FROZEN STATE TRANSITIONS IN RELATION TO FREEZE-DRYING

Freeze-drying is used as a gentle dehydration method for heat sensitiv e materials especially in food and pharmaceutical industries. Most mat erials, including dissolved sugars in water, do not crystallise during freezing prior to freeze-drying. Supersaturated, freeze-concentrated solutions are amorphous materials and they solidify into a glassy stat e when their temperature is depressed to below the glass transition te mperature, T-g. Differential scanning calorimetry has been used to sho w that maximally freeze-concentrated sugar solutions, when properly fr ozen, show during heating a glass transition, T-g', which is followed by ice melting endotherm with onset at T-m'. Low molecular weight mate rials are difficult to freeze-dry as they have low T-g' and T-m' sligh tly above T-g'. High molecular weight materials, such as carbohydrate polymers, exhibit improved dehydration characteristics and they have T -g' anti T-m' at about the same temperature close to the melting point of pure water. The amorphous, glassy structure typical of freeze-drie d materials is formed during prefreezing and retained after removal of ice and the unfrozen water from the freeze-concentrated material. Deh ydration temperatures below T-g' allow removal of ice within the solid , glassy solutes, but temperatures above T-m' result in collapse. The frozen state transitions and properties of freeze-dried materials can be shown in state diagrams which are used to derive proper freeze-dryi ng conditions and storage requirements for various materials.