Solvent removal from the frozen state by sublimation; commonly referred to as freeze-drying. In lyophilization, the dried material, a porous solid, is sealed under vacuum or inert-gas (liquid nitrogen) atmosphere and retains its physical and biological characteristics indefinitely. When reconstituted for use, the porous product readily readmits solvent—hence the term lyophile, or solvent-loving. See also: Solvent; Sublimation
Lyophilization is accomplished by freezing the material to be dried below its eutectic point (the lowest possible constant melting point of an alloy or solution) and then providing the latent heat of sublimation. Precise control of heat input permits drying from the frozen state without product melt-back. In practical application, the process is accelerated and more precisely controlled under reduced pressure conditions. See also: Eutectics
Several forms of apparatus are in use, ranging from small laboratory equipment (see illustration) to large chamber-type installations for preservation of serums, vaccines, blood plasma, soluble beverages, and foodstuffs.