Steviol glycoside

The steviol glycosides are responsible for the sweet taste of the leaves of the stevia plant (Stevia rebaudiana bertoni). These compounds range in sweetness from 40 to 300 times sweeter than sucrose. They are heat stable, pH stable, and do not ferment. They also do not induce a glycemic response when ingested, making them attractive as natural sweeteners to diabetics and others on carbohydrate-controlled diets.

Structure
The diterpene known as Steviol is the aglycone of stevia's sweet glycosides, which are constructed by replacing the bottom hydrogen atom (see figure) with glucose, and replacing the top hydrogen atom with combinations of glucose and rhamnose. The two primary compounds, stevioside and rebaudioside A, use only glucose: stevioside has two linked glucose molecules at the top hydrogen site, where rebaudioside A has three, with the middle glucose of the triplet connected to the central steviol structure.

In terms of weight fraction, the four major steviol glycosides found in the stevia plant tissue are:
 * 5–10% stevioside
 * 2–4% rebaudioside A — most sweet (250–300X of sugar) and least bitter
 * 1–2% rebaudioside C
 * ½–1% dulcoside A

Rebaudioside B, D, and E may also be present in minute quantities; however, it is suspected that rebaudioside B is a byproduct of the isolation technique. The two majority compounds stevioside and rebaudioside, primarily responsible for the sweet taste of stevia leaves, were first isolated by two French chemists in 1931.

Toxicity
A 1985 study reporting that steviol may be a mutagen has been criticized on procedural grounds that the data were mishandled in such a way that even distilled water would appear mutagenic. More recent studies appear to establish the safety of steviol and its glycosides. In 2006, the World Health Organization (WHO) performed a thorough evaluation of recent experimental studies of stevia extracts conducted on animals and humans, and concluded that "stevioside and rebaudioside A are not genotoxic in vitro or in vivo and that the genotoxicity of steviol and some of its oxidative derivatives in vitro is not expressed in vivo." The report also found no evidence of carcinogenic activity. The report also suggested the possibility of health benefits, in that "stevioside has shown some evidence of pharmacological effects in patients with hypertension or with type-2 diabetes", but concluded that further study was required to determine proper dosage.