Freeze-drying: from empiricism to predictability. The significance of glass transitions.

The physical and chemical principles which govern the behaviour of aqueous solutions during freezing and subsequent drying are discussed. They include undercooling, nucleation and growth of ice, the concomitant freeze concentration of the residual solution and its secondary effects, the role of water as plasticizer in amorphous organic materials, the removal of such water, and the over-riding importance of the glass/rubber transition. An understanding of the basic principles can remove most of the empiricism from freeze-drying and lead to more efficient process cycles and to products of superior quality and stability.