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The basic principle of supercritical extraction
1. Extraction agent
The extractant used for supercritical extraction is supercritical fluid.
Supercritical fluid is a state of matter that is neither gaseous nor liquid between gaseous and liquid, and can exist only when its temperature and pressure exceed the critical point.
Supercritical fluid has a higher density, similar to the liquid, and its viscosity is closer to the gas. Therefore, supercritical fluid is an ideal extractant.
2. The solvent strength of supercritical fluid depends on the extraction temperature and pressure
With this characteristic, the different components in the sample can be extracted successively according to the solubility in the fluid by changing the pressure and temperature of the extractant fluid.
(1) under low pressure, substances with weak polarity were extracted first. With the increase of pressure, substances with large polarity and large molecular weight and their basic properties were extracted. Therefore, supercritical extraction with different extraction components was carried out under the program pressure boost, which also played the role of separation.
(2) the temperature changes in the density of extraction solvent and solute the vapor pressure of the two factors, the low temperature zone (still above the critical temperature), temperature to reduce the fluid density, and solute the vapor pressure increases, therefore, the solution ability of extracting agent temperature can make the solute precipitation from fluid extraction agent, and further raise the temperature to high temperature, although further reduce the density of the extractant, but increased solute steam pressure, increased volatility, instead, not only will not reduce the extraction rate has a tendency to increase.
(3) in addition to pressure and temperature, the solute solubility of supercritical fluid can be changed by adding a small amount of other solvents. The mechanism of action is not yet fully understood. Generally, the amount of addition is no more than 10%, and the polar solvent methanol, isopropanol, etc. are the majority. The application of supercritical extraction technology can be extended to more polar compounds by adding a small amount of polar solvent.
Experimental device and extraction method of supercritical extraction
1. Pictures of experimental equipment for supercritical extraction
Multi-function supercritical fluid extraction, restructuring extraction step by step, there is a toxic ingredients extraction into, ultra trace ingredients extraction recovery, distillation, extractive distillation, reverse 溛 extraction, liquid-liquid extraction, solvent extraction of freezing crystallization, and diversity of the closed loop system and health food puffed, decoloring, desulfurization, trends of peculiar smell, coloring and flavoring and other refined processing industrial test device.
Complete equipment for supercritical CO2 extraction
2. The process of supercritical fluid extraction is shown in the attached figure, which includes:
(1) the source of supercritical fluid is composed of extractant bottle, high-pressure pump and other ancillary devices, whose function is to transform the extractant from normal temperature pressure state to supercritical fluid.
(2) the supercritical fluid extraction part is composed of sample extraction tube and accessory device, where the extractant in the supercritical state dissolves the extracted solute from the sample matrix. As the fluid flows, the fluid containing the extracted solute is separated from the sample matrix.
(3) the solute vacuum adsorption separation part, absorbed by the nozzle and tubes, extracted solutes and fluid, must by supercritical state by the nozzle decompression cooling transformation under normal condition, the fluid volatile escape, and solute transport in porous filler surface absorption tube, wash with a suitable solvent absorption tube, can put the solute elution collect spare.
High pressure pump -- extraction tube -- absorption tube -- collector -- supercritical fluid cylinder -- solvent eluting pump
2. Methods of supercritical extraction
Supercritical fluid extraction can be divided into:
A. Dynamic method: simple, convenient and fast, especially suitable for extraction of substances with high solubility in supercritical fluid extraction agent, and the sample matrix is easy to be infiltrated by supercritical fluid.
B. Static method: it is suitable for substances which are difficult to be separated from the sample matrix or are highly soluble in the extractant fluid; it is also suitable for situations where the sample matrix is relatively dense and supercritical fluid is not easy to penetrate, but extraction speed is slow.
Selection of supercritical fluid and extraction conditions
1. Selection of supercritical fluid
The basic principle is:
CO2 critical temperature (Tc) and critical pressure (Pc) were 31.05 ℃ and 31.05 MPa, when at the critical point above, at this time of CO2 has a dual characteristic of gas and liquid at the same time. It is similar to the gas, and the viscosity is similar to the gas. Similar to the liquid, the density is similar to the liquid, but its diffusion coefficient is much larger than the liquid. It is an excellent solvent that dissolves many substances through intermolecular interaction and diffusion. At the same time, in the region slightly higher than the critical point, the pressure changes slightly, which causes a large change in its density, thus causing a large change in solubility. Therefore, supercritical CO2 can be dissolved from the matrix to form the supercritical CO2 load phase, and then reduce the pressure of the carrier gas or increase the temperature. The solubility of supercritical CO2 is reduced, and these substances are precipitated (analyzed) and separated from CO2 to achieve the purpose of extraction and separation.
