The key difference between sorbitol and mannitol is that the hydroxyl group at the second carbon atom of sorbitol Fischer projection is coming out of the plane, whereas in mannitol, the hydroxyl group at the second carbon atom of mannitol is going behind the plane in its Fischer projection.
Sorbitol and mannitol are structural isomers. Both these are sweet-tasting sugar molecules and are useful as artificial sweeteners.
Sorbitol is a sugar alcohol having a sweet taste and occurs mainly in potato starch. It is slowly metabolized by the human body. We can obtain sorbitol via glucose reduction. Here, the aldehyde group of glucose is changed into a primary alcohol group. Therefore, sorbitol is an alcohol. We can find sorbitol as a naturally occurring compound; e.g. in apple, pears, peaches, etc. However, we mostly obtain sorbitol from potato starch.
When synthesized, sorbitol appears as a crystalline white powder. The major pathway of production is the glucose reduction reaction in which the aldehyde group is converted into an alcohol group. This reaction requires NADH and occurs in the presence of a catalyst – aldose reductase. Glucose reduction is a pathway of polyol production in glucose metabolism.
Figure 01: Chemical Structure of Sorbitol
There are several applications of sorbitol: as an artificial sweetener, as a laxative, as a bacterial culture media, in treating hyperkalaemia, in the manufacture of soft gel capsules, etc. Sorbitol is also useful in the cosmetic industry as a humectant and as a thickener. Besides, there are miscellaneous uses of sorbitol such as the manufacture of rocket fuel, production of biomass resources, etc.
What is Mannitol?
Mannitol is a sugar alcohol that is useful both as a sweetener and as a medication. Since it is poorly absorbed by the intestine, we can use mannitol in diabetic food. As a medication, we can use mannitol to decrease pressure in eyes and lower increased intracranial pressure. It can be administrated in the form of an injection in medical purposes.