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.
The diterpene known as Steviol is the aglycone of stevia's sweet glycosides, which are constructed by replacing steviol's carboxyl hydrogen atom (at the bottom of the figure) with glucose to form an ester, and replacing the hydroxyl hydrogen (at the top of the figure in the infobox) 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 hydroxyl site, whereas 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 (250–300X of sugar)
2–4% rebaudioside A — most sweet (350–450X 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.
The source of information regarding sugar free stevia for diabetic is from:
http://en.wikipedia.org/wiki/Steviol_glycoside
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