The extraction of chemical constituents from natural products is a time consuming, energy consuming and solvent consuming task. The traditional extraction methods include solvent extraction and steam distillation. The commonly used solvent extraction methods include Soxhlet extraction and reflux extraction. They all have disadvantages such as large loss of extracted components, long cycle, many processes, and low extraction rate. It has not adapted to the needs of the development of the times, especially in the extraction process of active ingredients of Chinese herbal medicines, which has exposed many shortcomings. With the development of science and technology, a number of new technologies and new equipment have emerged, such as ultrasonic extraction (SE) and supercritical fluid extraction (SFE). Ultrasonic extraction is still widely used in laboratories. SFE is limited in application due to complicated equipment, high operating cost, limited extraction range, and microwave extraction technology is a microwave-assisted extraction (MAA). It is a process in which a microwave is used to heat a solvent in contact with a sample to separate a desired compound from a sample matrix into a solvent. In 1986, Ganzler et al. first reported the use of microwaves for the extraction of natural product components. For more than a decade, this technology has been widely used in the analysis and extraction of food, biological samples and environmental samples. This paper will review the mechanism, characteristics and application of microwave extraction technology in natural product extraction, and look forward to its development trend and application prospects .
The microwave is an electromagnetic wave having a frequency between 300 MHz and 300 GHz, and has four basic characteristics of volatility, high frequency, thermal characteristics and non-thermal characteristics. The commonly used microwave frequency is 2450 MHz. Microwave heating uses the polar molecules of the substance to be heated (such as H2O, CH2Cl2, etc.) to rapidly steer and align in the microwave electromagnetic field, thereby generating tearing and mutual friction and heating. The heat transfer formula of the traditional heating method is: heat source → vessel → sample, and thus the energy transfer efficiency is restricted. Microwave heating is the direct action of energy on the substance to be heated. The mode is: heat source → sample → vessel. The air and the container do not substantially absorb and reflect the microwave, which fundamentally ensures the rapid conduction and full utilization of energy.
Pare et al. hypothesized that microwaves pass through the microwave-transparent solvent to reach the vascular bundles and glandular cells in the plant material, and the intracellular temperature suddenly rises. The continuous high temperature causes the internal pressure to exceed the cell space expansion ability, resulting in cell rupture; The substance inside the cell flows freely and is transmitted to the surroundings to be dissolved. The microwave can selectively heat the polar portions of different polar molecules and different molecules, thereby separating them into a solvent having a small dielectric constant and a relatively poor microwave absorption capability, so that the active components are extracted.
Since Pare proposed the assumption of microwave breaking, some scholars have raised objections. Some scholars have observed that the changes of microstructure of fresh Ginkgo biloba leaves after microwave-assisted extraction show that the plant cell structure changes significantly, mainly in the phenomenon of plasmo-separation, intracellular substances such as organelles and starch granules are destroyed, but microwave-assisted The extraction did not rupture the cell wall.
Regardless of whether the microwave is broken or not, the superiority of microwave extraction of polar substances has been recognized by many researchers.