Supercritical CO2 extraction
Supercritical fluid extraction (SFE) is a separation method developed in recent 20 years, which has the advantages of high solubility in organic matter, fast mass transfer and mild operating conditions. Because of the low critical value of CO2, safety and convenience, it is more advantageous to use supercritical CO2 fluid as extraction solvent. The application of supercritical CO2 extraction in tea deep processing began with the removal of caffeine from finished tea. Witzthum and Hubert treated tea with supercritical carbon dioxide, which reduced the caffeine content in tea from 3% to 0.07%. Caffeine in tea was separated and extracted by supercritical extraction technology, and then extracted and separated by dichloromethane, and the purity of caffeine was 95. 16%. The extraction rate and yield are 65438 06.85% and 0.55% respectively. Supercritical CO2 extraction process has strong selectivity, high efficiency, good product quality and high yield, but the production cost is high, which is difficult to be accepted by manufacturers, and it is still in the laboratory research stage. Tea polyphenols and caffeine can be obtained simultaneously by comprehensive extraction of tea, which reduces the cost.
adsorption method
Commonly used adsorbents are alumina, diatomite, activated carbon or XAD2 macroporous adsorption resin. Because of the unresolved problems such as the difficulty of regeneration caused by tea polyphenols polluting the resin, the separation of caffeine from tea by ion exchange resin has not played a role in actual production so far. In order to make the eluent penetrate into the resin and replace caffeine, some water-soluble impurities are often eluted with water-miscible eluent such as ammonia-ethanol or NaOH- ethanol, and it is also necessary to extract and purify with organic solvent, so the process is complicated and repetitive. Japanese scholars used 20 mL oolong tea juice to pass through an adsorption column filled with 30 g diatomite, and then eluted with 150 mL dichloromethane. 99.8% of caffeine in the column could be eluted. Activated carbon is often used as a trapping agent for caffeine. When caffeine is removed from plants by CO2 stripping, it is often used as an adsorbent to absorb caffeine in CO2.
Sublimation method
Based on the nature that caffeine can be sublimated in large quantities at 235℃ ~ 238℃, various sublimation devices were designed to extract caffeine. Chen Youren et al. designed a new caffeine extraction device, which is a method to directly prepare caffeine from tea by sublimation. The sublimation tank is directly connected with the condensation tank, and the bottom of the sublimation tank is heated by a far infrared heating furnace. The cooling separation tank consists of a separation tank, a collecting net, an annular water collecting tank, water-cooled jacket inlet and outlet pipes, a flue and a stirring transmission shaft. The device can not only complete the heating sublimation of extracting caffeine from tea, but also directly cool the sublimate to separate caffeine crystals, with short extraction time, high yield and good purity. The device improves the extraction rate of caffeine and avoids pollution. Ramaswamy recovered crude caffeine by electrostatic precipitation, and then concentrated, crystallized and purified to obtain pure caffeine. The crystallization box of wool pair was improved. Medical caffeine can be obtained by sublimation, but it is difficult to concentrate the sublimated caffeine in a targeted way, resulting in large losses and low extraction rate, which has been gradually eliminated in production.
Solvent method
Widely used in the production of caffeine, the product yield is higher than that of sublimation method. This method is usually used in combination with the industrial production process of tea polyphenols. At present, the most widely used method in China is organic solvent extraction. There are two main methods: the first method is to extract tea leaves with hot water, then extract them with chloroform and other organic solvents, and concentrate the organic phase to purify crystalline caffeine. Cheng Jinyao and others put the tea into an extraction tower and boiled it in water for 2 hours. The extractive solution is precipitated in a sedimentation tank, filtered and centrifuged, then separated, and then enters a clear liquid pool, evaporated and concentrated to 40% ~ 60%, and chloroform or dichloromethane is added according to the ratio of 3: 1 ~ 6: 1, and then is sent to an extraction tower for extraction. Let the extract stand at 0℃ ~ 4℃ for 30 minutes. Aft taking out that crude caffeine in the tank, keep the temperature at ≤ 65438 000℃ for 60-70 minutes, and finally putting it into a sublimation tank for sublimation to obtain pure caffeine. The second method is to denature tea by adding lime water and alkali, then extract with halogenated hydrocarbon such as chloroform, evaporate solvent, extract with hot water, purify and crystallize to obtain the product. These two methods have the characteristics of simple operation, low cost and high yield. However, due to the use and residue of dichloromethane or chloroform, the quality of products is affected. At the same time, if the tea leaves are soaked in alkaline water such as MgO or CaO, the organic solvent is not easy to enter and needs to be extracted repeatedly, which not only consumes a lot of organic solvent, but also has low efficiency.
Microwave radiation method
Zhang He and Lin Manbin extracted caffeine from tea by microwave radiation, put 65438 00.0 g powdered dry tea sample in a 250ml conical flask, add a certain amount of mixture of 98% ethanol and water, put it in a high-heat microwave oven, take it out after a certain period of radiation, take it out for suction filtration, evaporate the ethanol in the filtrate in a water bath, pour the residue into an evaporating dish, and add quicklime to stir it into a slurry. The method has the advantages of simple operation, energy saving and time saving, high extraction rate and high product purity.
Extraction sublimation synthesis method
Both extraction method and sublimation method need further improvement and perfection. Therefore, Hefei Coal Research Institute absorbed their advantages and organically combined them to develop a new technology. The important process of this process is: tea → pretreatment → sublimation → impurity treatment → sublimation → anhydrous caffeine.
Other preparation methods
Zhang Xiaolin and others chose PA resin and XDA macroporous adsorption resin to produce tea polyphenols and caffeine. This method avoids the use of toxic solvents and has the characteristics of simple operation, low energy consumption, less pollution and high selectivity. Zhou Zhi and others used middle and low grade green tea as raw materials to extract caffeine from tea by microwave water extraction combined with ethyl acetate extraction. The process has the advantages of short flow, high efficiency, non-toxicity and high product purity.