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Abstract: Since the last decade, there is enhanced interest for the use of Microwaves for the extraction of the constituents from plant material. The traditional techniques like maceration, percolation, decoction, etc., are time- and solvent- consuming. This review brings into prominence the importance of novel methods of extraction for delivering high quality product. The microwave-assisted extraction (MAE) technique is a promising technique which is highlighted by increased extraction yield, decreased time and solvent consumption; moreover the reproducibility is better. A brief theoretical background of the principles involved and the types of instruments used has been presented. The main parameters like microwave power, extraction time, solvents, temperature and matrix characteristics, etc., which influence the MAE have been discussed. The application of optimization techniques, such as Factorial Design, to MAE has been highlighted, with examples. The potential applications of this technique and its performance vis-à-vis that of classical techniques have been elucidated.
Introduction
The history of herbal medicine is as old as human civilization. The widespread use of herbal remedies and healthcare preparations, as those described in ancient texts such as the Vedas and the Bible, and obtained from the commonly used traditional herbs and medicinal plants, has been traced to the occurrence of natural products with medicinal properties. World Health Organization (W.H.O.) currently encourages, recommends and promotes traditional/herbal remedies in National Health Care Programmes because such drugs are easily available at low cost, are comparatively safe and the people have faith in such remedies.
Plants synthesize a tremendous number of so called secondary metabolites, in addition to compounds that are necessary for the growth and reproduction such as carbohydrates, proteins and lipids. As our understanding of chemistry and other natural sciences has increased, the active chemical compounds of the plants have been successfully isolated and identified. Extraction of crude drugs can be done by various processes depending on the physical nature of the drug and chemical properties of the constituents present in it. Various traditional methods used for the extraction of drugs include Infusion, Decoction, Digestion, Maceration and Percolation. Out of these Maceration and Percolation are of particular importance and most Pharmacopoeias refer to these processes for the extraction of crude drugs The conventional extraction processes are time consuming, e.g., maceration done for 2-7 days; involve bulk amount of solvents and ultimately there might be thermal decomposition of the target molecule like in the case of Soxhlet extraction.6 The demand for new extraction techniques has encouraged the development of alternative extraction techniques such as Ultrasonic Assisted Extraction (UAE), Microwave Assisted Extraction (MAE), Supercritical Fluid Extraction (SCF) and Accelerated Solvent Extraction (ASE). These techniques have enabled automation, shortened extraction time and reduced organic solvent consumption.
Microwave-assisted extraction (MAE) is a relatively new extraction technique, which utilizes microwave energy to heat the solvent and the sample to increase the mass transfer rate of the solutes from the sample matrix into the solvent. Many reports have been published on the application of microwaves for extracting Pesticide/Insecticide residues and Herbicides from the samples. MAE technique has also been used to extract contaminants present in the environmental samples. The usage of microwaves for extracting phyto-constituents is still in infancy. This paper deals with the MAE of phyto-constituents and the factors which considerably influence the extraction. The theory behind microwaves and the heating mechanism is also discussed. The principles behind the extraction process are elucidated. In order to optimize the extraction procedure various techniques are also stated. In nutshell, this paper presents the epitome of success of MAE technique in the medicinal plant research.