The clarified liquid extract is then passed through a column filled with weekly polar macro-porous synthetic resin granules. The resin absorbs steviol glycosides from the extract and the glycosides are stored within the resin. The spent extract, devoid of steviol glycosides comes out from the resin columns.
Then the adsorbed steviol glycoside is washed out from the resin column with alcohol – generally ethanol. Pure alcohol is fed to the column and alcohol with steviol glycoside dissolved into it comes out. Along with steviol glycoside, the alcohol is also carries plant pigments, and salts as impurities.
The alcoholic extract is then treated with activated carbon to remove most of the plant pigments. Then it is filtered to remove suspended activated carbon particles.
The activated carbon treated alcoholic extract is passed first through columns filled with strong cation exchange resin and then through another column filled with weak anion exchange resins. The resin columns remove salts and plant pigments from the alcoholic extract. After this process, the alcoholic extract becomes a highly purified solution of steviol glycosides.
Then the purified alcoholic extract is concentrated by separation of solvent by nanofiltration. The alcoholic extract is filtered through a membrane under high pressure. The pores of the filter membrane is so small, it can filter stevioside molecules from the alcohol. Thus, most of the alcohol is removed from the extract in this stage to make the extract highly concentrated. The separated alcohol is sent to a distillation tower to remove water from it and make it suitable to be used again in the process. In this process, most of the alcohol is recovered.
The concentrated alcoholic extract is then sent to a rising film evaporation tower. Here, more alcohol is removed from the extract by boiling the extract under partial vacuum. The evaporated alcohol is condensed and recycled. After this stage the extract become highly concentrated and becomes a syrupy liquid. To remove the last traces of impurity, the syrupy liquid is again treated with activated carbon and filtered. Then the purified syrupy liquid is sent to a spray drier. In the spray dryer, the syrupy liquid is sprayed as a fine mist against a stream of hot air. The hot air instantly dries the spray droplets and fine dust of glycosides form. This powdery material is separated from the hot air stream in a cyclone separator. This powder is about 90% pure Total Steviol Glycoside (TSG). This solid glycoside product is then sent to further purification through recrystallization.
Youtube video on the principle of ion exchange