Sorbitol, mannitol, xylitol and maltitol are marketed as white crystalline powders, with a range of particle sizes and crystalline forms. Sorbitol and maltitol are also available as aqueous solutions.
Most of the commercial syrups contain significant proportions of additional
polyol species which influence the solubility and have the effect of
Viscosity of solutions
The viscosity of a syrup depends on the molecular weight of the solute.
Monosaccharides (e.g. dextrose) and related polyols (e.g. sorbitol) give
low-viscosity solutions. Disaccharides (e.g. maltose) and related polyols
(e.g. maltitol) give slightly higher viscosities. The presence of
higher molecular weight polysaccharides leads to a greatly increased
viscosity. Those commercial syrups with the highest content of higher
molecular weight species have the highest viscosities.
There is no relationship between the hygroscopicity of a polyol in solution and that of the same product in the solid form. Sorbitol is an excellent humectant in solution owing to its high solubility. However, crystalline sorbitol is not very hygroscopic, particularly in the stable γ-form. Mannitol powder has very low hygroscopicity, taking up moisture only in excess of 98% relative humidity (RH). At 20°C pure crystalline maltitol (Maltisorb®) is less hygroscopic than sucrose.
The role of humectants is to limit the exchange of water to and from water-containing products. Depending on the application, in food or in cosmetics, sorbitol (Neosorb®) and liquid polyols used at concentrations of 3 to 60%, stabilize the moisture content, improve storage and give good consistency and plasticity.
Sorbitol is directly compressible because of the dentritic structure of its crystals. Special grades of other polyols for direct compression, notably mannitol, have also been developed (Serpelloni, 1985).
Polyols contain primary and secondary hydroxyl groups which are less reactive than the aldehyde and ketone groups of sugars. The polyols are all very stable to heat and melt without decomposition. Polyols do not react with amino acids. With certain grades some Maillard reaction browning, resulting from residual reducing sugars, may occur during the manufacturing process. Hydrolysis of maltitol and of the other higher molecular weight components of maltitol syrups is very slow under the mild conditions normally encountered in their use.
Values of melting points are sorbitol (γ-form) 99°C, maltitol 147°C, mannitol 165°C, and xylitol 94°C.
Boiling point, freezing point, and osmotic pressure
These colligative properties, as with their patent sugars, are dependent on the molecular weight and concentration of the solution.
The refractive index of polyol solutions increases linearly with concentration and is a method commonly used to specify the concentration of a given syrup.