The supercritical fluid refers to a fluid having a temperature and a pressure equal to or higher than a critical temperature and a critical pressure. This kind of fluid has the advantages of both liquid and gas. Its density is hundreds of times that of gas, close to liquid, and its fluidity and viscosity are also close to liquid. Its diffusion coefficient is about 1% that of gas and hundreds of times larger than that of liquid. Therefore, supercritical fluid has strong solubility, high extraction speed and good mass transfer performance, especially the change of temperature and pressure will lead to great change of supercritical fluid density, thus, the solubility of the solute in it can be considerably changed, and the solubility of the fluid can be easily improved by changing the physical parameters (temperature, pressure), so as to achieve the purpose of selective extraction and separation. CO2 is the most commonly used working medium for supercritical fluid extraction. The supercritical CO2 extraction process can be carried out at room temperature, and CO2 is non-toxic and has no residue, so it is particularly suitable for the separation and purification of unstable natural products and active substances.
In the past 30 years, the application of supercritical fluid extraction technology in the food industry has developed rapidly. For example, some developed countries have used this technology for industrial production such as hops extraction and coffee decaffeination. In addition, nicotine is removed from tobacco, and high-grade natural flavors, pigments and flavor substances are extracted from plants, the research and application of extracting polyunsaturated fatty acids from fish oil have made great progress. The research and application of this technology in tea has also attracted people's attention. Such as removing caffeine from tea, extracting aromatic substances from black tea, etc.
Removal of Caffeine from Tea
Tea is rich in caffeine, accounting for about 2% to 5% of dry matter. Caffeine is an alkaloid, which has a wide range of effects on the body's metabolism. Some are beneficial, some are not very desirable, but consumption Caffeine can affect health, and some people can't stand it even if they eat very little caffeine. In order to suit the needs of special populations, but also because of the increasing number of people who aspire to a healthy lifestyle in general, decaffeination methods have been valued. Decaffeinated black tea appeared as early as the 1950 s. At first, organic solvent method was used. This method will change the color, aroma, taste and shape of tea, especially the inevitable existence of organic solvent residues. With the deepening of the research and application of supercritical fluid extraction technology, people turned to supercritical CO2 extraction technology to produce decaffeinated black tea.
In the early 1970 s, SKW Company of the former Federal Republic of Germany put supercritical CO2 to remove caffeine from black tea into industrial operation. The process parameters were temperature 70~90 ℃, pressure 16 ~ 22MPa and time 7h. The process not only uses supercritical CO2 to remove caffeine, but also removes the aroma and flavor substances in black tea, which makes the quality of decaffeinated black tea produced significantly lower. Vitzthum and Hubert(1973) introduced a multilevel process that solved this problem. In the first stage, dry supercritical CO2 was used to extract the aroma and flavor substances in tea leaves at 50°C and 25 Mpa, and they were collected. In the second stage, the tea leaves from which the aromas and flavor substances were removed were humidified and decaffeinated by wet supercritical CO2 extraction, and the wet decaffeinated tea leaves were vacuum dried at 50°C. The third stage is to put CO2 dissolved in aromatic and flavor substances in the first stage into a solvent containing dry decaffeinated tea leaves to expand the CO2 and flavor the tea leaves. This process is also used to produce instant black tea that does not contain caffeine and preserves aromatic substances.

