What are the methods of extracting plant components?

1 ultrasonic enhanced extraction technology

Ultrasonic wave is a kind of mechanical wave, which propagates in elastic medium with a frequency greater than 20 kHz and is inaudible to human ears. The application of ultrasound mainly shows in two aspects: one is the application of energy ultrasound, that is, power ultrasound, and the other is the application of signal ultrasound. In the application technology of power ultrasound, an active branch in recent years is ultrasonic-assisted extraction-enhanced solvent extraction technology. Research and application have been carried out in this field, including: ultrasonic extraction of vegetable oil, pigments and spices in characteristic plants, effective components in Chinese herbal medicines, tetrandrine in hops, oils, toxins and residual pesticides in animal tissues.

1. 1 ultrasonic enhanced extraction principle ultrasonic wave has the duality of fluctuation and energy, and its vibration generates and transmits a lot of energy. The use of ultrasonic vibration energy can change the tissue structure, state and function of substances or accelerate the process of these changes. The action of ultrasonic wave on medium can be divided into thermal action and non-thermal action [1]. Thermal action refers to the transformation of mechanical energy into thermal energy of medium during vibration, and the calculated amount of thermal energy is directly proportional to the absorption coefficient of medium sound intensity, ultrasonic sound intensity and ultrasonic action time. Under a certain sound intensity, the heat generated and the heating effect are very limited, which has little significance for extraction. The non-thermal effect of ultrasonic wave plays a leading role in enhanced extraction. There are two main forms of non-thermal action, namely mechanical action and cavitation. The former refers to the alternating compression and stretching of medium particles caused by ultrasonic waves in the process of medium propagation. Although the vibration displacement and velocity of the particle change little, its acceleration may reach a particularly large order of magnitude. This large-scale acceleration can significantly increase the penetration of solvent into extraction cells and strengthen the mass transfer process, thus strengthening the extraction process. In contrast, the cavitation effect of ultrasonic wave is the most important reason for enhanced extraction. Ultrasonic cavitation refers to a series of dynamic processes such as oscillation, growth, contraction and even collapse of tiny bubble nuclei in liquid under the excitation of sound waves. According to the different manifestations, cavitation can be divided into two forms: steady cavitation and transient cavitation. Steady-state cavitation occurs under the action of low sound intensity, and cavitation bubbles oscillate in a nonlinear form in the medium for several periods. In the process of oscillation, the micro-flow around the cavitation bubble produces a large tangential force on other particles in the solution, which is beneficial to the penetration of solvent into cells. In addition, low-intensity ultrasound can not only make micro-flow around cells, but also make animal and plant cells produce intracellular circulation, thus improving the permeability of cell membrane and cell wall, without destroying the membrane or increasing the temperature of the medium to increase the mass transfer process. Ultrasonic wave will have transient cavitation under strong sound intensity. Gas (vapor) bubbles are rapidly generated, grown, compressed and collapsed within a sound wave period, and local hot spots as high as 5 000 K are formed when they collapse, and the pressure can reach hundreds or even thousands of atmospheres. With the release of high pressure, a strong shock wave (homogeneous) or high-speed jet (heterogeneous) will be formed in the liquid. In extraction, this powerful impinging stream can effectively reduce and eliminate the barrier between solvent and water phase, thus improving the mass transfer rate. At the same time, the impact flow produces a physical shear force on animal and plant cells, which causes them to deform and rupture, releasing inclusions and greatly speeding up the extraction process.

1.2 characteristics of ultrasonic enhanced extraction A large number of studies show that the strong vibration and cavitation effect produced by ultrasonic can improve the extraction efficiency, improve the quality of the extract, increase the yield, save raw material resources, and avoid the adverse effects of high-temperature extraction on some heat-sensitive components. This is of great significance to the extraction of traditional Chinese medicine.

In the process of extracting different components by traditional water boiling or alcohol extraction, there are some problems in different degrees, such as long leaching time, high temperature, long heating process of effective components, more impurities leaching, high energy consumption and low utilization rate of raw materials. Ultrasonic enhanced extraction technology can overcome the above defects and has high economic benefits.

