Stevia leaf extract comes from the leaves of the stevia rebaudiana Bertoni plant—more commonly known as “stevia”. Stevia has been a source of high-intensity sweetness for centuries, though the sweetness produced in the early years was often inconsistent and contained stray flavors. The stevia plant grows best in tropical and sup-tropical climates.
Stevia plants tend to be grown on small farms in Asia, South America or other tropical/sub-tropical climates. To extract the plant’s intense sweetness, stevia leaves are harvested and dried. The leaves are then steeped in hot water. After multiple stages of filtering and centrifuging to concentrate the sweetest components of the leaf, the resulting purified stevia leaf extract is ready to be sold commercially. Stevia leaf extract is comprised of steviol glycosides, which are the sweet-tasting parts of the leaves. Food grade stevia leaf extract must have at least 95% steviol glycosides. In the majority of countries where stevia leaf extract is sold, this is the standard.
Starting with the food grade steviol glycosides, a manufacturer may choose to package the stevia leaf extract in any number of different forms for various product uses. Because stevia leaf extract is intensely sweet (200-350 times the sweetness of sugar), much less is needed to produce a sweetness equivalent to sugar. However, very small amounts of stevia leaf extract would be difficult for people to use, especially for everyday sweetening applications.
Manufacturers help to make the intensely sweet stevia leaf extract easy to use by combining it with other ingredients to even out the sweetness. For instance, a product may contain stevia leaf extract along with dextrose, maltodextrin, inulin or erythritol to produce a stevia-based sweetener that can be used very much like sugar for sweetening foods and beverages.
 http://www.fao.org/fileadmin/user_upload/jecfa_additives/docs/monograph10/additive-442-m10.pdf. Accessed 11/9/2016.