Characterization of the Sucrose/Glycine/ Water System by Differential Scanning Calorimetry and Freeze-Drying Microscopy

The objective of this study was to characterize the thermal properties of systems containing various ratios of amorphous and crystalline components using both differential scanning calorimetry (DSC) and freeze-drying microscopy. The glycine/sucrose system was used as a model system, since it is routinely used in protein formulations. DSC analysis revealed that the addition of glycine to sucrose solutions resulted in a decrease in the glass transition (T'g) of the system. The T'g of a pure sucrose solution (7% w/v) decreased from -32.3 to -51.5°C for a mixture containing a sucrose/glycine ratio of 2:5. The glass transition of the sucrose/glycine mixture decreased linearly as more glycine was added to the system. This decrease in glass transition resulted in severe collapse during freeze-drying of these mixtures above T'g. However, collapse was not observed during freeze-drying if the DSC thermogram of the sucrose/glycine mixture exhibited a transition resulting from recrystallization of the amorphous glycine. Mixtures having a sucrose/glycine ratio of 3:4 and 2:5 had a glass transition of -48°C and -51.5°C, respectively. Despite their low glass transition temperatures, these samples freeze-dried readily at a product temperature Tg using a fast freeze-drying cycle (primary drying at a shelf temperature of +20°C and chamber pressure of 100 mlorr) without any sign of collapse. The crystallization of the amorphous glycine from the frozen mixture of sucrose and glycine provided support during freeze-drying which prevented the macroscopic collapse of the final product. Freeze-drying microscopy visually revealed the crystallization and allowed for prediction of cake quality upon lyophilization. Although the freeze-drying microscope is not as sensitive as the DSC in detecting all transitions (it cannot detect a glass transition), it clarifies the interpretation of DSC, and together they provide valuable information regarding the relevance of each of the transitions to the final freeze-dried product elegance.