1.3 Application of ultrasonic enhanced extraction The application of ultrasonic in the extraction of Chinese herbal medicine components is mainly in the following aspects [1, 2]: ① Extracting alkaloids from plants. Extracting alkaloids from plants by conventional methods is generally time-consuming, laborious and inefficient, but with the help of ultrasonic technology, remarkable results can be obtained. For example, compared with cold soaking method and Soxhlet method, ultrasonic method has the advantages of simple process, high extraction rate, fast speed and good effect. ② Extract glycosides from plants. In the quantitative analysis of traditional Chinese medicine, it is time-consuming and inefficient to determine the effective components and samples of traditional Chinese medicine by conventional decoction or reflux. The application of ultrasonic method can solve the problems of time and efficiency. For example, syringin is extracted from Acanthopanax senticosus, aromatic glycoside is extracted from Sophora japonica, gastrodin and gastrodin are extracted from Gastrodia elata. Compared with cold soaking method, ethanol solution reflux method (Soxhlet extraction method), heating cooking method with water as solvent and hot alkali extraction-acid precipitation method, the extraction rate can be greatly improved, and the process is simple and rapid. ③ Extraction of other pharmaceutical ingredients. The influence of ultrasonic wave on extraction is related to the fragmentation of tissue cells, which can better release soluble components in cells and make solvent molecules penetrate into tissue cells. Ultrasonic crushing is applied to some plant cells with firm cell walls, which makes them rupture in a very short moment below milliseconds. When cells break, fresh bioactive substances such as enzymes, hormones and vitamins are extracted. Ultrasonic wave can also be used to strengthen the extraction of contents of rosin, coffee, tea and ginkgo biloba leaves, such as flavonoids, tea polyphenols, animal and plant proteins and so on. In addition, ultrasound plays a special role in extracting natural spices from aromatic plants, that is, it can maintain the specific flavor of the extract to the maximum extent and inhibit the volatilization of these substances during the extraction process. Finally, the research and application of ultrasonic extraction in oil extraction is very active, and experiments and applications involving the extraction of star anise oil, almond oil, clove oil, perilla oil and evening primrose oil have been carried out.

Microwave enhanced extraction technology

Microwave is the shortest wave band (30 MHz ~ 300 GHz) in radio waves. Different from the traditional heat extraction from outside to inside by heat conduction and radiation, microwave-enhanced extraction accelerates the extraction process by dipole rotation and ion conduction from inside to outside. Microwave extraction is a new technology to extract effective components from Chinese herbal medicines.

2. 1 microwave enhanced extraction principle The absorption of microwave energy by a substance depends on the dielectric constant of the substance itself. When the dielectric constant is greater than 28, the net molecular dipole moment in the molecule is large, and the groups that generate dipole moment in the microwave field vibrate at the same frequency as the microwave, generating a lot of heat. These substances are "microwave self-heating substances"; However, substances with dielectric constant less than 28 generate little heat in the microwave field, which is called "microwave transparent substances". Microwave extraction technology requires that the extracted components of traditional Chinese medicine are microwave self-heating substances and the extraction solvent is microwave transparent substances. In this way, the difference of microwave absorption capacity makes some areas of the matrix material or some components in the extraction system be selectively heated, so that the extracted substances can be separated from the matrix or system and enter the extraction solvent with small dielectric constant and relatively poor microwave absorption capacity.

The mechanism of microwave-enhanced extraction comes from the action of microwave energy [3]. On the one hand, the process of microwave radiation is that high-frequency electromagnetic waves penetrate the extraction medium and reach the vascular bundle and glandular cell system inside the material. Because of the high water content in the vascular bundle and glandular cell system of the material, and water is a polar molecule that is particularly sensitive to microwave action, it absorbs microwave energy and heats up rapidly, which increases the pressure on cells. When the internal pressure of a cell exceeds the ability of the cell wall to expand, the cell will rupture. The effective components in cells escape, and the extraction of specific components is completed at a lower temperature. On the other hand, the electromagnetic field generated by microwave accelerates the diffusion speed of some extracted components to the extraction solvent interface. When water is used as the solvent, under the microwave field, water molecules are in an excited state, and do polarity exchange movement at a speed of 2.45 billion times per second, which is a high-energy unstable state, or water molecules vaporize, which enhances the driving force of extraction components; Or the energy released by water molecules themselves returns to the ground state, and the released energy is transferred to other substance molecules, thus accelerating their thermal movement, shortening the time for the molecules of extraction components to diffuse from the inside of the substance to the interface of extraction solvents, greatly improving the extraction rate, reducing the extraction temperature and ensuring the extraction quality to the maximum extent.

