Lyophilization

Abstract

Many labile, heat-sensitive products are stabilized by drying. Lyophilization, also called freeze-drying, avoids most of the problems of liquid-phase drying. A simple definition of lyophilization is that it is a means of drying, achieved by freezing the wet substance and causing the ice to sublime directly to vapor by exposing it to a low partial pressure of water vapor. In this context, ‘low’ means below about 6 mb (4.6 mmHg) — the triple point of water. Below this low pressure, by adding heat energy, ice can be sublimed directly into water vapor without first melting to liquid. Since the subliming ice crystals leave cavities, lyophilized dry material contains a myriad of interstices into which water can penetrate to give rapid and complete re-solution when needed. The name lyophilization is derived from the Greek for ‘to make solvent-loving’. Constituents of the material are kept immobilized during sublimation drying by the ice crystals. The shape of the dry substance is broadly the same as that of the frozen, wet substance and migration to the surface to form a skin is reduced. Since drying takes place at a low temperature, damage is minimized and volatile components are retained.