The methods of decaffeinated tea at home and abroad mainly include hot water extraction, organic solvent extraction, membrane separation, precipitation and column chromatography. Chloroform or dichloromethane extraction is widely used, but both of them are very toxic, and improper operation may be dangerous.
If an organic solvent cannot be used, the ideal solvent is water. The solubility of caffeine in tea is greater than that of other substances in hot water, but it is necessary to accurately determine how much caffeine is extracted.
Sublimation method: Caffeine begins to sublime above 120℃, and then sublimes to 180℃. After cooling, it becomes needle-like crystals. By using this attribute, it is extracted. At present, most universities in China use sublimation method for teaching, and the temperature is controlled at 182 ~ 185℃, and it will be 5 after cooling.
In addition, there is a simple hot water extraction method. According to the characteristic that caffeine is easily soluble in hot water above 80℃, some researchers at home and abroad started with the primary processing technology of tea and explored the feasibility of removing caffeine by water extraction in the primary processing of tea. Tsuchida Tengjiro et al. found that whether fresh tea leaves, steamed green leaves or withered leaves were directly soaked in hot water at 85℃, caffeine dissolved rapidly after about 1min. The dissolution rate reached 7 1%, and the dissolution rate of caffeine reached 83% after soaking for 3 minutes. However, in hot water at 60℃ for 5 minutes, the dissolution rate of caffeine in fresh tea and steamed green tea is very small, indicating that the dissolution rate of caffeine is restricted by the temperature of hot water. Wu's research shows that the extraction rate of caffeine can be increased from 60.566% to 90% when the extraction temperature is increased from 80℃ to 90℃.
Solvent extraction method: extracting tea with polar solvent, and then carrying out liquid-liquid extraction and separation to finally obtain the product. At present, this method is mainly used in industrial production. The product yield is 5% 10%, and the purity is about 80% ~ 98%. The organic solvents used are acetone, ether, methanol, hexane and chloroform. This method uses a variety of organic solvents.
Ion precipitation method: Ion precipitation method is to separate caffeine from metals, such as copper salt, lead salt or aluminum trichloride. This method uses heavy metals toxic to human body as precipitant, which affects the safety of products.
Column separation preparation method: The column separation preparation method includes gel column, adsorption column and ion exchange column. In recent years, people have begun to pay attention to improving purity, so there are many studies on chromatographic column separation. The key of this technology is the study of column packing and elution. It shows that the purity of column separation preparation method can reach 98. 1%, and the residue is only 0. 1% if it is separated by gel column.
Membrane extraction technology: Adding about 0.0 1% pectinase into tea juice, and keeping the temperature at 50℃ for 4 hours to decompose pectinase, the seepage velocity during ultrafiltration can be improved. Then it was separated by ultrafiltration membrane with molecular weight of 40,000 ~ 50,000. About 12% of tea polyphenols were retained, but caffeine was not retained by the membrane. The permeate is concentrated by reverse osmosis.
To sum up, the first two kinds of equipment have small demand and no toxic effect, but the output is low, which is suitable for teaching or experiment; The latter three chemicals have obvious toxic effects on human body and are suitable for industrial production; Membrane extraction technology should be the future development direction, but the production cost of ultrafiltration membrane needs to be further reduced.