacrol, and thymol contents

Origanum vulgare L. essential oil (OEO) is widely known for its good biological activity. However, different extraction methods with significant implications on the yield of OEO and the content of the thymol and carvacrol. As an efficient method for extracting essential oils, the supercritical carbon dioxide extraction (SC-CO2) can improve the yield of essential oils while protecting their main active components from loss. In this study, the process optimization of SC-CO2 of OEO was carried out. The effects of extraction pressure, temperature, time, and modifier concentration on the composite score of OEO extraction process were investigated. Response surface analysis was performed using a Box Behnken design with three levels and four independent variables. Steam distillation (SD) and lipophilic solvents (n-hexane) extraction (LSE) were compared with SC-CO2 for OEO yields. OEOs extracted by the three methods were qualitatively and semi-quantitatively analyzed by gas chromatography quadrupole-time-of-flight mass spectrometry and gas chromatography-flame ionization detector. The results showed that extraction pressure was the most significant factor affecting the OEO yield, thymol, and carvacrol content. In the optimal conditions (pressure: 217 bar, temperature: 54 °C, time: 2 h, modifier concentration: 14%), the yield of OEO reached up to 1.136%, and the contents of thymol and carvacrol reached 53.172 mg/g and 41.785 mg/g, respectively. SC-CO2 was the best extraction method compared to the other two methods. Under the optimal conditions, yield and the content of main components can be effectively improved. It can provide a theoretical basis for the industrial extraction of OEO. Taking the comprehensive score as the index, the interaction between the four independent variables in the supercritical fluid extraction process was evaluated by the response surface method. The effects of extraction parameters on the yield of essential oils and the contents of thymol and carvacrol were comprehensively investigated.