In this study, subcritical water extraction (SWE) was applied to extract the essential oil (especially curcumin) from Curcuma longa L. (turmeric) rhizome. Response surface methodology was used to evaluate the extraction conditions. Experiments were carried out using a three-factor Box–Behnken design under following conditions: temperature (90, 120 and 150C), water flow rate (1, 2.5 and 4 mL/min) and mean particle size (0.5, 1 and 1.5 mm). This is the first time that effects of mentioned variables were considered on SWE of curcumin. The optimum conditions were temperature of 150C, water flow rate of 1 mL/min and mean particle size of 0.5 mm. The maximum curcumin extraction yield was 90.1947 ± 0.26%, which was in a good agreement with the predicted value of 92.0097%. The extracts were analyzed by gas chromatography and gas chromatography-mass spectrometry. Also, results of SWE were compared with conventional methods. Curcumin is an important pharmacological agent that can be extracted from turmeric rhizome. In recent years, subcritical water has been used as a solvent for extraction of flavors, fragrances and antioxidant components from plant materials. Subcritical water is a good solvent for curcumin as well as other organic compounds but is safer than the organic solvents. Subcritical water extraction of curcumin from turmeric rhizome is an alternative method for increasing the quality of the extract.

Subcritical water extraction (SWE) was applied to extract essential oils from Trachyspermum ammi. Thymol as the main component of the extracted essential oils was quantified by reversed phase-high performance liquid chromatography with UV detection at 280 nm. The effects of various parameters on SWE such as temperature, pressure, water flow rate and mean particle size were screened by a fractional factorial design. The results showed that the temperature and water flow rate were significant (p<0.05), but the mean particle size and pressure were found to be insignificant. The response surface methodology (RSM), based on central composite design (CCD) was employed to obtain the optimum conditions of the significant parameters (temperature and water flow rate). The most suitable condition for extraction of essential oils was found to be at temperature 175oC, water flow rate 4 mL/min, mean particle size 1 mm and pressure 2 MPa. At these optimum extraction parameters, the maximum yield of thymol obtained experimentally (12.9634 mg/g dry sample) found to be close to its predicted value of 12.8921. The mathematical model developed was found to fit well with the experimental data. The SWE yield at optimized conditions was significant compared by hydrodistillation.

Many improved methods including subcritical water extraction (SWE) are being used in extraction technologies of food compositions to minimize use of organic solvents. It is faster, cleaner and cheaper compared to other conventional extraction methods that are in practice during these days. Subcritical water extraction (SWE) based on the use of water as an extractant, at temperatures between 100 and 374oC and pressures high enough to maintain the liquid state was applied for extraction of essential oil from Pimpinella anisum seeds. Trans-anethole as the main component of the extracted essential oils was quantified by reversed phase-high performance liquid chromatography with UV detection at 266 nm. Initially, extraction results of t-anethole was screened by Plackett-Burman design to determine the most important variables. The results showed that the water flow rate, temperature and mean particle size affected the essential oil extraction significantly (p<0.05), but the pressure and length-to-diameter ratio of extraction cell had insignificant effects. The response surface methodology (RSM), based on Box-Behnken Design was employed to optimize the significant parameters. The temperature of 170^oC, water flow rate of 2.8 mL/min and mean particle size 0.6 mm were determined as optimum for operating SWE. The SWE efficiency at optimized conditions was significant compared by Soxhlet extraction.