ABSTRACT:
In microwave assisted extraction (MAE) the sample preparation step was reduced from 1-2 h to 15 min, producing an overall reduction in analysis time. This work initiated the use of microwave energy as a heating source for the chemistry laboratory. The MAE times were a factor of 100 less than the traditional Soxhlet approach. In general, the mechanism involves localized heating of the free water present in the sample. Once the water is at or above its boiling point, the water causes the cell membrane to rupture. This water, as steam, transports the target analyte from the solid to the non absorbing solvent. In this type of work, the sample is a good dielectric while the solvent is a poor dielectric. The microwave process gave higher yields than the traditional steam distillation process. In addition, although in conventional extraction the heat is transferred from the heating medium to the interior of the sample, in MAE the heat is dissipated volumetrically inside the irradiated medium. During the extraction process, the rate of recovery of the extract is not a linear function of time: the concentration of solute inside the solid varies, leading to a non-stationary or unsteady condition In contrast, in MAE, the microwave energy is delivered directly to materials through molecular interactions with the electromagnetic field via conversions of electromagnetic energy into thermal energy . Microwave power and temperature are interrelated because high microwave power can bring up the temperature of the system and result in the increase of the extraction yield until it becomes insignificant or declines. There has been much research and many advances in development in the microwave assisted extraction of a number of plant compounds. This chapter showed the phenomena of mass and heat transfer of the MAE process as well the parameters that influence MAE extraction of bioactive compounds.