Freezing food preserves food from the time it is prepared to the time it is eaten. Since early times when farmers, fishermen, and trappers have preserved their game in unheated buildings during the winter season.Freezing food slows down decomposition by turning water to ice, making it unavailable for most bacterial growth. Freezing food is one of many kind of way to preserve food. So that it's shelf life would be longer than it should be.
Picture of Frozen Strawberries
Clarence Birdseye, an American inventor, developed the quick-freezing system. He discovered that the combination of ice, wind, and low temperatures in the Arctic froze anything that was exposed to it almost instantly. Birdseye's soon realized that the quick freezing effectively prevented large ice crystals from forming. Other attempts had resulted in the formation of large crystals, which destroyed the delicate cellular structure of the food. With only an electric fan, a few buckets of brine, and cakes of ice, Clarence Birdseye perfected his system of packing fresh food into waxed cardboard boxes and flash-freezing it under high pressure. He sold the patent to the Goldman-Sachs Trading Corporation (a subsidiary of Goldman Sachs & Company) and the Postum Company. In 1929 the first quick-frozen vegetables were sold to the public.
Manufacturers freeze foods by immersing them in very cold liquids, liquid nitrogen being the preferred medium. Nitrogen liquefies at about -320 °F (-195.5 °C), making it useful for quickly freezing foods.When food is submerged in liquid nitrogen, it rapidly freezes. The faster food freezes, the smaller the crystals that form within it.
High-Pressure Shift Freezing is another method used to manufacture frozen food. It uses the principles of water’s phase diagram. At a very high pressure, 900MPa, ice may be formed at room temperature.This is not an efficient way to create frozen foods, but it is being researched for future use.
Dehydrofreezing is a commercial method used to reduce the cost of shipping, handling, and storage of fruits and vegetables. During dehydrofreezing, food is first dehydrated to the desired moisture level and then frozen. Fruits and vegetables have a higher water content than meats, which makes them more susceptible to the formation of large ice crystals. Dehydrofreezing gives the manufacturer peace of mind and keeps produce fresher.
Picture of Frozen Carrots
Frozen foods don’t require many preservatives because the process of preparing the food for freezing kills much of the bacteria living on the food. Carboxymethylcellulose (CMC) is used as a stabilizer in frozen foods because of its tasteless and odorless properties.
Frozen food packaging must maintain its integrity throughout machine filling, sealing, freezing, storage, transportation, thawing, and often cooking.Most frozen foods are cooked in a microwave oven. To make it easier for the consumer, manufacturers have developed packaging that can go straight from freezer to microwave.
Picture of Frozen Food Products
In 1974, the first differential heating container (DHC) was sold to the public. A DHC is a sleeve of metal designed to allow frozen foods to receive the correct amount of heat. Various sized apertures were positioned around the sleeve. The consumer would put the frozen dinner into the sleeve according to what needed the most heat. This ensured proper cooking.
Today there are multiple options for packaging frozen foods. Boxes, cartons, bags, pouches, heat-in-bag pouches, lidded trays and pans, crystallized PET trays, and composite and plastic cans.
Scientists are continually researching new aspects of frozen food packaging. Active packaging offers a host of new technologies that can actively sense and then neutralize the presence of bacteria or other harmful sepcies. Active packaging can extend shelf-life, maintain product safety, and help preserve the food over a longer period of time. Several functions of active packaging are being researched:
- Oxygen scavengers
- Time Temperature Indicators and digital temperature dataloggers
- Carbon Dioxide controllers
- Microwave susceptors
- Moisture control: Water activity, Moisture vapor transmission rate, etc
- Flavor enhancers
- Oder generators
- Oxygen-permeable films
- Oxygen generators
- Validation of cold chain
With these new technologies, food may last longer and our knowledge about its safety will increase.
Effects on Nutrients
Vitamin Content of Frozen Foods
-Vitamin C: Usually lost in a higher concentration than any other vitamin. A study was performed on peas to determine the cause of Vitamin C loss. A vitamin loss of ten percent occurred during the blanching phase with the rest of the loss occurring during the cooling and washing stages. The vitamin loss was not actually accredited to the freezing process. Another experiment was performed involving peas and lima beans. Frozen and canned vegetables were both used in the experiment. The frozen vegetable were stored at -10 °F and the canned vegetables were stored at room temperature (75 °F). After 0, 3, 6, and 12 months of storage, the vegetables were analyzed with and without cooking. O'Hara, the scientist performing the experiment said, "From the view point of the vitamin content of the two vegetables when they were ready for the plate of the consumer, there did not appear to be any marked advantages attributable to method of preservation, frozen storage, processed in a tin, or processed in glass."
-Vitamin B1 (Thiamin): A vitamin loss of 25 percent is normal. Thiamin is easily soluble in water and is destroyed by heat.
-Vitamin B2 (Riboflavin): Not much research has been done to see how much freezing affects Riboflavin levels. One study found an 18 percent vitamin loss in green vegetables while another found a 4 percent loss.It is commonly accepted that the loss of Riboflavin has to do with the preparation for freezing rather than the actual freezing process itself.
-Vitamin A (Carotene): There is little loss of Carotene during preparation for freezing and freezing of most vegetables. However, there is a danger of losing the vitamin during a long-continued storage period.
Freezing is an effective form of food preservation because the pathogens that cause food spoilage are killed or do not grow very rapidly at reduced temperatures. The process is less effective in food preservation than are thermal techniques, such as boiling, because pathogens are more likely to be able to survive cold temperatures rather than hot temperatures.One of the problems surrounding the use of freezing as a method of food preservation is the danger that pathogens deactivated (but not killed) by the process will once again become active when the frozen food thaws.
Picture of Frozen Meat
Foods may be preserved for several months by freezing. Long-term frozen storage requires a constant temperature of -18 °C (0 °F) or less. Some freezers cannot achieve such a low temperature. The time food can be kept in the freezer is reduced considerably if the temperature in a freezer fluctuates; small ice crystals thaw as the temperature moves up, and refreeze onto larger crystals as the temperature declines. Fluctuations can occur by a small gap in the freezer door or adding a large amount of unfrozen food.