What makes Stevia sweet

The sweet compounds found in stevia leaves are diterpene glycosides (steviol glycosides) and are synthesized, at least in the initial stages, using much the same pathway as gibberellic acid, an important plant hormone. The steviol glycoside and gibberellin pathways diverge at kaurene. In stevia, laurene is converted to steviol, the "backbone" of the sweet glycosides, then glucosylated or rhaminosylated to form the principle sweeteners. The precursor compounds are synthesized in the chloroplast, then from there are transported to the endoplasmic reticulum, Golgi apparatus and then vacuolated.

The purpose of these compounds in the stevia plant is not yet clear, but their high concentration in the leaf and the conservation of the pathway within the species would indicate that, at some point in evoluntionary time, their presence conferred significant advantage upon those individuals that possessed them. Some researchers feel that they act to repel certain insects and others speculate that it is an elaborate means of controlling levels of gibberellic acid.

The four major steviol glycosides are:
stevioside
rebaudioside A
rebaudioside C
dulcoside A

Two other glycosides that may be present in plant tissue are rebaudioside D and E; rebaudioside B has been detected but is probably an artifact formed during isolation. The normal proportions (w/w) of the four major glycosides are: stevioside 5-10%, rebaudioside A 2-4%, rebaudioside C 1-2% and dulcoside A 0.5-1%. They range in sweetness from 40 to 250 times sweeter than sugar. A number of stevia genotypes with anomalous glycoside proportions have been reported in the Korean and Japanese scientific and patent literature. It has long been known that rebaudioside A has the best sensory properties (most sweet, least bitter) of the four major steviol glycosides. Steviol glycosides are heat and pH stable, non-fermentable and do not darken upon cooking and therefore have a wide range of applications in food products.

extract from www.practicalhippie.com