2.2 Characteristics of microwave-enhanced extraction Microwave-enhanced extraction has a series of advantages, such as selective extraction, short operation time, less solvent consumption, high yield of effective components, easy production control, environmental friendliness, simple production line composition, and low investment. Especially for the extraction of traditional Chinese medicine, it is easier to screen and determine the components by selective extraction with different polar solvents, and the microwave rapid solvent extraction has high repeatability, which can control the pharmaceutical process more quickly and discover, develop and utilize Chinese herbal medicines. At the same time, microwave extraction saves the sample drying treatment before extraction, so it has less influence on the matrix than other extraction technologies; By improving and adopting strong polarity activation technology, built-in polarized heating and polarized nonpolar reagent technology, nonpolar reagents can also be heated and polarized quickly under microwave field, so that microwave rapid solvent extraction technology can be widely used in various reagents including polar and nonpolar reagents. Microwave extraction is not limited by solvent affinity, and there are many solvents to choose from. At present, all mature solvent extraction methods can be carried out by microwave rapid solvent extraction. Usually polar solvents such as methanol and water are used for polar samples, and nonpolar solvents such as n-hexane are used for nonpolar samples.

Selective extraction-The characteristics of selective heating of different components in the extraction system make microwave extraction the only extraction process that can directly separate the target components from the matrix. Chemical solvent extraction method has the advantages of large energy consumption, many consumables, long time consumption, low extraction efficiency and great industrial pollution; Supercritical fluid extraction greatly improves the extraction efficiency, but its equipment is complex, the range of solvent selection is narrow, high-pressure vessels and high-pressure pumps are needed, and the investment cost is high. Compared with microwave extraction, its comprehensive advantages are very obvious.

2.3 Application of Microwave-enhanced Extraction At present, microwave extraction has been used in many Chinese herbal medicine extraction production lines, such as Pueraria lobata, tea leaves and Ginkgo biloba leaves. Researchers in Chinese medicine research institutions have used microwave extraction technology to treat hundreds of Chinese medicines, including clove oil, artemisinin, ephedrine, mint and garlic oil. In the process of extracting salidroside from Rhodiola sachalinensis, the methods of extracting salidroside by microwave treatment of soaked Rhodiola sachalinensis root and then leaching with water or organic solvent were compared with the methods of extracting salidroside by refluxing with ethanol solution (Soxhlet extraction) and heating cooking with water as solvent. The results showed that the microwave method greatly shortened the time of extracting the same amount of substances, and the quality of the extract was the best. The experimental study on the extraction amount of ephedrine from ephedra by microwave technology also shows that the extraction amount of ephedrine from ephedra by microwave technology is obviously better than that by conventional decocting method.

Because microwave has different effects on different plant cells or tissues, the release of intracellular products has certain selectivity, so different treatment methods should be selected according to the characteristics of products and their position in cells. Literature [4] takes anthraquinone components with different polarities in rhubarb and cassia seed, chlorogenic acid in honeysuckle and baicalin in scutellaria baicalensis as index components, and investigates the extraction rate by orthogonal experimental design. Through the analysis, it is considered that microwave extraction is selective for the extraction of traditional Chinese medicine with different morphological structures, but not significant for the extraction of traditional Chinese medicine with different polar components.

3 Supercritical extraction technology

Supercritical extraction technology is a new extraction technology developed rapidly in recent years, and its application in the research field of traditional Chinese medicine has also increased rapidly since the 1990s. Supercritical extraction is a technology that uses fluid under supercritical conditions as solvent to extract some components from liquid or solid and separate them. Supercritical fluid, also known as supercritical gas, is a substance that exists in the form of fluid above the critical temperature and pressure. The commonly used supercritical gas is carbon dioxide.