In addition, there are many decaffeination processes from tea leaves. The patent of Schuetz et al. (1985) is to decaffeinate black tea humidified to 15% ~ 50% water content with supercritical CO2 at 50~70 ℃ and 25 ~ 35MPa. Then CO2 containing caffeine passes through the activated carbon bed, and after caffeine and aromatic substances are adsorbed, CO2 is recycled to extract tea leaves, aromatic substances can be separated from activated carbon and then added to tea leaves. Using this process, 10kg of black tea containing 3.3 caffeine and 5.9 water content is humidified with 3.5L of water, then cyclically extracted with supercritical CO2 at 61 '℃ and 28Mpa for 2.5h. Finally, the black tea leaves are dried to 4,5% water content at 70 ℃, and the obtained decaffeinated tea contains only 0.08 caffeine.
The patent of Brunner et al. (1988) uses supercritical fluid such as C02 or N20 to extract tea at 40-100 ℃ and 15-35Mpa. Initially, a high mass ratio of extractant such as 200-250 kg/kg is used, and then the mass ratio is gradually reduced to 0-20 kg/kg, thus reducing the necessary extraction time by 0-50% and removing caffeine from tea more completely.
The patent of Klima et al. (1990) introduces the research results of regenerating CO2 used for decaffeination through the adsorbent installed in the extractor. Tea leaves and adsorbents such as activated carbon that have been adsorbed to contain 15% ~ 50% of water are alternately placed in the extractor in layers, and then extracted with wet supercritical CO2 under the conditions of 50~80 ℃ and 25 ~ 35Mpa, by passing the CO2 through one stage of the adsorbent bed before it finally leaves the extractor, the extraction time and the required CO2 flow rate can be greatly reduced.
Sato et al. (1990) conducted a study on the removal of caffeine from tea by supercritical CO2 plus entrainer. The results showed that 95% of caffeine could be removed from dry green tea when humidified supercritical CO2 was used as entrainer. Acetone was used as entrainer, and the removal rate was 50%, while the removal rate was only 25% when no entrainer was used.
The patent of Sebald et al. (1995) is to extract black tea with supercritical CO2 first, and then wash CO2 dissolved with caffeine and other components with water before circulation. This washing only removes caffeine and does not remove other components. These components (aromatic and flavor substances) are continuously adsorbed by tea leaves in CO2, thus the net loss of aromatic and flavor substances entering CO2 from tea leaves is very small, this process results in a high quality decaffeinated black tea.
The above decaffeination processes are all aimed at finished tea. The patent of German HVG Company (1985) introduces the process of removing caffeine from fresh tea leaves. Compressed or liquefied gas, such as fresh tea leaves before extraction or after steaming with liquid or supercritical CO2, is used to process fresh tea leaves into black tea or green tea according to regular methods after caffeine is removed, extracting 2h, 4h and 4.5h with supercritical CO2 at 30Mpa, 80 ℃, 40 ℃ and 20 ℃, respectively, can remove about 97% of caffeine from wet fresh tea leaves.
2. Extraction of aromatic substances from tea
Tea has a unique aroma and flavor, the composition of its aroma and flavor substances is very complex, most of these components are unstable substances, easy to heat deterioration or volatilization, so the operation temperature of low supercritical CO2 extraction method is particularly superior.
Vitzthum(1975) was the first to carry out research in this field. Supercritical CO2 was used to extract black tea and separate its volatile components. 56 kinds of black tea aroma components from volatile components enriched by Porapak Q adsorption were successfully identified. The extraction pressure was 10 ~ 30Mpa and the separation pressure was 5 ~ 7Mpa. In addition, since the early 1980 s, the former Federal Republic of Germany used supercritical CO2 to extract aromatic substances from black tea as food additives. The extraction process conditions were temperature 40 ℃, pressure 30Mpa and time 4h.

3. Extract pigment from tea

In recent years, the research of extracting pigment from tea has been carried out more, generally using organic solvent method, which can obtain the crude mixture of pigment, but the solvent consumption is large, the efficiency is low, and the cost is high. The author (1996) extracted green pigment from green tea with supercritical CO2 extraction, and the selection of suitable entrainer can significantly improve the yield of green pigment.
4. Extraction of tea polyphenols from tea leaves
Tea polyphenols are important physiological active ingredients in the human body, which have high medicinal value and strong antioxidant capacity, and have been widely valued by people. In recent years, the development and utilization of tea polyphenol products has become a hot spot in the deep processing of tea. Now the production of tea polyphenols from tea is extracted by organic solvent method. Li Jun et al. (1996) tried to study the extraction of tea polyphenols in tea by supercritical CO2, and the results showed that the solubility of tea polyphenols in supercritical CO2 was very low, and its weight fraction was only 10-6 orders of magnitude. Supercritical CO2 in 80%, 21Mpa under the condition of adding 3.63 ethanol aqueous solution as entrainer, extraction of green tea 1H, the extraction rate is only 0.215 6.
In addition to the above-mentioned aspects, supercritical fluid extraction technology has also been applied to the extraction of essential oil from tea flowers, the extraction and refinement of tea seed oil, and so on.
In short, supercritical fluid technology has shown great potential and will promote the further development of tea processing.