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For food to remain fresh if eaten at a later time, one common preservation method is freezing. Agricultural products like fish and grain, have been preserved during the winter months, in heat-free buildings, over the ages. Keeping food frozen stalls decomposition, by solidifying residual moisture into ice. This inhibits the survival and growth of microbes. The food commodity industry employs two processes, to keep food frozen:
- cryogenic (flash freezing)
Freezing is core to keep food quality and texture intact. Freezing the item, quickly produces smaller ice crystals, and cells retain their structure. Cryogenic freezing is the quickest freezing technique available. It is dependent on the ultra-low temperature of liquid nitrogen employed – -196 °C (-320 °F). Freezers are used in domestic kitchens to preserve food today. People were advised to freeze food on the day of purchase, but there are indications that food should be stored as soon as possible, and right up to the product’s expiry (best before) date. The Food Standards Agency backs this with the caveat that the food has been appropriately stored before that time.
Food preservatives, refrigeration, and freezing
Refrigeration and freezing are very popular forms of food preservation. Refrigeration impedes bacterial action, so that food takes longer to spoil. Freezing however, causes any bacterial action to cease, because frozen bacteria are completely inactive. Since microorganisms cannot survive when food temperatures dip below -9.5 °C (15 °F), frozen products do not need additional preservatives. Freezing is adequate by itself, to prevent food spoilage. Rather, a tasteless stabiliser that also has no odour, carboxymethylcellulose (CMC), is typically added to frozen food, since it has zero impact on the quality of the product. To preserve food for the long haul, significantly lower temperatures may be required. Refrigeration and freezing are used on all food types (fruits, vegetables, meat, and so forth), except a few. Food texture and taste is preserved during refrigeration, whereas freezing has no effect on taste or texture of meat, completely alters fruits, while having negligible effect on vegetables. The minimal effect of refrigeration, is the reason for its widespread adoption.
A brief historical tour of freezing food
In cold climes, tribes through the ages have employed natural food freezing, using winter frosts. Darling Harbour in Sydney, Australia was established by Thomas Sutcliffe Mort in 1861. It was the first freezing works in the world. It would later become the New South Wales Fresh Food and Ice Company. Mr Mort funded Eugene Dominic Nicolle’s experiments. Dominic Nicolle was a French-born engineer, who got to Sydney in 1853, and successfully registered his first ice-making patent in 1861. Frozen meat was transported to London from Sydney, in 1868. It was a trial shipment. Mort and Nicole’s machinery was never employed in the frozen meat trade, but they developed systems for domestic trade that were importantly commercially-viable, though the financial return on that venture did not make too much sense, for a businessman of Mort’s stature. Chicken and geese, albeit in small numbers, comprised shipments to London from Russia, by 1885. They were packed in insulated cases, using this technique. By March 1899, the British Refrigeration and Allied Interests reported that Baerselman Bros, a food importing concern, was shipping around 200,000 frozen geese and chickens, each week to New Star Wharf, London, from three Russian depots. This happened over a short three to four winter months. This frozen food commercial activity was made possible due to Linde freezing plants, introduced in the Russian depots and London warehouse. The warehouse held the frozen items, until shipment to Manchester, Liverpool, Birmingham, and London markets. The techniques were later expanded into the meat-packing industry. Americans embraced flash freezing in 1929, when Clarence Birdseye introduced it. Birdseye took fur-trapping expeditions, to Labrador in 1912 and 1916, and developed interest in food freezing, where he saw the natives use natural freezing, to keep food integrity intact. The Icelandic Fisheries Commission was established in 1934, to initiate innovation in the industry, encouraging fishermen to quick-freeze their haul. Íshúsfélag Ísfirðinga, a pioneer frozen fish company, was formed in Ísafjörður, Iceland as the result of a 1937 merger. Eleanor Roosevelt’s food on her trip to Russia, was frozen using more advanced techniques, while the United States military experimented with orange juice, ice cream, and vegetables, towards the end of World War II.
