The leaves of the Stevia rebaudiana Bertoni plant are a source of sweetness. The root and stem of the stevia plant are not known to have a sweet taste.
Stevia leaves carry the sweetness of the plant, but a closer look reveals some parts to the leaves tend to taste better than others. This realization provided a challenge to those working to develop stevia-based sweeteners: how to isolate and extract the best tasting parts while removing the less pleasing off-tastes.
It took years to learn how to uncover the best-tasting parts of the stevia leaves. Eventually, product developers learned to isolate different great-tasting steviol glycosides from others, and this work continues.
Turning the Best-tasting Parts of the Stevia Leaf into Something you can Enjoy
The very sweetest and best-tasting steviol glycosides—rebaudioside A, for example—are extracted and purified into what we call stevia leaf extract. This extract is GRAS (“Generally Recognized as Safe”), is safe for human consumption and is the primary source of sweetness in many stevia-based sweeteners.
 Kinghorn, AD. Stevia: The Genus Stevia (NY: Taylor & Francis, 2002) 75