Freeze-Dried Food



Background

Freeze drying is a relatively recent method of preserving food. It involves freezing the food, then removing almost all the moisture in a vacuum chamber, and finally sealing the food in an airtight container. Freezedried foods can be easily transported at normal temperatures, stored for a long period of time, and consumed with a minimum of preparation. Once prepared, freeze-dried foods have much the same look and taste as the original, natural products.

The freeze-drying process was developed during World War II as a method of preserving blood plasma for battlefield emergencies without requiring refrigeration or damaging the organic nature of the plasma. The technology was applied to consumer food products after the end of the war. Coffee was one of the first freeze-dried products to be marketed on a large scale. Today, many fruits, vegetables, meats, eggs, and food flavorings are freeze-dried.

Freeze-dried food has many advantages. Because as much as 98% of the water content has been removed, the food is extremely lightweight, which significantly reduces the cost of shipping. This also makes it popular with boaters and hikers who have to carry their food with them. Because it requires no refrigeration, shipping and storage costs are even further reduced. Freeze-dried food is also relatively contamination-free since the dehydration process makes it virtually impossible for yeast and potentially harmful bacteria to survive. Finally, since the physical structure of the food is not altered during the freeze-drying process, the food retains much of its color, shape, texture, and flavor when it is prepared for consumption by reintroducing water. This makes it more attractive to consumers than food preserved by some other methods.

One of the major disadvantages of freeze-dried food is its cost. The equipment required for this process requires a large investment of money, and the process itself is time consuming and labor intensive. These costs are usually passed on to the consumer, which makes freeze-dried food very expensive when compared to other methods of food preservation such as canning or freezing.

Raw Materials

Some foods are extremely well-suited to the freeze-drying process, others do not fare so well. Liquids, thin portions of meat, and small fruits and vegetables can be freezedried easily. Coffee is the most common freeze-dried liquid. Chunks or slices of shrimp, crab, lobster, beef, and chicken can be freeze-dried. They are often mixed with vegetables as part of soups or main course entrees. Almost all fruits and vegetables can be freeze-dried, including beans, corn, peas, tomatoes, berries, lemons, oranges, and pineapples. Even items like olives and water chestnuts can be processed this way.

Thick portions of meat and larger, whole vegetables and fruits cannot be freeze dried with any success. With many other foods, it is simply not economical to preserve them by freeze drying.

The Manufacturing
Process

A freeze-drying processing facility is usually a large plant with modern equipment. Its

Freeze-Dried Food
food-handling areas must be approved by the United States Department of Agriculture, and the company and its employees must adhere to government regulatory procedures. The plant may include a receiving and storage area for raw foods that arrive at the plant in bulk; a food cooking area for those foods that must be cooked before processing; a large area with several large freezing and drying chambers; and a packaging area. The facility may also include a research area where improved methods of freeze-drying foods are developed, and a test kitchen where new preparation techniques to improve the final taste, quality, and texture of the food are tried. Some plants are dedicated to freeze-drying only one product like freeze-dried coffee. Others process a wide range of meats, vegetables, and fruits. Nonfood products such as chemicals and pharmaceuticals are usually processed in separate plants from food products.

The freeze-drying process varies in the details of temperatures, times, pressures, and intermediate steps from one food to another. The following is a generalized description of the process with several specific exceptions noted.

Freeze-Dried Food

Testing and preparation

Freezing

Drying

Sizing and blending

Packaging

Quality Control

Each food has different processing, storage, and rehydration requirements. Some of the variables include the sizing of the raw food products before freezing, the cooking or blanching time and temperature, the rate of freezing and final freezing temperature, the rate of application of vacuum and the final vacuum pressure during drying, the rate and method of application of heat and the final dried product temperature, the allowable residual moisture content after drying, the storage temperature and atmosphere (vacuum, nitrogen, etc) after drying, and the rehydration procedures. At large freeze-drying facilities, electronic microprocessors regulate the times, temperatures, and pressures throughout each step of the process. A central computer collects this data, analyzes it using statistical quality control methods, and stores it for later reference. This assures that the food sent out to the public for consumption has been through a strictly controlled process that meets government guidelines and varies only slightly from batch to batch. The computer also collects data on the bacterial and moisture levels of the raw, bulk food products coming into the plant as well as the final freezedried products. Special equipment may include computerized gas chromatographs and oxygen analyzers. Even the packaging materials are tested for their ability to prevent water vapor and oxygen transmission.

The Future

Food is not the only material that is freeze-dried. Pharmaceutical products such as antibiotics and vaccines are often preserved this way. Specialty chemicals, pigments, and ceramics powders are also produced using freeze-drying. Currently, there is development work on freeze-drying various aerosol sprays. One of the most interesting applications is freeze-drying flowers to produce bouquets that can be stored for many months before being reactivated to make "fresh" flowers. This would be especially beneficial for those who want flowers that bloom only during a short season.

Where To Learn More

Books

Considine, Douglas M., Ed. Food and Food Production Encyclopedia. Van Nostrand Reinhold Company, 1982.

Hui, Y.H., Ed. Encyclopedia of Food Science and Technology, Volume 2. John Wiley and Sons, 1992.

Carol Brennan



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abuzar
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Aug 29, 2006 @ 6:06 am
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