In the extraction process the first step involves extraction of dried leaves with a liquid extractant. The most common extractants are water, primary and secondary alcohols. Sometimes, uncommon solvents like dioxane is also used. Steviol glycosides are soluble in polar solvents and virtually insoluble in non-polar solvents. Often, this property is used for separation of fats and waxes from steviol glycoside extracts. Dried leaves are often pre-treated with organic non-polar solvents like chloroform, ethylene chloride or hexane to remove fats and waxes from the leaves. Then, glycosides are extracted from the defatted leaves.
The actual engineering set up for the extraction process also differs greatly among extraction facilities. Most of the traditional Chinese units use an open trough fitted with an agitator for extraction. In this system, leaves are dumped in a long open trough; warm water is fed to the trough to submerge the leaves. Then the leaves are agitated in the water. After a certain time, the extract is drained from the trough and the extracted leaves are discharged from the trough by opening trap door at the bottom of the troughs.
The absorbed extractant from the leaves are squeezed out either by basket centrifuges or screw press. The extracted and spent leaves then go for solid waste disposal. The separated extractant is pooled with that drained from the trough.
As an alternative process for extraction in a trough, often the leaves are extracted in vertical stirred reaction vessels provided with steam jacket to heat the water or solvent used in extraction. These vessels are closed and offer more process hygiene, but, this setup is costly and more suitable for smaller units.
We have been working for process development of this extraction stage for long time and developed a percolation extraction process. This process is very energy efficient and the extract concentration is also much higher. In this process, a large quantity of leaves can be extracted with little amount of water. Till now, all the primary extraction processes being used in different extraction facilities are batch processes. We are also working out a counter current continuous solid-liquid extraction process for making this step more efficient.
Traditionally, horizontal mixing vessels are used for water extraction of stevia leaves
Alternatively, Vertical jacketed stirred vessels are also sometimes used.