Freeze drying (lyophilization) is a dehydration technique where a product is dried under vacuum at low temperature. The water that is contained in the sample is thereby frozen to a solid (ice) and then removed by turning the ice into vapour. Conducting this process under vacuum allows the water to be evaporated without having to pass through the liquid phase.
The major advantage of freeze drying is that thermo-labile components, like proteins, flavours or colours, are preserved and the original size and shape of the sample is maintained. This is only possible by keeping the material in a frozen state at low temperatures during the entire drying process.
The absence of water in the final dried product minimizes the effects of oxidation and other degradation processes, thus allowing it to be stored over long periods without the risk of infections by micro-organisms or compositional changes (genetically or enzymatically). Freeze drying has therefore become a widely used method in the pharmaceutical and food industry to process heat sensitive materials that require long-term storage at temperatures above freezing.
Freeze drying process: There are three stages in the complete freeze-drying process:
- Primary drying
- Secondary drying
Freezing : The freezing process consists of freezing the material. In a lab, this is often done by placing the material in a freeze-drying flask and rotating the flask in a bath, called a shell freezer, which is cooled by mechanical refrigeration, dry ice and methanol, or liquid nitrogen. On a larger-scale, freezing is usually done using a freeze-drying machine. In this step, it is important to cool the material below its eutectic point, the lowest temperature at which the solid and liquid phases of the material can coexist. This ensures that sublimation rather than melting will occur in the following steps.
Larger crystals are easier to freeze-dry. To produce larger crystals, the product should be frozen slowly or can be cycled up and down in temperature. This cycling process is called annealing. However, in the case of food, or objects with formerly-living cells, large ice crystals will break the cell walls (discovered by Clarence Birdseye). Usually, the freezing temperatures are between −50°C and −80°C. The freezing phase is the most critical in the whole freeze-drying process, because the product can be spoiled if badly done. Amorphous (glassy) materials do not have a eutectic point, but do have a critical point, below which the product must be maintained to prevent melt-back or collapse during primary and secondary drying. Large objects take a few months to freeze-dry.
Primary drying : During the primary drying phase, the pressure is lowered (to the range of a few millibars), and enough heat is supplied to the material for the water to sublimate. The amount of heat necessary can be calculated using the sublimating molecules’ latent heat of sublimation. In this initial drying phase, about 95% of the water in the material is sublimated. This phase may be slow (can be several days in the industry), because, if too much heat is added, the material’s structure could be altered. In this phase, pressure is controlled through the application of partial vacuum.
The vacuum speeds sublimation, making it useful as a deliberate drying process. Furthermore, a cold condenser chamber and/or condenser plates provide a surface(s) for the water vapor to re-solidify on. This condenser plays no role in keeping the material frozen; rather, it prevents water vapor from reaching the vacuum pump, which could degrade the pump’s performance. Condenser temperatures are typically below −50°C (−60°F). It is important to note that, in this range of pressure, the heat is brought mainly by conduction or radiation; the convection effect can be considered as insignificant.
Secondary drying : The secondary drying phase aims to remove unfrozen water molecules, since the ice was removed in the primary drying phase. This part of the freeze-drying process is governed by the material’s adsorption isotherms. In this phase, the temperature is raised higher than in the primary drying phase, and can even be above 0 °C, to break any physico-chemical interactions that have formed between the water molecules and the frozen material.
Usually, the pressure is also lowered in this stage to encourage desorption (typically in the range of microbars, or fractions of a pascal). However, there are products that benefit from increased pressure as well. After the freeze-drying process is complete, the vacuum is usually broken with an inert gas, such as nitrogen, before the material is sealed. At the end of the operation, the final residual water content in the product is around 1% to 4%, which is extremely low.
Since the freeze drying process works by removing water from the product and rendering the food product shelf stable, these items can be kept safe from spoilage for many years. In addition to the time extension, freeze-dried products have many other benefits such as:
- Untouched Nutrients: Freeze dried foods retain nearly 100% of the health benefits of the original food product. Active enzymes, nutrients, antioxidants, amino acids, and more remain “locked in”. When frozen, vital molecules remain in place and so the overall integrity of the nutritional composition remains intact. The freeze dried food is still considered a “raw food” that remains vibrant and alive in a dormant state just waiting for moisture to reactivate.
- Taste & Appearance: Freeze-fried products retain their original color, shape, size, form, taste, texture, and structure. When placed in water, they can reconstitute to their original state quickly.
- Long Shelf Life: Cold storage is not required for freeze-dried products. They can be stored at room temperature for many years without spoilage. It is important to keep them in a dry place to avoid absorption of environmental moisture.
- Light Weight: Freeze-dried food products weigh less than fresh as over 98% of water is removed. This makes it easier to handle them and less costly to transport.