It can be envisaged that the continuous operation of decaffeinated tea concentrate or instant tea can be realized by connecting a production-type supercritical CO2 extraction device on the instant tea production line.
During the test, the prepared green tea concentrate was injected into the non-return distributor and then installed in the extraction kettle.
The extraction kettle was closed, and the CO2 steel bottle was opened. After the CO2 was filtered, the compressor was driven into the extraction kettle through the heat exchanger.
Extraction is performed at a set operating pressure and temperature, and then the supercritical CO2 carries the extract flowing out of the top of the kettle, and then enters the separation kettle after decompression.
After releasing the caffeine, it passes through the filter and enters the compressor for recycling. When the set extraction time is reached, the compressor is turned off, CO2 is recovered, and the remaining CO2 is released, so that the pressure in the system is reduced to normal pressure.
Then the extraction kettle is opened, the non-return distributor is collected, and the concentrated concentrate is extracted. test.
During the test, the green tea concentrate is injected into the entrainer container.
After the system works normally, the high-pressure metering pump is turned on and the tea concentrate is poured into the non-return distributor in the extraction kettle to achieve continuous feeding and The material is to open the sampling valve under the extraction kettle without releasing the pressure to release the extracted tea concentrate.
Results and analysis:
From the orthogonal test results, it can be seen that the caffeine removal rate increases significantly with the extension of the extraction time, mainly because the extraction time selected in the experimental design is not long enough, and in a short period of time, the caffeine in the concentrated liquid is difficult to be fully extracted In addition, the effect of extraction time is the largest.
The influence of operating pressure is second, mainly due to the constant temperature. The density of supercritical CO 2 increases with the increase of pressure, and the permeability and transmission capacity are enhanced.
The solubility of caffeine is also enhanced, so it is necessary to fully remove green tea.
The greater the operating pressure of the caffeine in the concentrate, the better the operating pressure, but excessive operating pressure will place high requirements on the entire system.
The effect of operating temperature is again, the temperature change has both positive and negative effects.
On the one hand, the pressure is constant, the density of supercritical CO2 decreases with increasing temperature, and the permeability and transmission capacity are weakened accordingly.
Solubility is also weakened; on the other hand, in coffee concentrates, caffeine is usually combined with tea polyphenols and proteins in the form of hydrogen bonds, and the lower the temperature, the greater the binding capacity.
The stronger it is, the higher the temperature, the higher the entropy of the system, the lower the free energy, the ligands bound by hydrogen bonding tend to depolymerize, and the caffeine is more easily extracted by supercritical CO2.
The interaction of the two aspects makes the relationship between the operating temperature and the caffeine removal rate relatively complicated. However, to fully remove the caffeine in the green tea concentrate, a higher operating temperature should be used.
Considering the longer extraction process, the excessively high operating temperature is The quality of green tea concentrate is not good, and the operating temperature should not be too high.
Concentration of the concentrated solution
The effect of the concentration of the concentrated solution is the smallest, mainly because the concentration value itself has little difference and sufficient water is present, which can ensure the formation of a co-solvent with supercritical CO2, so as to effectively extract the caffeine, but the higher the concentration, the more concentrated the viscosity of the concentrated solution Large, it is not conducive to the penetration and mass transfer of supercritical CO2.
The lower the concentration, the smaller the binding force of caffeine with tea polyphenols and proteins, which is beneficial for the extraction of caffeine by supercritical CO2, but the lower the concentration, the more effective the amount of treatment. The less it is, the less it is economically disadvantageous.
The use of supercritical CO2 to effectively remove caffeine from the green tea concentrate is another important reason. The non-return distributor was used in the experiment.
Supercritical CO2 evenly penetrated the tea concentrate in the form of bubbling. There is tea saponin in tea. Tea saponin is a non-ionic surfactant.
It has a strong foaming ability and good foam stability. During extraction, the green tea concentrated solution will continue to form bubbles under the action of supercritical CO2.
After the supercritical CO2 penetrates the liquid layer of the concentrated liquid, it must pass through layers of bubbles for layered extraction.
When the tea liquid bubbles rise to the top of the extraction kettle and contact the sintering plate, they will rupture. After the bubbles are broken, the tea liquid flows downward. It forms a countercurrent extraction with supercritical CO2, making the extraction process fully effective.
Control the operating pressure of 30MPa and the operating temperature of 56 ° C. Turn on the high-pressure metering pump and pour the concentrated liquid into the extraction kettle, then turn off the pump.
After 4 hours of extraction, turn on the high-pressure metering pump, and open the sampling valve under the extraction kettle to achieve continuous feeding.Continuous discharge.
As a result, it was found that the caffeine removal rate of the solid sample of 15 mL of the extracted green tea concentrate just collected at the beginning was 70. 35%, and the subsequent rate of caffeine removal was decreasing.
Mainly due to continuous feeding, the newly added green tea concentrate is mixed with the concentrate that has been extracted for a long time in the extraction kettle, and some are discharged without sufficient extraction, so that the later collected samples have a lower caffeine removal rate.
In order to overcome this problem, instead use a high-pressure metering pump to drive the concentrated liquid into the extraction kettle, and then turn off the high-pressure metering pump. After extraction for a certain period of time, open the sampling valve at the bottom of the extraction kettle, discharge all the concentrated liquid in the kettle, and then close the sampling. Valve, open the high-pressure metering pump and pour in the new green tea concentrate, each time control the extraction time for 6h, continuous operation, collect the extracted concentrate 2.62 L, and the freeze-dried solid sample has a caffeine removal rate of 84. 26 %.
This continuous operation significantly improves the efficiency, and also reduces the consumption of CO2 gas.
There is no need to use the orthogonal test or supercritical CO2 extraction of tea as before. Instead, it is charged at normal pressure and then extracted under pressure, and then released. Unloading.
It can be envisaged that the continuous operation of decaffeinated tea concentrate or instant tea can be realized by connecting a production-type supercritical CO 2 extraction device on the instant tea production line.