Similarly to ultrasound technique, interest in microwave-assisted extraction has recently increased significantly. The main difference in these two techniques is in acceleration mechanism. Microwaves oscillate in the frequency range from 0.3 to 300 GHz, penetrate into materials and interact with polar groups, causing heat generation. Microwave heating is based on ionic conduction and dipole rotation, which cause friction and collisions between ions and dipoles. It could be concluded that microwave heating is selective and target materials can be heated based on their dielectric constants. Moreover, in microwave-assisted extraction, both gradient of heat and mass transfers are in the same direction (from solid to liquid phase), which is not the case in other novel extraction techniques. This phenomenon causes significant reduction of extraction time. Two main types of microwave-extraction systems are open and closed (Figure 1). Besides two commonly used microwave systems, various modifications such as vacuum, nitrogen-protected, ultrasonic and dynamic microwave-assisted extraction, have been developed. Moreover, this technique is compatible with other extraction techniques (enzyme-assisted extraction, hydrodistillation, etc.) and could be easily coupled with them.
Figure 1. Closed (a) and open (2) type microwave extraction system
Besides common parameters affecting solid–liquid extraction in relation to plant matrix and solvent characteristics, microwave-assisted extraction is affected by irradiation power, temperature, extraction time and cycle of heating, so all parameters must be adequately optimized for each extraction system. The main limitation of this technique is its application at industrial scale which is still at developing level. Since microwaves mostly affect polar compounds, this technique is mainly used for the extraction of proteins, polysaccharides, low-molecular phenolics and flavonoids. In our research, we have successfully applied microwave-assisted extraction for the recovery of different groups of bioactive compounds from compounds from medicinal plants, food industry by-products and agricultural waste).