Freeze drying is a rather new way of preserving food. The food is first frozen, then a majority of the food’s moisture is removed in a vacuum, and finally the food is finally sealed in its packaging. Freeze dried foods can be stored for very long durations, moved at normal room temperatures, and be consumed with very little preparation. Freeze-dried foods appear and taste the same as the original meals. Freeze drying technology was first developed in the height of World War II as a way of storing blood to be used for injuries in the battlefield, without requiring refrigeration systems. This newly discovered technology was later applied to food products following the War to end all wars. Coffee was one of the many products to be produced using freeze drying methods.
Usually, ninety-eight percent of the water content is removed, which makes the weight of the freeze dried food light. This reduction in weight also significantly lowers the cost of shipping the freeze dried food — adding to many of its known advantages: Long shelf life, low storage costs, great taste, and appearance). Freeze dried foods are popular because they require no additional refrigeration, while shipping and storage costs are very low. In addition, freeze-dried food is unstained, because the dehydration process precludes the survival of yeast and other harmful bacteria during and after the manufacturing process. Lastly, the food keeps most of its texture and flavor as it is prepared by adding water to it. A minor drawback of freeze-dried foods are the costs involved in its production. The equipment utilized in the process is rather costly, and the process is both labor-heavy and time consuming. Consumers usually bear the brunt by paying high prices for freeze dried foods as the costs are passed on to them.
Moreover, freeze dried foods are more expensive when compared to other foods that were preserved using different methods.
Which foods can be freeze-dried?
Not all foods were created equally and made to be freeze-dried; only thin slices — as larger portions are not economical to preserve — of meat (chicken, beefs, and certain sea foods), small fruits, vegetables, and liquids can be freeze dried very easily. These ingredients are generally blended with vegetables as part of soups and/or main course entrees. Moreover, nearly most fruits and vegetables can be freeze-dried.
Preparation
The food is controlled for contamination and purity and tested for bacterial counts and whether it is spoiled or not. The food processing facility relies on the harvest season for each food. In other words, the plant processes foods available for that season only.
Foods like seafood and meats have to be cooked before they are freeze dried. These foods are likely to have been already purchased and cut into very small pieces. They are put in large, industrial-sized containers and cooked. In addition, Fruits and vegetables are generally procured cut, peeled, pitted and washed with plenty of water. Certain vegetables, such as peas and corn, are steamed before freezing.
Freezing the food
The pieces of food are spread out on flat, thin metal, trays. The food, which has been pre-cooked and frozen, is pre-chilled to stop thawing during handling. The carts are transported into a large coldroom where the temperature can be as low as -40 degrees Fahrenheit. In these temperatures, the food is promptly frozen. There can be more than a dozen or more coldrooms in operation at the food processing plant, and the carts remain there until it is time to transport them into the drying area.
Drying the food
The carts are moved out of the coldroom and into a chamber where they are dried in a vacuum. The drying chamber is a big, horizontal cylinder, where one end is made to open and shut. The chamber is closed and sealed when the trays of frozen food pieces are placed in it.
A freeze dryer
The drying method incorporates a process known as sublimation, in which a solid material is made to change state into a gaseous one, bypassing the liquid state. In the process of freeze drying food, the solid ice crystals in the frozen food are made to change directly into water vapor. This is achieved by removing all the air using a vacuum pump to reduce the pressure to approximately 0.036 psi. The temperature of the food is increased to 100 degrees. As the air is removed from the chamber, the pressure falls well below the point at which water can simultaneously exist in a solid, liquid, and gaseous state; the threshold is defined as the triple point of water. The moment pressure falls below this point, the heat allows the ice crystals trapped in the frozen pieces of food to change directly to water vapor. The water vapor is then gradually drawn off and condensed, while the food is left behind. The freeze dried food is replete with tiny empty wholes, just like a sponge, where the ice crystals were once present. This not only makes it easier for the food to re-uptake the water when it is prepared to be eaten, but the dried food maintains its original size, shape and flavor. Freeze-dried liquids can take a few hours to to prepare, whereas semi-solids and solids can take up twelve hours or more to make.
Packaging
The dried foods are removed from the drying chamber. Then they are tested for moisture content and purity. A portion of the food pieces can be ground to a smaller size or transformed to a powder.
Freeze-dried foods have to be sealed in airtight containers to preclude them from absorbing moisture from the air. A few types of food containers may be used:
– Metal/plastic cans
– Plastic foil pouches
Some of the freeze-dried food is vacuum packed, where the air is removed from the container before sealing. At times an inert gas like (like nitrogen) is introduced into the container prior to being sealed to displace the oxygen and prevent oxidation and the subsequent spoilage of the food. The packaging is done in the freeze-dry facility just as the foods come out of the drying chamber.
The food plant can design, load, and seal the packages to the desired weight and specifications for the consumer. Packages of freeze dried foods are sold directly to the consumer in packed cartons.
The control of the end product
All freeze-drying food processing facilities have electronic microprocessors that regulate each step of the process. A central computer garners all the data, analyzes it by employing statistical quality control methods, and keeps it for later reference. This guarantees that the food sent out to the consumers has been through a meticulously controlled process that meets government food safety guidelines. The quality control software also collects data and analyzes the bacterial and moisture levels of the raw materials entering the plant just as it does the freeze dried food products. Last but not least, the packaging materials are tested for their capacity to avert water vapor and oxygen transmission.
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