Application of Supercritical Fluid Extraction in Tea Processing

Application of Supercritical Fluid Extraction in Tea Processing

The so-called supercritical fluid refers to the fluid whose temperature and pressure are above the critical temperature and pressure. This fluid has the advantages of both liquid and gas. Its density is hundreds of times as close to liquid as gas, its fluidity and viscosity are also close to liquid. Its diffusion coefficient is about 1% of gas and hundreds of times larger than liquid. Therefore, supercritical fluid has strong solubility, higher extraction speed and better mass transfer performance. Especially, the change of temperature and pressure will lead to a great change in the density of supercritical fluid, which can change the solubility of solute considerably. By changing the physical parameters (temperature and pressure), the solubility of fluid can be improved conveniently, so as to achieve the purpose of selective extraction and separation. CO2 is the most commonly used working medium in supercritical fluid extraction. The supercritical CO2 extraction process can be carried out at room temperature, and CO2 is non-toxic and non-residual, so it is especially suitable for the separation and purification of unstable natural products and active substances.

In recent 30 years, the application of supercritical fluid extraction technology in food industry has been developing rapidly. Some developed countries have used this technology in the industrial production of hops extraction and decaffeinated coffee. In addition, removing nicotine from tobacco and extracting high-grade natural flavors and pigments and flavoring substances from plants, Great progress has been made in the research and application of extracting polyene unsaturated fatty acids from fish oil. The research and application of this technology in tea also attracted people’s attention. Such as removing caffeine from tea, extracting aromatic substances of black tea, etc.

1. 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 human metabolism. Some are beneficial, some are not very suitable, excessive consumption of caffeine will affect health, and some people can not bear to eat very little caffeine. In order to meet the needs of the special population, and because the number of people who want a healthy lifestyle in general is increasing, decaffeinated methods have been paid attention to. As early as the 1950s, decaffeinated black tea appeared. At first, organic solvent method was used. This method can change the color, aroma, taste and shape of tea, especially the unavoidable presence of organic solvent residues. With the development of supercritical fluid extraction (SFE) technology, people turn to supercritical CO2 extraction technology to produce decaffeinated black tea.

In the early 1970s, SKW Company of the former Federal Republic of Germany put into industrial operation the removal of caffeine from black tea by supercritical CO2. The technological parameters were temperature 70-90 C, pressure 16-22 MPa and time 7 h. The process not only removes caffeine with supercritical CO2, but also removes aromatic and flavoring substances in black tea, which makes the quality of decaffeinated black tea decreased significantly. Vitzthum and Hubert (1973) introduced a multistage process to solve this problem. In the first stage, dry supercritical CO2 was used to extract aromatic and flavoring substances from tea at 50 C and 25 Mpa and collect them. The second stage is to humidify the tea which removes butadiene aroma and flavor substances, then remove caffeine by wet supercritical CO2 extraction, and then vacuum drying the wet tea which removes caffeine at 50 C. In the third stage, CO2 dissolved in aromatic and flavor substances in the first stage is fed into a tea dissolver containing dry decaffeinated tea, which expands CO2 and fragrances tea. This process is also used to produce instant black tea without caffeine and preserves aromatic substances.

In addition, there are a variety of decaffeinating processes from tea. Schuetz et al. (1985) patented the use of supercritical CO2 to decaffeinate black tea, which is humidified to 15%-50% water content, at 50-70 C and 25-35 MPa, and then let the caffeine-containing carbon dioxide pass through the activated carbon bed, flower caffeine and aromatic substances to be absorbed, and then CO2 is recycled to extract tea. Leaves, while aromatic substances can be separated from activated carbon and added to tea. With this process, 10 kg black tea containing 3.3% caffeine and 5.9% water is first moisturized with 3.5 L of water, then extracted by supercritical CO2 at 61 ~28 Mpa for 2.5 hours. Finally, black tea is dried to 4.5% water at 70 ~C, and the dehydration is achieved. Caffeine tea contains only 0.08% caffeine.

Brunner et al. (1988) patented the use of supercritical fluids such as CO2 or N2O to extract tea at 40-100 C and 15-35 Mpa. Initially, high-quality extractants such as 200-250 kg/kg were used, and then the mass ratio was gradually reduced to 0-20 kg/kg, thus reducing the necessary extraction time by 0-50%, and removing caffeine from tea more completely.

The patents of Klima et al. (1990) introduced the research results of regeneration of CO2 used for decaffeinating by adsorbents packed in extractors. Tea and adsorbents such as activated carbon, which have been adsorbed to 15%~50% water content, were placed alternately in extractors by layers, then extracted with wet supercritical CO2 at 50~80 C and 25~35 Mpa, and passed through one stage before CO2 finally left the extractor. The adsorbent bed can greatly reduce the extraction time and the required CO2 flow rate.

Sato et al. (1990) studied the removal of caffeine from tea by supercritical CO2 with entrainer. The results showed that 95% of the caffeine could be removed by extracting dry green tea with humidified supercritical CO2 as entrainer. Using acetone as entrainer, the removal rate was 50%. Without entrainer, the removal rate was only 25%.

