A model for freeze concentration and glass transition of carbohydrate solutions as determined by differential scanning calorimetry

Differential scanning calorimetry (DSC) is a common method for the determination of certain thermophysical properties of matter, such as phase transition heat, specific heat, or phase and glass transition temperatures. In particular, the freeze concentration and glass transition of carbohydrate solutions have been extensively studied by DSC. In order to verify the reliability of differential scanning calorimetry in the characterisation of such processes, the correct assignment of the features of the DSC curves to real physical effects is essential. Based on the results of one-dimensional numerical calculations of the material redistribution upon cooling, a new approach is proposed for the interpretation of the double-step feature appearing in the DSC curves. Because of the strong dependence of the molecular diffusitivies on temperature and composition, below a certain temperature the homogeneous distribution of the more and more concentrated solute cannot be established in the unfrozen volume upon fast cooling, and a thin, high-concentration (maximally freeze-concentrated) boundary layer develops at the ice surfaces. This boundary layer acts as a diffusion barrier for the molecules and inhibits water freezing upon cooling and delays ice melting upon heating. The model is illustrated by experimental data obtained by DSC.