Ultrasound-enhanced subcritical water extraction of essential oils from Kaempferia galangal L. and their comparative antioxidant activities

•An ultrasonic probe was introduced into the subcritical water extraction device.•Ultrasound-enhanced subcritical water extraction (USWE) has high recovery yields.•USWE sample shows a better effect of antioxidant activity.

Kaempferia galangal L., belonging to family Zingiberaceae, has many bioactivities beneficial to human health. The main purpose of this study was to extract essential oils from K. galangal using ultrasound-enhanced subcritical water extraction (USWE) and investigate the antioxidant ability of the essential oils. An ultrasonic probe was introduced into the subcritical water extraction device to help the extraction process. The effects of temperature, extraction time, pressure, ultrasonic power density and frequency on the extraction yield was investigated. The USWE method was also compared with the other two methods, subcritical water extraction and steam distillation extraction, in terms of the extraction conditions, recovery yields and composition. The antioxidant activity of the essential oils was evaluated by the assays of the DPPH scavenging ability and the superoxide anion radical scavenging activity. The maximum yields (25.80 mg/g) of ethyl trans-p-methoxycinnamate were obtained at an extraction condition of temperature 120 °C, extraction time 20 min, extraction pressure 10 Mpa, ultrasonic power density 250 W/L, and ultrasound frequency 20 kHz. USWE was found to be superior to the other extraction methods in terms of its higher recovery yields of essential oil and its stronger antioxidant ability.