Microwave Assisted Extraction for Phytoconstituents-An Overview

At the present time, there are a number of non-conventional extraction methods in use that are all, in principle, solid-liquid extractions (SLE) but which introduce some form of additional energy to the process in order to facilitate the transfer of analytes from sample to solvent. These methods include fairly inert, insoluble, and often polymeric material, such as cellulose of plants or fungi and the microbial cell wall. The first step of the extraction is therefore to release and solubilize the smaller secondary metabolites in the matrix, resulting in the initial extract. Forced-flow solid-liquid extraction (FFSLE) techniques, such as medium-pressure solid-liquid extraction (MPSLE) and rotation planar extraction (RPE), in these methods the extraction solvent is forced through the sample bed either by means of pressure or by centrifugal force, thus increasing the efficiency of the extraction process. Even extraction by electrical energy has been studied. The main advantage of these non-conventional methods compared to conventional SLE methods is the increase extraction efficiency, which leads to increased yields and/or shorter extraction times. Indigenous cultures have learnt to exploit the properties of secondary metabolites in many ways, e.g. specific plants or parts of them have been used as poisons, analgesics, stimulants, preservatives, colorants, tanning agents for tanning leather etc. As our understanding of chemistry and other natural sciences has increased, the active chemical compounds of these traditionally used plants have been successfully isolated and identified. There is an increasing trend of using pure compounds instead of crude extracts prepared from plant material, irrespective of their intended use.