The relation between material properties and supra-molecular structure of water-soluble food solids

Polar water-soluble molecules can form different supra-molecular structures. In an amorphous matrix the molecules are randomly distributed while a crystalline structure is characterised by a symmetric arrangement of ions or molecules. Linked to the different molecular mobility in these structures, water-soluble solids behave differently while they are exposed to increasing humidity, temperature or stress.The article reviews the state of the art concerning the relation between supra-molecular structure and the physical material properties such as hygroscopicity, hygrosensitivity and mechanical properties.The following conclusions can be drawn:With increasing relative humidity of the surrounding air the water content in crystalline matrix only slightly increases until it dissolve at a substance-specific humidity. Accordingly variations of the relative humidity below the critical water activity do not change the mechanical properties of the particles. However, small moisture quantities liberated in a closed system lead to a partial dissolution of the material at the surface of the crystals. Furthermore, temperature variations below the melting point do not affect to a great extend the mechanical properties of crystalline structures. Crystalline substances dissolve slower due to the limited permeability of the crystalline matrix and the endothermic dissolution process.Amorphous water-soluble substances absorb significant quantities of water while they are exposed to increasing relative humidity. With increasing moisture content or temperature the visco-elastic properties of amorphous matrices vary greatly. Viscosity and elasticity might change by magnitudes due to the absorbed water quantities. Thus, any variations of the relative humidity affect the material behaviour while small quantities of added water do not change the material properties significantly. Dissolution of amorphous structures is comparable fast due to the high permeability of the amorphous matrix and the exothermic dissolution process.