What is freeze-drying?

Freeze-drying is also known as lyophilization. This is a technique used for preservation of perishables 1 such as food, or items which deteriorate if not refrigerated. 2

Freeze-drying can be performed under conditions of decreased temperature as well as low pressure. 3

Freeze-drying is different to dehydration in the sense that dehydration involves exposure of the material to hot air which absorbs the moisture, leading to dryness. In freeze-drying, the moisture is taken out by placing the material on racks which are exposed to vacuum. In these vacuum conditions, the temperature is lowered to and then slowly raised. This causes the moisture to move from the gaseous state (steam) to solid (frozen). The nutritional value of the material is kept intact. 4

Need for freeze-drying 5

1. The process of freeze-drying involves lower temperatures which act as a protective agent

2. The process of freeze-drying can be carried out under completely sterile conditions, thereby preventing microbial contamination

3. Dehydration of the material takes place in a rapid manner

4. The process of freeze-drying for food materials is completely approved by food safety regulations and is safe for consumption for users. Reconstitution of freeze-dried food material (in the form of a porous low-density plug) is extremely simple

Principles or Fundamentals involved

The process of lyophilization or freeze-drying includes the conversion of the material directly from the solid phase to the gaseous phase, without going through the liquid phase (sublimation).

For perishables, freeze-drying is the most appropriate method of preservation –

1. Refrigeration not necessary

2. Material can be stored at room temperature

3. Reconstituted with water within a short period of time

4. Long-term stability (for about two years)

Lyophilization involves three main stages – 6

  • Freezing – Conversion of liquid material to be frozen into solid form. Rate at which the material freezes has an impact on the drying procedures. Hence this is the most crucial step in the process of lyophilization. 6 The temperature of freezing ranges around -50oC and -80 oC. In order to avoid crystallization during the freezing process, the process of freezing should be carried out in a slow and cyclical manner, known as annealing. 7
  • Primary drying – During the drying process, formation of vapour takes place. The vapour so created surrounds the material. This vapour must be at a pressure lower than the pressure in its solid form. Heat energy should be applied at a temperature lower than the sublimation temperature. 8
  • Secondary drying – This involves the elimination of moisture in its minimal amount. The pressure of vapour is in the most reduced levels. 8
The final product formed is completely dry, but retains its form as well as its properties. Reconstitution can be done by water. 8

Applications of Freeze-Drying

The process of freeze-drying can be employed in the following fields – 9

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. 10

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. 11

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. 12

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. 13

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. 14 Pharmaceuticals subjected to the freeze-drying process include vaccines, hormones, proteins, plasma, antibiotics, etc. 15

6. Research – Botanical samples are preserved by freeze-drying to be used for research purposes. 16 Laboratory samples which can be preserved by freeze-drying include active pharmaceuticals, ingredients, pathological samples, microbiological cultures, viruses, bacteria, antibodies, etc. 17

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. 19

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. 20 Flowers which can be preserved using freeze-drying include aster, carnation, daffodil, hyacinth, rose, etc. 21

9. Taxidermy – Freeze-drying is not an alternative to taxidermy but can be considered as an asset to taxidermy. 22 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. 23

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. 24 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. 25


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About Author / Additional Info:
I am a post-graduate in Biochemistry from the University of Mumbai