Frozen food technology
Globally, the frozen food market is rapidly evolving. The same can be said of the freezing technique, which is racing towards faster, more efficient, and more affordable. Mechanical freezers were the first to be employed in the food industry. Many freezing/refrigerating line operations still engage them. They distribute a refrigerant (often ammonia), around the system, pulling heat awayfrom the food product. This heat is then transferred to a condenser, and dissipated into air or water. The refrigerant in the mechanical freezer is at this point, a high-pressure, hot liquid, directed into an evaporator. As it makes its way through an expansion valve, it is cooled and vaporised. This paves the way for low-pressure gas at low temperature, to be reintroduced into the system. More recent is the flash freezing (cryogenic freezing) of food. Leading food manufacturers worldwide, use this technique. It involves very low temperature gases (often solid ice – carbon dioxide – or liquid nitrogen), applied directly to the food product. Meat is an obvious candidate for flash freezing.
Packaging for frozen food
For food to remain in pristine conditions as it is filled, sealed, frozen, stored, transported, thawed, and cooked, care must be taken to preserve it. Microwave ovens can now handle most packaging that frozen foods come in. Differential heating containers (DHC), became available in 1974. It is a metal sleeve, designed to give frozen foods the ability to receive the appropriate amount of heat. Apertures of various sizes, were positioned around the sleeve, providing the user with a place to put frozen dinner, based on what needed the most heat. This guaranteed proper cooking. These modern times, there are plenty of options, for packaging frozen foods. These include but are not limited to cartons, boxes, pouches, bags, lidded trays and pans, composite and plastic pans, boil-in-bags, and crystallised PET trays. Frozen food packaging is an interesting area of research, for many scientists. Common disciplines of interest are:
- Microwave susceptors
- Flavour enhancers
- Oxygen (O2) scavengers
- Oxygen-permeable films
- Odour generators
- Oxygen generators
With active packaging, there is an entire set of new technologies that actively sense the presence of, and neutralise harmful microbes. Active packaging has many benefits, including the following:
- extended shelf life
- undiminished product safety
- long-term preservation of food
Effectiveness of freezing
Freezing is undoubtedly effective in preserving food, as food pathogens are either decimated or cannot thrive at such low temperatures. Thermal techniques like boiling, are more effective to preserve food, as the pathogens survive better at cold, than at hot temperatures. One issue with freezing is that pathogens may be deactivated and not killed, meaning they will become active upon thawing. Freezing can preserve foods for months on end, and long-term storage by freezing, needs a constant temperature of -18 °C (0 °F).
Frozen foods many times require defrosting before cooking. Meats should be defrosted first, preferably. This presents the best results as the meat is more evenly cooked, and is of good texture. Defrost in a refrigerator, to limit significant proliferation and growth of any pathogens. This can take time, and defrosting can be achieved in a few ways:
- expose frozen food to room temperature
- within a refrigerator
- using a microwave oven
- place in cold water or under cold running water after wrapping in plastic
Time constraints and ignorance are reasons people defrost at room temperature. Such foods must be consumed promptly after cooking, or discarded, and never be refrozen or refrigerated, as pathogens are not frozen to death.
Frozen food quality
How quickly food is frozen directly influences the sizeand quantityof ice crystals, within the food item’s cells and extracellular space. Slow freezing makes for fewer,but larger ice crystals, while fast freezing yields just the opposite – smaller, more numerous crystals. Large ice crystals can pierce the walls of the cells of the food product, causing a deterioration in the product’s texture and loss of its natural juices, during thawing. Using mechanical freezing, non-ventilated mechanical freezing, and flash freezing using liquid nitrogen, there is a marked difference in quality of the food product, in question. Frozen food will continue to remain popular in any case, as increasingly superior techniques will keep being developed. If you want to buy a used refrigerated van, a used freezer van, a new refrigerated van or a new freezer van call, Glacier Vehicles on 0208 668 7579.