Frozen state transitions in relation to freeze drying

Freeze-drying is used as a gentle dehydration method for heat sensitive materials especially in food and pharmaceutical industries. Most materials, 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 state when their temperature is depressed to below the glass transition temperature,T _g. Differential scanning calorimetry has been used to show that maximally freeze-concentrated sugar solutions, when properly frozen, show during heating a glass transition,T _g , which is followed by ice melting endotherm with onset atT _m . Low molecular weight materials are difficult to freeze-dry as they have lowT _g , andT _m slightly above T _g . High molecular weight materials, such as carbohydrate polymers, exhibit improved dehydration characteristics and they have _g and _m at about the same temperature close to the melting point of pure water. The amorphous, glassy structure typical of freeze-dried materials is formed during prefreezing and retained after removal of ice and the unfrozen water from the freeze-concentrated material. Dehydration temperatures belowT _g allow removal of ice within the solid, glassy solutes, but temperatures aboveT _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-drying conditions and storage requirements for various materials.