Steviol glycosides is a chemical compound responsible for the sweetness of the leaves of South America plant Stevia rebaudiana (Asteraceae) and the main ingredients (or precursors) of many sweeteners marketed under the generic name stevia and some trade names. They also occur in related species Stevia phlebophylla (but no other species Stevia ) and at the Rubus chingii plant (Rosaceae).
Steviol glycosides from Stevia rebaudiana have been reported between 30 and 320 times sweeter than sucrose, although there is some disagreement in the technical literature on these figures. They are hot-stable, pH stable, and not fermenting. In addition, they do not induce a glycemic response when ingested, because humans can not metabolize stevia. This makes them attractive as a natural sugar substitute for diabetics and others on a carbohydrate-controlled diet. Steviol glycosides stimulate insulin secretion through the potentiation of cells, preventing high blood glucose after eating. Adjustable daily intake (ADI) for steviol glycosides, expressed as steviol equivalents, has been established for 4 mg/kg body weight/day, and is based on no observed effect of a 100-fold higher dose in rat studies.
Video Steviol glycoside
Structure
These compounds are steviol glycosides, the dihydride compounds. In particular, their molecules can be seen as steviol molecules, with carboxyl hydrogen atoms replaced by glucose molecules to form esters, and hydrogen hydroxyl with a combination of glucose and rhamnose to form acetals.
Steviol glycosides are found in S. rebaudiana leaves, and the percentage by weight, including:
The last three are present only in minute quantities, and rebaudioside B has been claimed as a by-product of isolation techniques. The commercial steviol glycoside mixture extracted from the plant was found to have about 80% stevioside, 8% rebaudioside A, and 0.6% rebaudioside C.
The Chinese factory Rubus chingii produces rubusoside, a steviol glycoside not found at Stevia .
Stevioside and rebaudioside A were first isolated in 1931 by French chemists Bridel and Lavielle. Both compounds have only a subgroup of glucose: stevioside has two glucose molecules connected at the hydroxyl site, whereas rebaudioside A has three, with triplet glucose connected to the central steviol structure.
Initial sensory tests led to claims that rebaudioside A was 150 to 320 times sweeter than sucrose, 10-10 to 270 times sweeter, 10 cauliflower rebaudioside C 40 to 60 times sweeter, and dulcoside A 30 times sweeter. However, a newer evaluation found rebaudoside A to be about 240 times sweeter, and stevioside about 140 times. Rebaudioside A also has a bit of bitterness and aftertaste. Relative delicacy appears to vary with concentration: a mixture of steviol glycosides in natural proportions is found 150 times sweeter than sucrose when matching a 3% sucrose solution, but only 100 times sweeter when matching 10% sucrose solution.
Maps Steviol glycoside
Biosynthesis
In Stevia rebaudiana , glucoside biosynthesis occurs only in green tissue. Steviol was first produced in plastids and in the endoplasmic reticulum is glucosylation and glycosylation in the cytoplasm, catalyzed by UDP-glucosyltransferase. Rebaudioside A, in particular, is formed from stevioside.
Although there are several molecules that fall under the category of steviol glycoside, synthesis follows the same route. The synthesis of steviol glycoside begins with an isoprene unit made through the DXP or MEP pathway. Two molecules derived from primary metabolism, Pyruvate and Glyceraldehyde 3-Phosphate, are the starting molecules for this pathway.
After forming IPP and DMAPP, GGPP diterpene is formed through the addition of head-to-tail by the Sn1 mechanism. Elongation begins when IPP and DMAPP form Geranyl Pyrophosphate (GPP). GPP extends through the same Sn1 mechanism to create Farnesyl Pyrophosphate (FPP), and FPP extends to form GGPP.
With the formation of GGPP cyclization occurs by the enzyme copalyl diphosphate synthase (CDPS) and Kuarene Synthase (KS) to form - (-) Kuarene. Several steps of oxidation then occur form steviol.
Steviol's biosynthetic glycosides then follow some modifications of the steviol selected regioselectively for the sugar molecules to be placed.
Once these molecules are fully glycosylated, the glycosides are then stored in a vacuole.
See also
- Glycosides
- Stevia
- Substitute sugar
References
Source of the article : Wikipedia
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