Freeze drying involves the removal of water or other solvents from a given product by a process called sublimation. This occurs when the ice of a frozen product converts directly to the gaseous state without passing through the liquid phase. This enables the preparation of a stable product that is easy to use and store at ambient temperatures.

A low pressure environment is pre-requisite to allow this process to take place. In order to start the removal of water, the pressure inside the freeze dryer must be below the “triple point value” for the product, whilst also maintaining the temperature of the sample below its freeze point in the lyophilization process.

Pre-freezing – first stage of the lyophilization process

The sample is frozen, which means the water in the product is converted to ice, thereby the phase has changed from liquid to solid.
Slow pre-freezing will produce lager ice crystals, which are easier to lyophilize, whilst fast pre-freezing results in smaller crystals.

Primary drying – Second stage of the lyophilization process

In the second stage of lyophilization the sublimation process starts. The ice formed in the pre-freeze step is removed from the sample by the direct transition of the “solid” ice to a vapour without passing through a liquid phase. The resultant vapour is collected by the condenser, which has a lower temperature and pressure than the product. The vapour is thus converted back to ice on the condenser surface.
The “energy” required for this process to occur is provided by a gentle heating of the sample, which will start the sublimation process and eventually the sample will dry.

If too much energy (heat) is applied to the sample during this stage the condenser of the lyophilizer may not be able to condense the volume of vapours fast enough, the ice condenser temperature will subsequently rise along with its vapor pressure, thus increasing the risk of the sample melting.

Secondary drying – Third stage of the lyophilization process

Finally, any residual water present, which is strongly bound to the molecules of the sample, is converted to vapour and removed from the sample.

This water has invariably a vapour pressure lower than that of water in its “free” form.Removal of the water in this final stage of lyophilization will be performed at higher product temperatures, consequently, any biological activity of the sample will not be impaired or affected. This usually involves increasing the temperature and lowering the pressure to provide enough energy to break down the molecular bonding. A process called desorption.

The process of freeze-drying can be employed in the following fields

1. Food industry – Food needs to be preserved by freeze-drying for consumption by individuals in the fields of space exploration, hikers, military personnel, as well as the availability of dehydrated foods such as noodles, soups, etc. The most common form of freeze-dried food is instant coffee.

2. Dairy industry – Dairy products which usually required refrigeration were preserved by freeze-drying during World War II. Dairy solids and liquids both can be preserved by this technique, and do not require the use of chemical preservatives. The decreased volume of the product after freeze-drying proves to be an added advantage in the event of transportation of products. Dairy products which can be preserved by freeze-drying include milk, yogurt, ice-cream, cheese, etc. These products can then be supplied to bakeries, dairies, restaurants, etc.

3. Nutraceuticals – In the case of nutraceuticals, the process of freeze-drying used for stabilization as well as increasing the shelf-life of the products. Liquid nutraceuticals are converted to the powder form, which helps in preserving them for a longer time. Nutraceuticals which can be preserved by freeze-drying include seaweeds, aloe vera, tea, etc.

4. Starters and Cultures (edible cultures) – In the case of regular drying methods, the resultant product loses its quality. This might bring about adverse changes in the properties of the product. Products preserved using this technique include probiotics, buttermilk, etc.

5. Pharmaceuticals – Pharmaceutical companies use freeze-drying as a tool to increase the shelf-life of drugs and vaccines. If a liquid drug is converted to its powdered form and stored in a vial, it can be easily reconstituted as necessary. Pharmaceuticals subjected to the freeze-drying process include vaccines, hormones, proteins, plasma, antibiotics, etc.

6. Research – Botanical samples are preserved by freeze-drying to be used for research purposes. Laboratory samples which can be preserved by freeze-drying include active pharmaceuticals, ingredients, pathological samples, microbiological cultures, viruses, bacteria, antibodies, etc.

7. Document Recovery – The process of freeze-drying can be used for recovery and saving of documentation facing damage through fire, floods, etc. ¹⁸ Freeze drying using vacuum can be used to restore books damaged by water as well as paper containing water-soluble inks.

8. Floral – The moisture content present in flowers is eliminated through vacuum extraction, followed by freezing at low temperatures. This prevents floral shrinkage, and maintains the structure and quality of flowers.  Flowers which can be preserved using freeze-drying include aster, carnation, daffodil, hyacinth, rose, etc.

9. Taxidermy – Freeze-drying is not an alternative to taxidermy but can be considered as an asset to taxidermy. Animals possessing a large quantity of lipid content need to be processed accordingly in order to achieve optimal quality of freeze-drying. Animals which can be preserved using freeze-drying in taxidermy include birds, fish, dogs, cats, museum specimens, etc.

10. Pet Food – Removal of moisture is necessary for long-term preservation of pet food. However, this removal of moisture can have an adverse impact on the nutrition and quality of the product. Freeze-drying helps in preservation as well as maintenance of product quality. Pet foods which are subjected to freeze-drying can be regarded to be as close to a naturally-occurring diet. These foods can be shaped accordingly for transportation and convenience purposes. Reconstitution of pet food can be done using water, or can be consumed by pets in the powder form.

1. Taste fresh Freeze-dried foods, like frozen, retain virtually all their fresh-food taste and nutritional content. Freeze-drying removes the water, not the flavor.

2. Look fresh Freeze-dried foods maintain their original shape and texture, unlike dehydrated foods, which shrink and shrivel due to high-temperature processing. Freeze-drying removes water under low temperatures which keeps intact the moisture channels and food fibers. Just add water, and in minutes every fresh food detail returns.

3. Weigh less than fresh Freeze-dried foods have 98% of their water removed. This reduces the food’s weight by about 90%. Because of its light weight per volume of reconstituted food, freeze-dried products are popular and convenient for hikers. More dried food can be carried per the same weight of wet food, and remains in good condition for longer than wet food, which tends to spoil quickly. Hikers reconstitute the food with water available at point of use.

4. Stay fresh Freeze-dried foods can be stored at room temperature, without deterioration or spoilage. This is because freeze-drying and packaging remove both water and oxygen – the two primary causes of food deterioration.