Microwave Extraction

• Under the influence of this pressure, the cells stretch and ultimately rupture, thus leaching out the active constituents into the surrounding solvent and improving the yield of phytoconstituents.

• This phenomenon can be more strengthened by saturating the plant matrix with solvents having higher heating efficiency under microwave.

• The higher temperature achieved by microwave radiation hydrolyses the ether linkages of cellulose (the main plant cell wall constituent) and converts them into soluble fractions within 1 -2 minutes.

• The higher temperature achieved by the cell wall during MAE facilitates cellulose dehydration, reduces its mechanical strength, and thus helps the solvent to easily enter the cellular components

• The electromagnetic radiation which have a frequency 0.3-300 GHz are called microwave.

• Industrial microwaves are generally operated as 2.45 GHz so that it avoid the interferences of domestic instruments like radio and other.

• Due to their electromagnetic property microwaves contain magnetic and electric field.

• They are perpendicular to one another.

• Electric field generates heat through two simultaneously method i.e. ionic conduction and dipolar rotation.

• Microwave transfer the energy to solid matrix and solvent homogenously and very efficiently.

• Substance (solid matrix and solvent) absorb the energy as per dielectric constant.

• The plant material which is present in the microwave transport solvent absorbs the heat of microwave which causes the heating of moisture present inside the drug.

• Evaporation occurs due to heating of moisture and this will produce high vapour pressure.

• This high vapour pressure crack the cell wall of plant drug and release the active constituent into the solvent.

• The extraction property of the solvent and their interaction with microwave can be altered by using solvent mixtures.

Advantages MAE is a much potential substitute to the traditional solid-liquid extraction method due to the following advantages over Soxhlet:

1) It is less time-consuming as it takes only a few seconds to few minutes (15-20 minutes) to complete the extraction process.

2) It requires less solvent (only a few milliliters).

3) It improves the extraction yield.

4) Since it is an automatic process, it provides better accuracy and precision.

5) It can be used for the extraction of thermolabile constituents.

6) It can be used for extracting minute traces of constituents including heavy metals and pesticide residues from a few milligrams of plant sample.

7) It provides agitation during extraction, which improves the phenomenon of mass transfer.

8) Since its instrumental setup (like Soxwave) combines the features of Soxhlet as well as benefits of microwave, it makes extraction even more attractive.

Instruments

• The closed extraction vessels and focused microwave ovens (or Focused Microwave Assisted Soxhlet or Solvent Extraction, FMASE) are the two types of commercially available MAE systems.

• The former is used for extraction under controlled pressure and temperature, and in the latter, only a part of the extraction vessel containing the sample is irradiated with microwave.

• However, both the systems are available as multimode and single-mode or focused systems.