Lyophilisation (revision 6)

Lyophilisation, also called freeze-drying, is a dehydration process in which a solution is frozen and the ice is then removed by sublimation at a pressure below the triple point of water, leaving a porous solid whose volume approximates that of the original fill. It is the standard final step in the isolation of synthetic peptides after preparative purification, and the reason a research peptide is distributed as a dry cake in a sealed vial rather than as a solution.^[1]

The process is used because peptides in aqueous solution degrade by hydrolytic routes — deamidation of asparagine and glutamine, backbone cleavage, aggregation — whose rates depend strongly on molecular mobility and on the availability of water. Removing water and holding the residue in an amorphous glass suppresses those routes by orders of magnitude, converting a product with a shelf life of days at ambient temperature into one with a shelf life measured in years.^[2][3]

Water can pass directly from solid to vapour only at pressures below its triple point, which lies at 611.657 Pa and 0.01 °C. Above that pressure, warming ice produces liquid; below it, warming ice produces vapour. Every lyophilisation cycle therefore operates at a chamber pressure well beneath 611 Pa, typically between 5 and 20 Pa, so that heat supplied to the product drives sublimation rather than melting.^[1]

The driving force for sublimation is the difference between the vapour pressure of ice at the product temperature and the partial pressure of water vapour in the chamber. Because the vapour pressure of ice falls steeply with temperature, small changes in product temperature produce large changes in drying rate.