Due to different solubility in CO2 or different solubility of the same substance under different pressure and temperature, this extraction and separation process is highly selective.
CO2 is the most used supercritical fluid at present. It is not only a strong solvent, but can extract a wide range of compounds in food processing. Moreover, it is relatively stable in nature, cheap in price, non-toxic, non-combustion and reusable. Therefore, it is particularly suitable for extraction of volatile and heat-sensitive substances. Compared with traditional solvents of n-hexane and dichloromethane, it has obvious advantages.
In terms of solvent strength, supercritical ammonia is the best choice, but ammonia easily reacts with other substances, corrodes the equipment severely, and is too dangerous for daily use. Supercritical methanol is also a good solvent, but because it has a high critical temperature and is a liquid at room temperature, complex enrichment steps are required after extraction, and low hydrocarbons are less widely used than CO2 because they are flammable and explosive.
2. Selection of extraction conditions
Extraction conditions can be selected in several cases:
(1) the extraction conditions were changed by selecting different pressures for the same fluid to extract different types of compounds;
(2) select appropriate extraction conditions according to the solubility of the extract in supercritical fluid under different conditions;
(3) the analytical material is deposited on the adsorbent and eluted by supercritical fluid for the purpose of classification and selection of extraction;
(4) is a component with relatively high polarity, methanol can be directly added to the sample and extracted with supercritical CO2, or another pump can be used to pump methanol and supercritical CO2 in a certain proportion to achieve the purpose of increasing the strength of the extractant.
In addition to the pressure, composition and extraction temperature of the extractant fluid, the extraction process time and the temperature of the absorber will affect the extraction and collection efficiency. Extraction time depends on two factors:
(1) is the solubility of the extracted substance in the fluid. The higher the solubility, the higher the extraction efficiency and the faster the extraction speed.
(2) the higher the mass transfer rate of the extracted material in the matrix, the more complete the extraction and the higher the efficiency. The temperature of the collector or absorber will also affect the recovery rate, and lowering the temperature will improve the recovery rate.
Following supercritical fluid decompression, there are two main methods for collecting extracts:
(1) offline SFE: the operation is simple, only the extraction steps need to be understood, and the sample extract can be analyzed by other appropriate methods.
(2) online SFE or online SFE: not only need to know about SFE, but also understand the chromatographic conditions, and does not apply to other methods to analyze the sample extract, its advantage mainly eliminates the extraction and chromatography analysis between the sample processing, and is due to extract transfer directly to chromatographic column and could reach the maximum sensitivity.
Examples of application of supercritical fluid extraction in food industry
The application of supercritical fluid extraction in food is a matter of nearly 20 years. In food processing, CO2 is used as extractant.
1. Extraction of vegetable oil (seeds of soybean, sunflower, cocoa, coffee, palm, etc.)
2. Extraction of animal oil (fish oil, liver oil, etc.)
3. Decaffeination from tea and coffee, extraction of hops (to eliminate pesticide pollution)
Tea is rich in caffeine, accounting for about 2 to 5 percent of dry matter. Caffeine is an alkaloid that has a wide range of effects on human metabolism.
Decaffeinated black tea emerged as early as the 1950s. At first, organic solvent method was used, which could change the color, aroma, taste and shape of tea, especially the inevitable organic solvent residue. With the development of supercritical fluid extraction technology, people turn to supercritical CO2 extraction technology to produce decaffeinated black tea.
Extraction in the development of tea decaffeination
4. Non-fat (non-fat starch, Fried food, etc.)
5. Extraction of spices
6. Extraction of plant pigments and decolorization and deodorization of various substances of supercritical CO2 are similar to the polarity of n-hexane, so it is particularly suitable for extraction of fat-soluble components. For example, Chinese paprika, capsaicin, tobacco tree orange, lutein and so on. In addition, the polarity of CO2 can be changed by the use of different entrainers to expand the extraction range.
Supercritical CO2 was used to extract carotene from algae. Acetone as entrainer can improve the extraction rate.
Table 1 shows the solubility and extraction rate of carotene in CO2 under different acetone contents.