Essential oil extraction using microwave

Currently, the essential oil extraction using microwave is studied intensively to replace conventional technologies such as steam distillation (hydrodistillation), isolation with fat (enfleurage), etc. The primarily advantage of this method is the less time of volatile compounds extraction than previous methods.
Electromagnetic waves transmitted through radiation will penetrate the transparent substances like glass as a container of the raw material. It is able to achieve the gland and vascular system in the plant material. Moisture content and other components, including volatile compounds in plant parts absorb the radiation and it will turn it into heat energy. This leads to an increase in temperature in the raw material rapidly. Temperature increasing continues until the internal pressure exceeds the expansion capacity of cell wall. In this condition, the walls will rupture and the substances inside, including volatile substances will come out freely.
After the aromatic compounds went out of the wall, it will be mixed with moisture which is also contained in the raw material. These mechanisms facilitate the the evaporation of volatile compounds and water through a hydrodiffusion mechanism.
Microwave-Assisted Extraction (MAE)
MAE is a simple technique for the essential oil extraction using microwave. This technique can be applied both in the liquid phase (which liquid is used as a solvent) or gas phase (gas is used as a medium for extracting).
The first technique is based on the principle of differences in the ability to absorb microwave energy at each of the compounds contained in plant parts. Parameters commonly used to measure the physical properties are referred as the dielectric constant.
MAE technique also depends on the dielectric constant of the solvent used. Alfaro et al (2003) said that the ethanol produces the best yield of ginger oil than the other solvent such as hexane, petroleum ether, and dichloromethane. Further in his research said that the use of MAE can reduce the processing time to 60 seconds instead of using the soxhlet extraction.
Microwave-Assisted Hydrodistillation (MAH)
This method is basically a combination between the essential oil extraction using microwave radiation with dry distillation system. Therefore, the MAH are not required additional solvents such as MAE. To ensure ongoing process of diffusion of volatile compounds and water, so the raw material must be fresh (containing moisture).
In general, the method is as follows: plant parts put in the distillation flask made of glass that can be penetrated by radiation so that those materials will absorb it then it reach the gland and vascular systems in the cell wall. These mechanisms generate heat so that the walls will rupture and the aromatic substances inside can freely come out. Water in the raw substance will also become hot due to the absorption of electromagnetic energy and diffuse into the volatile oil causing hydrodiffusion. Aromatic substances and moisture evaporate together based on the principles of distillation and then condensed.
Finally, essential oil extraction using microwave needs to be further studied, especially its composition or its contents, whether there is a change of natural substances in the essential oils or not.

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