Freeze drying is a recently industrialized method of preserving food stuffs. It involves the icy of the food, then removing most of the foods moisture within a vacuum, and lastly sealing the food in air-tight storage. Ice dried foods can be conveyable at general temperatures, stored for very long periods of time, and be eaten with minimum preparation. Once prepared to eat, freeze-dried foods look and taste the same as the original, natural products.
Freeze-dried food was industrialized while World War Ii as a method of preserving blood for battlefield injuries, without needing refrigeration or damaging the organic nature of the plasma. The technology was later applied to food products after the end of the war. Coffee was one of the first freeze-dried products to be produced on a large scale. Many fruits, vegetables, meats, eggs, and foods are freeze-dried.
Dried Fruit
Freeze-dried food has many advantages. Because as much as 98% of the water content has been removed, the food is highly lightweight, which reduces the cost of shipping. This also makes it beloved with whatever that has to carry their food with them. Because it requires no refrigeration, shipping and warehouse costs are minimal. Freeze-dried food is also relatively clean, as the dehydration process makes it virtually impossible for yeast and harmful bacteria to survive the process. Finally, since the bodily structure of the food is not altered while the freeze-dried process, the food retains much of its color and flavor when it is prepared by adding water. This makes it far more keen to people, than food preserved by other methods.
One of the major disadvantages of freeze-dried food is its cost. The equipment required for this process is expensive, and the process itself is time keen and labor intensive. These costs are commonly passed on to the consumer, which makes freeze-dried food very high-priced when compared to other methods of food preservation such as canning or freezing.
Raw Materials
Some foods are highly well-suited to the freeze-drying process. Liquids, thin portions of meat, and small fruits and vegetables can be Ice dried easily. Coffee is the most tasteless 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 Ice dried with any success. With many other foods, it is plainly not frugal to withhold them by Ice drying.
The Manufacturing Process
A freeze-drying processing installation is commonly a large plant with modern equipment. Its food-handling areas must be stylish by the United States department of Agriculture, and the business and its employees must cleave to government regulatory procedures. The plant may include a receiving and warehouse 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 any large icy and drying chambers; and a packaging area. The installation may also include a study area where improved methods of freeze-drying foods are developed, and a test kitchen where new establishment 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 commonly processed in cut off 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 record of the process with any specific exceptions noted.
Testing and establishment
The food is first checked for contamination and purity. Fruits, meats, and some other edibles are tested for bacterial counts and spoilage. Much of the work of the plant is dependent on the harvest season for each food. In January, for example, the plant would be processing celery, olives, lemons, oranges, and pineapples. In July, it would process green beans, peas, and strawberries, among others.
Some kinds of food, like seafood and meats, must be cooked before Ice drying. They are commonly purchased already cut into small pieces. If they have not been pre-cooked and frozen, these foods are settled in large, industrial-sized kettles and properly cooked. Fruits and vegetables are commonly purchased already cut, pitted, and peeled. These foods are plainly washed with sprays of water. Some vegetables, like peas and corn, are swiftly scalded, or blanched, before freezing. Coffee is purchased as a pre-brewed concentrated liquid. Because the aroma of coffee is leading to consumers, a small amount of coffee bean oil may be added to the liquid. Unlike the water, the oil is not removed while the drying process.
Freezing
The food pieces are spread out on flat, metal trays which are stacked 20 to 30 high in slots in a wheeled cart. With food that has been pre-cooked and frozen, the trays are pre-chilled to prevent partial thawing while handling. With liquids like coffee, the pre-brewed coffee is poured into shallow pans. The carts are wheeled into a large, walk-in coldroom where the climatic characteristic can be as low as -40F (-40C). In this highly cold temperature, the food is swiftly frozen. There are commonly a dozen or more coldrooms in operation, and the carts are kept there until it is time to move them into the drying chamber.
Drying
The carts are wheeled out of the coldroom and into a vacuum drying chamber. In the case of liquids like coffee, the icy coffee is first ground up into small particles in a low-temperature grinder. The drying chamber is a large, long, horizontal cylinder with semi-elliptical ends. One end is hinged to open and close. When the trays of icy food pieces are inside, the chamber is terminated and sealed. In a large plant, there may be 20 to 30 drying chambers in carrying out at any time.
The drying course involves a process known as sublimation. In sublimation, a solid material is forced to turn state into a gaseous material without ever becoming a liquid. In the case of freezedried food, the solid ice crystals trapped in the icy food pieces are forced to turn into water vapor without ever becoming liquid water. In the drying chamber, this is terminated by evacuating the air with a vacuum pump to sacrifice the pressure to about 0.036 psi (0.0025 bar). The climatic characteristic of the food is raised to about 100 degrees by direct conduction through the lowest of the trays, radiation from heat lamps, or microwave heating. When the chamber is evacuated of air, the pressure is below the threshold at which water can simultaneously exist in a solid, liquid, and gaseous (vapor) state. This threshold is known as the triple point of water. Once the pressure falls below this point, the heat causes the ice crystals trapped in the icy pieces of food to turn directly to water vapor. The vapor is drawn off and condensed within the chamber leaving the food behind. The dried food is filled with tiny voids, like a sponge, where the ice crystals were once present. Not only does this make it easier for the food to reabsorb water when it is prepared for consumption, but the dried food retains its traditional size and shape. The time for this drying process varies. Freeze-dried liquids make take only about four hours to prepare, while semi-solids and solids like soup and sliced meats may take 12 hours or more.
Sizing and blending
The dried food pieces are removed from the drying chamber and tested for moisture content and purity.
Some food pieces may be ground to a smaller size or may be reduced to a powder. Others may be screened to cut off them by size. Two or more dissimilar products may also be blended together to meet a customers specific specifications.
Packaging
Freeze-dried foods must be sealed in airtight packaging to prevent them from keen moisture from the air. any types of packaging may be used: plastic laminated foil pouches, metal and plastic cans, or metal and fiber drums for bulk packaging. Some freeze-dried food is vacuum packed, in which the air is evacuated from the package before sealing. Other food has an inert gas like nitrogen injected into the package before sealing to displace the oxygen in the air and prevent oxidation or spoiling of the food. The packaging is done in the freeze-dry plant Almost as soon as the foods come out of the drying chamber. The plant can form, fill, and seal the packages to the desired weight for the end user. Packages that are to be sold directly to the buyer are packed in cartons, stacked on pallets, and conveyable to the grocery warehouse. Other freeze-dried food is packaged in bulk and sold to a secondary processor for incorporation into other food products. Freeze-dried blueberries, for example, may be sent to a business that makes pancake and muffin mixes.
Quality Control
Each food has dissimilar 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 icy and final icy temperature, the rate of application of vacuum and the final vacuum pressure while drying, the rate and method of application of heat and the final dried product temperature, the permissible residual moisture content after drying, the warehouse climatic characteristic 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 market it for later reference. This assures that the food sent out to the group 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 advent into the plant as well as the final freezedried products. Extra equipment may include computerized gas chromatographs and oxygen analyzers. Even the packaging materials are tested for their quality 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 porcelain powders are also produced using freeze-drying. Currently, there is amelioration work on freeze-drying various aerosol sprays. One of the most keen 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 useful for those who want flowers that bloom only while a short season.
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