Phase changes: What conditions influence lyophilisation?

Lyophilisation, or freeze-drying, is a technique of dehydration which utilizes low pressure, low temperature environments to induce the sublimation of water content from a material. Sublimation is the process converting a substance from a solid directly to a gas, as shown in Figure 1.

Figure 1. Generalized diagram indicating the phase change of sublimation.

Perhaps the most recognizable form of freeze-drying is freeze-dried ice cream, sometimes also called ‘astronaut ice cream’ due to its development and use in the Apollo space missions; however, lyophilisation also has many applications in the pharmaceutical agricultural industry. Freeze-drying is ideal for preservation because low water content prevents enzymes and microorganisms from spoiling or degrading the substance. Additionally a freeze-dried substance can be rehydrated easily because of the microscopic pores left behind by the ice crystals when they sublimate away. This makes lyophilisation ideal for the long term storage and relatively fast reconstitution of substances, e.g. pharmaceuticals from an inert to an active form.

There is a three stage process to lyophilisation. First the substance must be frozen, usually at very low temperatures between $-50$‍°C and $-80$‍°C. Once frozen, the substance is placed in a vacuum on plates, and a small amount of heat is added to help sublime the water directly from solid ice to water vapor. This is known as the primary drying stage, which removes $95$‍% of a material’s water content. It can last up to several days, and requires rigorously maintained conditions due to the sensitivity of the sublimating structure. Finally, there is a secondary drying stage, which involves the temperature being raised again slightly and pressure being lowered further to sublime as much of the last $5$‍% of moisture as possible.

Researchers have found in their own astronaut ice cream making experiments that varying the temperature conditions can influence the rate of sublimation, as well as the consistency of the dehydrated ‘cake’ product. Table 1 shows how various temperatures affect the overall structure of freeze-dried ice cream during the primary drying stage by increasing temperature.

Table 1. Results of lyophilisation of ice cream under varying temperature conditions.