What is Freeze Drying ?
There are many definitions of freeze-drying, some of which incompletely define the process. The term ‘lyophilisation’ (‘lyophilization’ in the US) is also used to describe the same process, although, technically speaking, to make a material into a ‘lyophile’ (from Greek ‘lyo’ [solvent] and ‘philos’ [seeking], which is one of the principal attributes of a successfully freeze-dried product) could theoretically be achieved using many different methods.
Freeze-drying is a unit operation that is widely used in the production of a wide range of products, including diagnostics, vaccines, pharmaceuticals, biotech products, foods and beverages.
Operationally, we could define the freeze-drying process as a controllable method of dehydrating labile materials, often of biological origin, by desiccation under vacuum. This may be to achieve better stability compared to the initial formulation, or simply because the freeze-dried form is preferred by the end user, or a combination of these.
As a stepwise process, freeze-drying might typically involve:
- Cooling of the initial formulation (which may be a solution, suspension, emulsion, slurry or a more complex mixture or structure)
- Conversion of freezable solvent (usually water) into a solid
- Crystallisation of crystallisable solutes, with the persistence of an amorphous matrix composed of non-crystallising solutes and non-freezable solvent
- Sublimation of the solid solvent (usually ice) under vacuum
- “Evaporation” of unfrozen solvent from the amorphous matrix
- Desorption of chemi-sorbed solvent resident in the apparently dried product (often referred to as a ‘cake’ or ‘plug’)
- Sealing of the product in its container if appropriate, either under vacuum or ‘backfilled’ with a non-reactive gas such as nitrogen or argon
Freeze-drying may be carried out in a number of formats. The most common formats are vials and ampoules - usually when freeze-drying is the final stage of the production process – or trays (known as ‘bulk drying’) – usually where freeze-drying is used as an intermediate step in the production process (e.g. to concentrate an active ingredient prior to further processing such as tabletting).
Ampoules
Vials
Learn more about Freeze Drying by taking a look at our other articles
8th March 2005