A new method for the determination of the unfrozen matrix concentration and the maximal freeze-concentration

During the freezing process, water is partially separated as ice and the solutes are concentrated in the unfrozen matrix (UFM). With further lowering of the temperature, the UFM becomes highly viscous. The high viscosity of the UFM prolongs ice formation and makes it difficult to accurately determine the glass transition (T_g′) and the concentration (C_g′) of the maximally freeze-concentrated matrix. In this study, a new method for the determination of the concentration of the UFM was developed using differential scanning calorimetry (DSC). Sugar solutions were frozen, annealed at temperatures slightly above the expected T_g′, rapidly cooled and then heated to 20 °C. The UFM concentrations of the annealed samples were obtained by estimating the solute concentration corresponding to the T_g at the respective annealing temperature. The dependence of the T_g on experimental conditions such as the annealing time, annealing temperature and cooling rate was studied in detail. Values for C_g′ and T_g′ were obtained by linear and quadratic extrapolations of the experimental data over a short temperature and solute concentration range. The maximal freeze-concentrations of glucose, sucrose and maltose were determined to be 79.9, 80.9 and 80.3% (w/w), respectively. Results of this study were in good agreement to previously published data.