Melting and crystallization of sugars in high-solids systems.

Crystalline structures of sugars, particularly that of sucrose, depend on crystallization conditions and the presence of impurities. Sugar crystals show melting that often occurs at low temperatures with time- and temperature-dependent characteristics. Melting at low temperatures can be accounted for by the presence of impurties and defects. Sugar crystals also contain noncrystalline regions that may undergo decomposition and subsequent dissolution at the decomposition interface and acceleration of decomposition reactions. Such processes with melting establish a supersaturated condition for the remaining crystals, leading to a time-dependent melting point depression and subsequent melting of the remaining crystals. Decomposition of sugars, as well as dissolution and melting of sugar crystals, are separate phenomena, although they are commonly found to coincide. Decomposition of sugars requires the presence and mobility of molecules for reactions outside the crystal lattice; that is, the molecular mobility of amorphous or melted regions is a prerequisite for decomposition, whereas melting of sugar crystals occurs as a separate thermodynamic process with no chemical change of the molecules.

Solubility and diffusion of nitrogen in maltodextrin/protein tablets.

The gas transport properties of compacted tablets consisting of an amorphous mixture of maltodextrin and sodium caseinate were studied by dissolving nitrogen gas in the tablets and then determining the gas release over time as a function of temperature and water activity. Gas was dissolved in the tablet matrix by heating the tablets under pressure, generally to temperatures above the glass transition temperature of the matrix, holding them at these conditions for a specified time and then rapidly cooling them while maintaining the external pressure. The solubility of nitrogen was found to be largely determined by the free volume of the matrix, which in turn can be influenced to some degree by thermal and pressure treatments during gas loading. At the levels of free volume studied, the dissolved nitrogen is densely packed in the free volume, the packing density being virtually independent of the externally applied pressure. Release of gas from the tablets at temperatures below the glass transition temperature is generally well described by Fickian diffusion. The effective diffusion coefficient of gas release is strongly dependent on the microstructure and porosity of the tablet matrix, and an approximate model describing the relationship between tablet structure and rate of gas release is formulated. The model is in semiquantitative agreement with the rates of gas diffusion obtained for tablets and dense granules. Owing to the structural heterogeneity and variability of the tablets and the history-dependent properties of the tablet matrix, the effective diffusion coefficients of gas release from the tablets showed a relatively large spread. The temperature dependence of diffusional release follows an Arrhenius relation below the glass transition temperature. This allows the prediction of the nitrogen retention in the tablets as function of time, temperature and pressure.