The clarified liquid extract is then passed through a column filled with activated carbon granules. Activated carbon absorbs residual plant pigments from the water extract.
Then the clarified water extract is passed through a de-salination column filled with strong cation exchange resins. This step removes all the salt cations and cationic organic impurities.
After this cation removal stage. The extract is passed through an anionic resin bed to remove salt anions and anionic organic impurities. This stage also removes the last traces of plant pigments.
The extract coming out from anionic resin bed is then passed through special macro-porous resin bed. The special resin used in this bed has high affinity specifically towards steviol glycosides and it adsorbes steviol glycosides very efficiently on its surface. The resin takes up the steviol glycosides from the water extract. Water, which is stripped off of stefviol glycosides, comes out as effluent from this stage - which is discarded.
After a certain period of time. The macro-porous resin gets saturated with steviol glycosides. Then the adsorbed steviol glycoside is eluted 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.
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