Glass transitions in frozen sucrose solutions are influenced by solute inclusions within ice crystals

Modulated temperature–differential scanning calorimetry (MT–DSC) and microscopy were used to examine the glass transition behavior of frozen 40% sucrose solutions under various freezing conditions, and to correlate this behavior to observed microstructural differences in the samples. Freezing procedures that resulted in deep undercooling followed by rapid nucleation led to two glass-like transitions upon warming. Micrographs of the structure resulting from this freezing process revealed the presence of sucrose inclusions within the ice crystals. It is believed that the lower transition represents the glass transition of the bulk phase, Tg′ after annealing to approach maximal freeze-concentration, while the second, warmer transition is related to the presence of the solute inclusions. However, maturation of these crystals leading to a pure crystalline phase and a homogeneous bulk phase led to a change of the warmer baseline shift during warming from a glass-like transition to a non-reversing transition. Thus, the freezing procedure dictates the structure within the sample, which then dictates thermal behavior.