Title: Alternative Sweeteners

Sorbitol and Mannitol 319
Figure 2 The planar configurations of sorbitol and mannitol.
The major difference between the two isomers is that sorbitol is hygroscopic
and mannitol is nonhygroscopic. Therefore, sorbitol is used as a humectant
because of its affinity for moisture, and mannitol is used as a pharmaceutical and
nutritional tablet excipient because of its inertness and stability against moisture.
Sorbitol solution is hygroscopic, attracting and releasing moisture under
varying humidity conditions, but it does so very slowly. Polyols of lower molecular
weight, such as glycerin, tend to gain and lose water more rapidly. Sorbitol
provides improved moisture control and is more likely to maintain equilibrium
in the surrounding environment. This slower rate of change in moisture content
protects the food products in which sorbitol is used, thus maintaining the asmade
quality of the product and extending the shelf-life (Fig. 3).
There are many different polymorphs of both sorbitol and mannitol. The
most stable polymorph of sorbitol is (γ)-sorbitol. The properties of γ-sorbitol and
the other polymorphs are contained in Table 1 (2). The most stable polymorph
of mannitol is the (β)-polymorph. Working with the most stable polymorph is
important because one needs to be certain the product being used will not change
during processing, (i.e., an unstable polymorph converting to the stable polymorph).
Use of the most stable polymorph ensures that the properties of the
finished product will not change (5). Most of the commercially available sorbitol
and mannitol products are of the most stable polymorphs. One of the characteristics
affected by polymorph is solubility. The solubility in water of γ-sorbitol is
very high (235 g/100 ml at 25°C). The solubility in water of β-mannitol is lower
(22 g/100 ml at 25°C).