How to reduce the error in the extraction of tea polyphenols

At present, the extraction methods of tea polyphenols mainly include solvent extraction, ion precipitation, microwave extraction, ultrasonic extraction, supercritical fluid extraction and resin adsorption separation [2-3]. 2. 1 solvent extraction solvent extraction method is to separate polyphenols and caffeine in tea by using the solubility difference in different solvents, and extract and purify tea polyphenols with a solvent-insoluble tea polyphenol extractant. Its technological route is: tea crushing-extraction-organic solvent decolorization, decaffeination-ethyl acetate extraction-solvent recovery and drying-tea polyphenols [4] The common extractant is water or water and other organic solvents, and the interaction effect between water and organic solvents is better than pure water as extractant. The advantages of solvent extraction method are: (1) easy to realize industrialization, and the whole operation process can be produced intermittently or continuously, which is stable and reliable; (2) Organic solvents can be recycled. The main disadvantages are: (1) the yield is low, and other means are often used to improve the yield; (2) The high-temperature extraction conditions make the product easy to be oxidized during the extraction process, and the product purity is relatively low; (3) The content of caffeine in tea polyphenols is a very important index. Chloroform is used to remove caffeine. The use of toxic organic solvents such as chloroform affects the quality and application scope of tea polyphenols (if it cannot be used in food industry), and a large number of toxic and flammable organic solvents have certain hidden dangers to production safety; (4) Multi-stage extraction operation is generally adopted in the extraction process, and multi-stage distillation is needed for solvent recovery, which is complicated in process operation and high in energy consumption. 2.2 ion precipitation ion precipitation method is to make use of the characteristics that tea polyphenols can form complex precipitation with metal ions Al3+, Bi3+, Ca2+, Ag+ and Hg2+ under neutral or alkaline conditions, so that they can be separated from the leaching solution, and then separated from caffeine, monosaccharides, amino acids and other components in the aqueous solution, thus obtaining high-purity tea polyphenols [5]. The general process route is as follows: tea raw material-ethanol extraction-vacuum filtration-metal ion precipitation-acid dissolution-ethyl acetate extraction-concentration-vacuum drying-refined tea polyphenols [6] The advantages of purifying tea polyphenols with tea polyphenols and precipitants are: (1) high product purity; (2) the use of a large amount of organic extractant is reduced, and certain energy consumption is reduced; (3) In terms of process flow, including the recycling of organic solvents and precipitants, the process is simple and easy to realize, and the cost of auxiliary materials such as precipitants is low. The disadvantages are: (1) The use of metal ions limits the application scope of tea polyphenols, and some toxic metal ions remain in tea polyphenols with high content, so its products cannot be accepted by the pharmaceutical industry; (2) In the production process, the production of a large amount of waste residue undoubtedly causes great pressure on the environment; (3) The process of solubilization is complicated, which leads to the oxidation of some polyphenols and reduces the yield; (4) The conditions of production equipment are harsh, which requires high corrosion resistance of production equipment, auxiliary equipment and pipelines. 2.3 Microwave Extraction The basic principle of this method is that molecules move at high frequency in the microwave field, and the diffusion rate increases, so that tea polyphenols and other extracts can be quickly extracted under the action of microwave. The optimum conditions of microwave extraction are 80℃, solid-liquid ratio 1: 15, time 1.5 minutes [7]. Wang Ying used microwave-assisted extraction technology to extract tea polyphenols from Rizhao green tea, and the extraction rate reached 14.2 1% [8]. Ezzohra Nkhili and others compared the effects of microwave-assisted water extraction (MWE) and traditional heating-assisted water extraction (CWE) on the extraction of tea polyphenols. The results showed that the content of tea polyphenols extracted by MWE was higher than that of CWE, especially the content of EGCG reached 77. 14 mg/g, which was higher than that of CWE. Microwave-assisted extraction can shorten the extraction time, avoid the oxidation of tea polyphenols and effectively protect the effective components in raw materials; The extraction rate is high, the solvent is saved, and the extraction efficiency is greatly improved; Avoid using toxic solvents, and the product is safe. Microwave-assisted extraction technology has the advantages of energy-saving, labor-saving, time-saving, environmental protection, suitable for batch extraction, easy to popularize and so on, and it is a sustainable development technology. 2.4 Ultrasonic-assisted extraction of tea polyphenols Ultrasonic extraction method, the best conditions are 70℃, under 100 w ultrasonic radiation, and 80% ethanol extraction according to the ratio of material to liquid for 20min [10-1]. The content of extracted tea polyphenols is high, which can prevent tea polyphenols from being oxidized and maintain high quality. The technological process is as follows: tea+ethanol-ultrasonic extraction-filtration-evaporation concentration-chloroform extraction-ethyl acetate extraction-evaporation recovery of crude tea polyphenols-purification [12] Ultrasonic extraction of tea polyphenols is simple, especially combined with solvent method, precipitation method and resin adsorption method, and good results have been achieved. The method has the advantages of low extraction temperature, short extraction time, good extraction effect, avoiding the possibility of long-term high-temperature oxidation of tea polyphenols, high recovery rate, small oxidation loss, time saving, energy saving and high extraction rate. At the same time, the use of toxic solvents is avoided, and the extraction process has low cost, obvious comprehensive economic benefits and good industrial promotion value. 