Abstract
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.
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Roos, Y.H. Frozen state transitions in relation to freeze drying. Journal of Thermal Analysis 48, 535–544 (1997). https://doi.org/10.1007/BF01979500
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DOI: https://doi.org/10.1007/BF01979500