3. 1 Supercritical extraction principle (SFE) SFE is a technology that uses the special properties of supercritical fluid to separate chemical substances. Under supercritical conditions, supercritical fluid not only has low viscosity and diffusion coefficient of gas, but also has high density of liquid, so it has good heat and mass transfer and permeability, and has strong solubility in many substances. The physical and chemical properties of SCF are very sensitive to the changes of temperature and pressure near the critical point, that is, the properties of fluid can be continuously adjusted by pressure and temperature. A small amount of solvent, such as a small amount of entrainer, may also greatly change the properties of SCF [6, 7]. Therefore, with its good permeability and strong solubility, supercritical fluid can contact with the substance to be treated and selectively dissolve some components, and the density and dielectric constant of supercritical fluid increase with the increase of pressure in the closed system, and the polarity increases. Components with different polarities can be extracted in stages by programmed pressurization. After the extraction is completed, the temperature or pressure of the system is changed, so that the supercritical fluid becomes common gas and is dispersed, and the extracted components in the material can be completely or basically separated, thus achieving the purpose of extraction separation.

3.2 Characteristics of Supercritical Extraction Compared with traditional pharmaceutical methods, the research and development of Chinese medicine by supercritical extraction technology has the following unique advantages [8]: ① Strong extraction ability and high extraction rate of effective components, which greatly improves the product yield and resource utilization rate. ② There is a certain selectivity to the extract, which is mainly related to the polarity, boiling point and molecular weight of the substance. Selective extraction is conducive to the separation of various substances in traditional Chinese medicine, reducing impurities, enriching the effective components of traditional Chinese medicine, and is conducive to quality control. ③ It can run at normal temperature, especially suitable for the extraction of volatile and heat-sensitive substances, which can ensure the "purity" of the extract and effectively prevent the oxidation and dispersion of heat-sensitive components. ④ The extraction speed is fast and the time is short. Supercritical fluid extraction device integrates extraction and separation, which greatly shortens the technological process and has the advantages of fast extraction speed, high efficiency and simple operation. ⑤ The operation parameters are easy to control, and the effective components and product quality can be guaranteed. High-purity products without solvent residue can be obtained. ⑥ Many substances that can't be extracted by traditional methods can be extracted, and it is easy to find new components from traditional Chinese medicine, thus discovering new pharmacological effects and developing new drugs. ⑦ It can be used as an effective analysis method to analyze the quality of traditional Chinese medicine. ⑧ The process flow is simple, energy consumption is saved, and pollution is suppressed.

3.3 Application of Supercritical Extraction In recent 10 years, SFE-CO2 technology has been widely used in the production of traditional Chinese medicine. Because CO2 fluid is nonpolar, it is especially suitable for extracting volatile components. Volatile oil, alkaloids, phenylpropanoids, flavonoids, organic phenolic acids, glycosides and natural pigments can be extracted from traditional Chinese medicine by adjusting temperature and pressure and adding appropriate modifiers. This technology can not only improve the extraction efficiency, but also retain a large number of thermally unstable and easily oxidized components and components with low extraction content. Its application in the extraction and separation of traditional Chinese medicine is shown in several aspects: the extraction and separation of effective components of a single Chinese herbal medicine, including the extraction and separation of a single component or several components with similar polarity; Extraction and separation of compound Chinese herbal medicine preparation extract: combined with other unit operations to extract and separate active ingredients; Combining spectral and chromatographic analysis methods, the effective components of traditional Chinese medicine can be quantitatively analyzed more accurately and effectively. Supercritical fluid extraction (SFE) has been successfully applied to the extraction of medicinal components from more than 30 kinds of Chinese herbal medicines, such as Ginkgo biloba, honeysuckle, white peony root, Solidago, ginger, angelica, radix aucklandiae, garlic, seabuckthorn, salvia miltiorrhiza and pilose antler grass.

It is particularly noteworthy that supercritical extraction will develop more rapidly in the following fields of pharmaceutical industry. Extracting effective components from pharmaceutical compounds; Effective components or intermediate materials of Chinese herbal medicine, bark and rhizome extracts; Extracting antibiotics from the fermentation broth; Extracting alkaloids from natural medicines.

To sum up, due to its unique advantages, supercritical extraction technology can not only ensure that the original color, aroma and taste are not destroyed by heat, but also ensure heat sensitivity. Oxidizable substances are not destroyed, and the extract can be separated and purified at the same time in the extraction process, so the significance and role of supercritical extraction technology in modern Chinese medicine extraction will become more and more important.