PROBLEMS

Basic engineering such as the amount of water to be removed from a specimen and the temperature and time to be used for drying are fundamental problems to be overcome before the specimen can be freeze-dried. Some specimens, such as microbial cells must retain a certain amount of water at all times to be able to survive. Other specimens can survive with all of the water that is economically feasible for the process having been removed. Food specimens are dried at a relatively high temperature but the time used to dry must be carefully chosen to prevent loss of taste. Individual cells, such as bacterial cells must be dried at a lower temperature because of heat sensitivity.

During the freeze-drying process, product changes must be prevented at all costs. If freeze-dried from the liquid instead of the frozen state, one can end up with a shrunken and insoluble product. As the water leaves the liquid specimen, the surface tension increases, and the specimen can collapse under this tension. If this happens, the specimen becomes grossly shrunken and no porous, interstitial openings are left in the specimen for rehydration. As a result, the product is useless.

Mechanical injury, especially to living cells, is a very perisitent problem in freeze-drying. As the cells are frozen, they rupture as a result of the growth of the ice crystals from the water which constitutes much of a living cell. Slow freezing can sometimes prevent rupture of the cell membranes by giving time to diffuse water out of the cell or to alter the shape of the membrane.

Chemical injuries are yet another common injury to living cells. As the water is removed from the cell, very high residual concentrations of other chemicals are left, which can be toxic to the cell. A very effective solution to this problem is to introduce an additive to the specimen prior to freezing, such as glycerol. Glycerol is a natural antifreeze agent that protects the cell by preventing the final concentration of solutes from becoming too high.

During rehydration, certain precautions must be taken, or physical injury of the cell can result. As water is reintroduced into and moves throughout the cell, it carries along with it solutes from other parts of the cell. This results in a concentration gradient of solutes in the cell and changing osmotic pressure could disrupt the cell's metabolic processes that are dependent on the solutes. To prevent this from occuring, an isotonic water solution must be used for rehydration.

APPLICATIONS AND CONCLUSIONS

The biggest market for freeze-drying is the food industry. Freeze-dried food is used by hikers, hunters, astronauts, the military, as well as being used in the food industry for dehydrated soups and meals for consumers in the supermarket. The largest application is freeze-dried coffee. Yet compared to the quantity of food that is annually preserved, freeze-dried food still only represents less than one per cent of the total.

Another important application of freeze-drying is freeze-dried microorgranisms, frequently used for fermentation reactions. The dried microorganisms are reconstituted, cultured, and used in bioconversion reactions. Also, the freeze-dried microorganisms are stored for research. After decades, the strains can be revived.

The pharmaceutical industry has been revolutionized by the freeze- drying process. Life saving medicines that have very short shelf lives are now being freeze-dried, shipped and stored in many places that could never previously receive or store these medicines, thus saving many lives. Another life saving substance in which freeze-drying has made progress is in that of blood. Although freeze-drying blood is still extremely difficult due to the very delicate nature of the blood cell, parts of the blood, as well as blood mixed with glycerol, have been successfully freeze-dried, thus also saving lives.

Although freeze-drying has been a useful process for as much as fifty years now, its commercial applications can be limited by high capital cost for a freeze-drying unit and high operating costs for the vacuum, heater, and condenser. Coffee making has provided strong incentives for improving freeze drying. One inovation has been radiant heating with infra red bulbs or a dielectric heater inside the freeze dryer. Dielectric heat is produced internally in the frozen portion of the specimen, thereby reducing costs and time required to sublime the specimen. Another advance is microwave freeze-drying; microwaves induce the molecular vibrations.

Question #1 True or False. The technical name for freeze-drying is lyophilzation. (t or f)

True

Question #2 - The vacuum on the drying chamber is the force that causes the water molecules to diffuse through the specimen and sublime.

False

No, it is only the concentration gradient of the water which drivesdiffusion.

Question #3 - The purpose of the 'COLD FINGER' is to condense the sublimed water vapors.

True

Question #4 - Anything that has been freeze-dried has an infinite shelf life.

False. No, delicate materials such as blood still have finite shelf lives.

Question #5 - Glycerol is used as a natural antifreeze agent to protect the cell specimens from toxic solute concentrations upon freeze-drying.

True

References

'THE FREEZE-DRYING TECHNIQUE MAKES FOR MOVABLE FEASTS'. J. Hopson. bibl (p. 126). il., Smithsonian magazine, 14:90-2+, July 1983, pages 91-96.

National Academy of Sciences and National Research Council, Freeze-Drying of Foods (book), Ed. Frank R. Fisher, Proceedings of a Conference - Shoreland Hotel, Chicago, Illinois, April 12-14, 1961.

K. N. Shukla, Diffusion Processes During Drying of Solids (book), World Scientific Publishing Co., New Jersey, 1990, pp. 6-7, 128-144.

Professor Henry R. Bungay, Discussion on Freeze-Drying, December 2, 1992.

converted to html 2-June-96