2.5 Supercritical fluid extraction method Supercritical fluid extraction method is a new separation method, which uses the fluid between gas and liquid whose temperature and pressure are slightly higher than or close to the critical value as extractant to extract some components with high boiling point and heat sensitivity from solid or liquid, so as to achieve the purpose of separation and purification [13]. The technological process is: tea-tea juice-concentration-supercritical CO2 extraction-organic solvent extraction-ester phase concentration and drying-supercritical CO2 extraction-tea polyphenols [4]. Compared with other extraction and separation technologies, the extraction of tea polyphenols by supercritical CO2 fluid extraction has the following advantages: (1) With CO2 as the extractant, the environmental pollution caused by the process is greatly reduced, and there are almost no toxic metals and organic solvent residues in the product. At present, supercritical fluid is the only substance that can replace toxic organic solvents to extract caffeine. (2) The extraction conditions are mild, which effectively prevents the oxidation of tea polyphenols. Its main disadvantages are: (1) catechin has low solubility in nonpolar supercritical CO2 fluid, and alcohol solution is often needed as an entrainer, so the extraction rate and purity are not high; (2) The conditions of extraction equipment are relatively strict, which often requires special design, large process investment and high production cost. At present, it can only be industrialized on a small scale, and the extract can only be purified after repeated refining. 2.6 resin adsorption separation method The principle of resin adsorption separation method is to realize the separation of tea polyphenols from other extract components according to the characteristics of selective adsorption and desorption of tea polyphenols by adsorption resin. Resin adsorption method requires higher types and structures of resins. The specific surface area, porosity and polarity of porous resin will affect the adsorption capacity of tea polyphenols. Polar polyphenols are easily associated with polar resins through hydrogen bonds, and then the resin adsorbed with tea polyphenols is eluted with appropriate organic solvents, and then the eluent is extracted with ethyl acetate, so that tea polyphenols can be selectively separated from its leachate. The process flow is as follows: tea leaves-hot water extraction-concentration-adsorption by adsorption column-desorption agent elution-vacuum distillation of tea polyphenols eluent to recover solvent-vacuum drying-powder crude tea polyphenols-water dissolution into solution-ethyl acetate extraction-vacuum distillation to recover solvent-vacuum drying-powder crystallization of tea polyphenols [4]. The advantages of separating tea polyphenols by resin adsorption method are: (1) One or more catechins in tea polyphenols products have strong targeting, and the target catechins can be selectively adsorbed and separated; (2) high purity and high caffeine removal rate; (3) the product has a wide application range; (4) The resin adsorption-desorption process is generally carried out at a relatively mild temperature, thus avoiding the loss of tea polyphenols; (5) Avoid using toxic organic solvents, which is easy to recycle. This method is the main research direction of clean production of tea polyphenols at present. The main disadvantages are: (1) The reuse rate of resin regeneration limits the scale of producing tea polyphenols by resin adsorption, so it is often only suitable for small-scale production; (2) The high price of resin brings pressure to the production cost; (3) A lot of organic solvents are used in the whole process, and the desorption process is often complicated. Conclusion and Prospect As a natural antioxidant, the demand of tea polyphenols in food, cosmetics, health care products and other application fields is gradually expanding. At present, some industrialized extraction and purification processes are relatively backward and have certain disadvantages. Toxic organic solvents and some heavy metal ions will limit the application of tea polyphenols in food or medicine. Macroporous resin adsorption and supercritical CO2 fluid extraction are popular methods at present, but the huge cost and operating expenses limit the large-scale production of tea polyphenols. The ideal extraction and purification technology of tea polyphenols should meet the requirements of simple process, convenient operation, good safety, high extraction rate, good product purity and no toxicity or less toxicity, and meet the needs of industrial production. Therefore, new extraction and purification technology of tea polyphenols needs further development and research. Development of (1) new extractant and its application in the extraction of tea polyphenols. Aqueous two-phase extraction is a new and promising separation technology, which, together with ionic liquids and supercritical fluids, is called the three green solvents in the 26th century. Compared with the traditional organic solvent extractant, aqueous two-phase extraction technology has shorter phase separation time, which is helpful to the mass transfer of active substances in the two phases, and there is almost no solvent residue problem. The product is harmless to human body, and the process is easy to be amplified and operated continuously, which can provide a mild environment for bioactive substances and is not easy to oxidize and inactivate tea polyphenols. The types and composition of aqueous two-phase system, as well as the distribution relationship between tea polyphenols and caffeine in aqueous two-phase system, are the focus of future research on extracting tea polyphenols from aqueous two-phase system. (2) Purifying tea polyphenols in a super-large continuous bed. There are many super-large pores in the super-large porous continuous bed, which can form hydrogen bonds with tea polyphenols through chemical modification, so that solid phases such as cell fragments in the raw material solution can pass through smoothly and associate with the target, thus effectively separating the target tea polyphenols. The cost is low, the steps of centrifugation, filtration, concentration and chromatographic separation can be integrated, and the residence time of the target in the bed is short and the selectivity is strong. The preparation and chemical modification of super-large porous continuous bed have always been the key to new biological separation, and the strength, size and regeneration of the bed need further study.