Kinetics of Microwave Extraction of Rosemary Leaves in Hexane, Ethanol and a Hexane plus Ethanol Mixture

Extraction experiments were carried out at atmospheric pressure in an electrically and mechanically modified domestic microwave oven, with temperature changes monitored by a gas thermometer. The rates of extraction were measured of six essential oil components of rosemary (Rosmarinus officinalis L.) and detailed results are given for alpha-pinene and for 1,8-cineole. Major differences of behaviour were found according to whether the solvent employed was hexane, ethanol, or a mixture of 10 mol% hexane + 90 mol% ethanol. Simulated microwave extraction plots generated from the data of a series of thermostatted solvent extractions showed faster extraction rates than expected from the temperature changes. The rates of extractions were affected by the intensity of the microwave energy, the dielectric properties, size and composition of the workload, and the shape of the extracting vessel. Scanning electron micrographs showed that the kinetics and mechanism of the extractions were related to structural changes in the oil-containing glands.