Selective extraction of oxygenated compounds from oregano with sub-critical water.

Background: The essential oil of oregano is composed of numerous substances that exhibit various properties (e.g. antioxidants). The innovative and promising method of extraction with sub-critical water (subcH₂O) has been applied to the Greek oregano.

Results: The sub-critical water extraction experiments were performed at various conditions of pressure, temperature and water flow rate. Extracts collected at different extraction times were examined by gas chromatography. The oil has been processed by super-critical carbon dioxide (scCO₂) followed by steam distillation or sub-critical water extraction. The conventional method of steam distillation was also performed. The main component of the plant is carvacrol. The favourable oxygenated compounds (carvacrol, thymol, borneol and thymoquinone) have been extracted preferentially and faster with sub-critical water. This method was selective for thymoquinone, which was not present in the oil from steam distillation. The oil yield obtained was much higher in the case of sub-critical water extraction compared to the one of super-critical carbon dioxide. The latter method resulted in oil with the highest concentration in carvacrol.

Conclusion: Compared to the classical steam distillation, the sub-critical water extraction is superior in terms of higher yields, less energy consumption (as it was a faster process), and better composition/selectivity of the extracts controlled by the extraction parameters.