Lyophilization, also known as freeze-drying, is a drying method that removes water from a material without it ever passing into the liquid state. The sample is frozen first, and the ice is then converted straight into vapour under low pressure. The pharmaceutical industry, biological sample banks, microbiology laboratories and the food sector all apply the same principle at different scales.
This article describes the method itself: the physics behind it, the parts of the unit and how a drying cycle is designed. Why venom is turned into a powder, and how the two product forms compare, were covered in the article on venom lyophilization.
The physical basis of the method
There is one single temperature and pressure at which the solid, liquid and gas states of water can exist side by side; this is called the triple point. For water it lies at roughly 0.01 °C and 611 pascals, that is, about 6 millibar. Once the pressure is taken below that value, ice will not melt even if it is heated, but passes straight into water vapour. The entire logic of the method is to hold the sample in this region and draw the water off without it ever entering the liquid phase.
Sublimation calls for energy. The heat required to turn ice into vapour is supplied from the shelves in a controlled way; if the energy falls short the process slows, and if it is excessive the product temperature climbs past the safe limit. Freeze-drying is, in short, the business of holding heat input and vapour removal in balance.
The method costs time and energy. Evaporating the same amount of water with heat takes far less time, whereas freeze-drying calls for cycles measured in hours and at times in days. The method is therefore reserved for heat-sensitive or high-value materials. Proteins, vaccines, bacterial cultures, enzymes and reference standards are typical examples.
Which parts make up the unit?
| Part | Function |
|---|---|
| Drying chamber | The sealed, vacuum-resistant compartment that holds the samples |
| Shelves | Surfaces carrying a heat-transfer fluid, which first cool the product and then heat it under control |
| Condenser | A cold surface that traps the water vapour leaving the product, refreezing it as ice at a very low temperature |
| Vacuum pump | The unit that brings the chamber pressure below the triple point and holds it there |
| Measurement and control unit | Software that logs shelf and product temperature, pressure and time |
The condenser is kept markedly colder than the product. Vapour flows towards this cold surface and freezes on it, which both preserves the vacuum and keeps water away from the pump. The condenser's ice capacity sets the upper limit on how much water can be removed in a single cycle.
The steps of a cycle
1. Filling
The solution is divided in equal volumes between the containers it will be dried in. In vial-type containers the stopper is seated only halfway; the slots in its side let vapour escape. Fill height matters: the thicker the layer, the further the vapour has to travel and the longer drying takes.
2. Freezing
The shelves are cooled and the solution freezes. The liquid often fails to form ice at once even below its freezing point; because this state, known as supercooling, ends at random, containers within one batch may freeze at different moments. As the ice crystals grow, the dissolved substances become concentrated in the narrow channels between them. Where this concentrated phase is crystalline it has a eutectic temperature, and where it is amorphous a glass transition temperature; this is the origin of the critical temperature the product must not exceed during drying.
In some cycles the frozen product is held briefly at a slightly higher temperature. In this step, called annealing, small crystals turn into coarse ones; when coarse crystals sublime they leave wider channels behind, and vapour escapes more easily.
3. Primary drying
The chamber is placed under vacuum and the shelf temperature is raised step by step. The ice sublimes from the upper surface of the container downwards, and as the dry layer above thickens, the vapour finds it harder to get out. This stage is usually the longest part of the cycle.
The end of the stage can be recognised by comparing two different pressure gauges. A gauge based on thermal conductivity reads higher than the true value while water vapour is present, while a capacitance gauge is unaffected by the type of gas. Once the two readings converge, no ice is left subliming in the chamber. Isolating the chamber briefly from the condenser and watching the pressure rise serves the same purpose.
4. Secondary drying
Once the free ice has gone, water bound to the solid structure remains. The shelf temperature is raised once more and this bound water is drawn off slowly. The target is not zero moisture but a low residual moisture level suited to the product; over-drying is reported to strain the structure of some proteins. Residual moisture is mostly measured by Karl Fischer titration.
5. Stoppering
At the end of the cycle, dry nitrogen is usually admitted to the chamber and the shelves are brought together so that the stoppers seat fully while still inside. The containers are thus closed without ever meeting ambient air or the moisture it carries.
Points watched in cycle design
- Critical temperature: The temperature at which the frozen solution collapses is measured beforehand and taken as the design limit of the cycle.
- Safety margin: The product temperature is held below the critical value; leaving a wider margin than needed only lengthens the process for nothing.
- Loading pattern: Containers at the shelf edge dry faster because they also take heat from the chamber walls; this difference is followed through the records.
- Batch record: The shelf temperature, chamber pressure and time curves are added to the record of every batch.
- Scale: A cycle developed on a small unit is not carried over unchanged to a large one; because heat transfer differs, it is revalidated.
How is the dried product judged?
A well-dried product sits in the container as a cake with an even surface that holds its volume. A collapsed or shrunken appearance may point to the critical temperature having been exceeded during primary drying. Alongside appearance, reconstitution time, residual moisture and the analytical profile of the reconstituted sample are assessed together. The work does not end when drying does, since the dry product is affected by moisture and temperature too. Storage is covered in the article on the cold chain, and the changes that emerge over time in the article on venom stability.
The powder form of our products is prepared on this same principle; the specifications of each form appear on the products page, and use is confined to research and industrial purposes.