The patent of Sebald et al. (1995) is to extract black tea with supercritical CO2, then wash the dissolved caffeine and other components of CO2 with water before recycling. This washing only removes caffeine, but does not remove other components. These components (aromatic and flavor substances) are continuously absorbed by tea in CO2, so the net loss of aromatic and flavor substances from tea into CO2 is very great. In addition, high quality decaffeinated black tea can be obtained by this process.

The above decaffeinating processes are all for finished tea. The patents of HVG Company of Germany (1985) introduced the process of removing caffeine from fresh tea leaves. The fresh tea leaves were processed into black tea or green tea by conventional methods after removing caffeine with compressed or liquefied gases, such as liquid or supercritical CO2 before extraction or after steaming. The tea leaves were processed with supercritical CO2 at 30 Mpa and 80 C. About 97% of caffeine can be removed from wet fresh tea leaves by extracting for 2 h, 4 h and 4.5 h at 40 and 20 respectively.

2. Extraction of Aromatic Substances from Tea

Tea has unique aroma and flavor, and its composition of aroma and flavor substances is very complex. Most of these components are unstable substances, which are vulnerable to heat deterioration or volatilization. Therefore, the supercritical CO2 extraction method with low operating temperature is particularly superior.

3. Extraction of Pigment from Tea

In recent years, the research of extracting pigment from tea has been carried out a lot, generally using organic solvent method. This method can obtain the crude mixture of pigments, but the solvent consumption is large, the efficiency is low, and the cost is high. In 1996, supercritical CO2 was used to extract green pigments from green tea. The yield of green pigments could be significantly improved by choosing suitable entrainers.

4. Extraction of Tea Polyphenols from Tea

Tea polyphenols are important physiological active ingredients in tea, 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 polyphenols products has become a hot spot in tea deep processing. Tea polyphenols are extracted from tea by organic solvents. Li Jun et al. (1996) tried to extract tea polyphenols from tea by supercritical CO2. The results showed that the solubility of tea polyphenols in supercritical CO2 was very low, its weight fraction was only 10-6 orders of magnitude, and the solubility could only be increased to 10-6 orders of magnitude by adding some entrainers. Supercritical CO2 was used to extract green tea with 3.63% ethanol aqueous solution as entrainer at 80 C and 21 Mpa for 1 h, and the extraction rate was only 0.215%.

In addition to several aspects mentioned above, supercritical fluid extraction (SFE) technology has also been applied to the extraction of essential oil from tea fragrant flowers and the extraction and refining of tea seed oil.

5. Application Prospect of Supercritical Fluid Extraction in Tea Processing

Supercritical fluid extraction (SFE) meets the trend of natural development in food industry because of its own characteristics. It is entering the stage of large-scale production and application at a relatively fast speed. It is believed that SFE has good application prospects in the following aspects of tea processing.

5.1 Production of decaffeinated tea

At present, the vast majority of decaffeinated tea is produced and sold in Europe and North America, and its consumption is increasing rapidly, with a good momentum of growth. China has not yet produced decaffeinated tea, but China has the capacity to manufacture large-scale supercritical extraction equipment and is constantly improving, fully able to industrialize the production of better quality decaffeinated tea, which is very meaningful to meet the needs of the elderly, children and special groups, expand the demand for tea, expand the consumption of tea, and solve the problem of tea backlog. Supercritical fluid extraction of caffeine from tea is also a good method to prepare natural caffeine.

5.2 Processing scented tea

Flower tea is an important tea variety in China. The current processing method is to combine tea and flowers to make tea absorb flower fragrance. The shortcomings of this method are long processing time, low utilization rate of aromatic substances in flowers, especially the processing and production are seriously restricted by the flowering period of flower source. Supercritical CO2 is used to extract aromatic substances from fresh flowers and collect them, which is conducive to storage and transportation. The aromatic substances in pre-collected flowers can be dissolved by supercritical CO2. By decompressing and expanding CO2, aromatic substances can be added to tea. At this time, the aromatic substances of flowers can penetrate into all parts of tea quickly, and their utilization rate will be greatly improved.

5.3 Production of Natural Food Additives

Different kinds of tea have different aroma, flavor and color, which are usually pleasant. Supercritical fluid extraction technology can be used to obtain a series of high-quality flavor food additives with natural flavor and color from different kinds of tea.

5.4 Removal of impurities from tea polyphenol products

At present, tea polyphenol products are obtained by organic solvent method. Generally, it contains more caffeine (about 3%~8%) and a certain amount of residual organic solvents, which will bring harm to the manufacture of tea polyphenols into medicines. Using the significant difference of solubility between tea polyphenols and caffeine in supercritical fluids, the appropriate technological parameters were controlled. Supercritical fluid extraction (SFE) can remove caffeine and residual organic solvents from tea polyphenols to obtain high purity tea polyphenols. In addition, catechin products can be purified by this technology, thereby obtaining pure catechin with high added value.

It will be of great significance to study these aspects more deeply and obtain better technological parameters, especially to carry out necessary pilot test, and to analyze the technical and economic feasibility of industrialized production.

In a word, supercritical fluid extraction technology has shown great potential and will promote the further development of tea processing.

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