In this article we will discuss about:- 1. Introduction to Industrial Freeze Drying 2. The Freeze-Drying Process 3. Applications 4. Advantages 5. Disadvantages.
- Introduction to Industrial Freeze Drying
- The Freeze-Drying Process
- Applications of Industrial Freeze-Drying
- Advantages of Industrial Freeze Drying
- Disadvantages of Freeze Drying
1. Introduction to Industrial Freeze Drying:
Freeze-drying is a special form of drying that removes all moisture and tends to have less effect on a food’s taste than normal dehydration process. In freeze-drying; food is frozen and placed in a strong vacuum. The water in the food sublimates, it turns straight from ice into vapor.
Freeze-drying is most commonly used to make instant coffee, but also works extremely well on fruits such as apples. Freeze-drying (also known as lyophilization or cryodesiccation) is a dehydration process typically used to preserve a perishable material or make the material more convenient for transport.
The alternative name for freeze drying favored by pharmaceutical industry is “Lyophilization” which comes from the Greek “to make solvent loving”. Freeze-drying works by freezing the material and then reducing the surrounding pressure and adding enough heat to allow the frozen water in the material to sublime directly from the solid phase to gas.
There are the following three stages in the complete drying process in industrial processes:
The freezing process consists of freezing the material. In a lab, this is 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 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. But large ice crystals will break the cell walls of the individual cells present in the food. Generally 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. Large objects take a few months to freeze-dry.
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) and 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.
Moreover a cold condenser chamber and/or condenser plates provide a surface for the water vapor to re-solidify on. This condenser plays no role in keeping the material frozen and it prevents water vapor from reaching the vacuum pump, which could degrade the pump’s performance. Condenser temperature is typically below -50°C (-60°F).
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 the primary drying phase and can 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. 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 extremely low.
3. Applications of Industrial Freeze-Drying:
The most important application of industrial freezing drying is explained below:
Pharmaceutical companies often use freeze-drying to increase the shelf life of products, such as vaccines and other injectable. By removing the water from the material and sealing the material in a vial, the material can be easily stored, shipped and later reconstituted to its original form for injection.
2. Food Industry:
Freeze-drying is used to preserve food and make it very light weight. The process has been popularized in the forms of freeze- dried ice cream; an example of astronaut food. The coffee is often dried by vaporization in a hot air flow, or by projection on hot metallic plates. Freeze-dried fruit is used in some breakfast cereal. Culinary herbs are also freeze-dried, although air-dried herbs are far more common and less expensive. However, the freeze-drying process is used more commonly in the pharmaceutical industry.
In chemical synthesis, products are often lyophilized to make them more stable, or easier to dissolve in water for subsequent use. In bio separations, freeze-drying can be used also as a late- stage purification procedure, because it can effectively remove solvents. Also it is capable of concentrating substances with low molecular weights that are too small to be removed by a filtration membrane.
Organizations such as the Document Conservation Laboratory at the United States National Archives and Records Administration (NARA) have done studies on freeze-drying as a recovery method of water-damaged books and documents. In bacteriology freeze- drying is used to conserve special strain. Advanced ceramics processes sometimes use freeze-drying to create a formable powder from a sprayed slurry mist.
Freeze-drying creates softer particles with a more homogeneous chemical composition than traditional hot spray-drying, but it is also more expensive. Freeze drying is also used for floral preservation and wedding bouquet preservation has become very popular with brides who want to preserve their wedding day flowers.
The most important advantages of freeze-drying are listed below:
1. Minimum damage to the heat labile material.
2. Creation of porous friable structures.
3. Speed and completeness of rehydration.
4. The ability to sterile filter liquids just before dispensing.
5. The substance may be stored at room temperature without refrigeration and be protected against spoilage for many years. It would greatly reduce water content inhibits the action of microorganisms and enzymes that would normally spoil or degrade the substance.
6. Freeze-drying does not usually cause shrinkage or toughening of the material being dried.
7. The flavors, smells and nutritional content generally remain unchanged, making the process popular for preserving food.
8. Freeze drying is useful when the product meets one or more of the following criteria-
(i) It is unstable
(ii) It is heat stable
(iii) Quick and complete rehydration is required
(iv) The product of high value
(v) Weight must be minimized, and
(vi) Frozen or chilled storage is not appropriate.
The principle disadvantages are:
1. High capital cost of equipment
2. High energy cost
3. Lengthy process time (typically 4-10 hrs. per drying cycle)
4. Possible damage to products due to change in pH and tonicity.