FOOD PRESERVATION TECHNIQUES
Food preservation is the process of treating and handling food in such a way as to stop or greatly slow down spoilage to prevent foodborne illness while maintaining nutritional value, density, texture and flavOUr.
One of the oldest methods of food preservation is by drying, which reduces water activity sufficient to delay or prevent bacterial growth. Most types of meat can be dried. This is especially valuable in the case of pig meat, since it is difficult to keep without preservation. Many fruits can also be dried; for example, the process is often applied to apples, pears, bananas, mangos, papaya, and coconut. Zante Currants, sultanas and raisins are all forms of dried grapes. Drying is also the normal means of preservation for cereal grains such as wheat, maize, oats, barley, rice, millet and rye.
Meat, fish and some other foods may be both preserved and flavoured through the use of smoke, typically in a smoke-house. The combination of heat to dry the food without cooking it, and the addition of the aromatic hydrocarbons from the smoke preserves the food.
Freezing is also one of the most commonly used processes commercially and domestically for preserving a very wide range of food stuffs including prepared food stuffs which would not have required freezing in their unprepared state. For example, potato waffles are stored in the freezer, but potatoes themselves require only a cool dark place to ensure many months' storage. Cold stores provide large volume, long-term storage for strategic food stocks held in case of national emergency in many countries.
Salting or curing draws moisture from the meat through a process of osmosis. Meat is cured with salt or sugar, or a combination of the two. Nitrates and nitrites are also often used to cure meat.
Sugar is used to preserve fruits, either in syrup with fruit such as apples, pears, peaches, apricots, plums or in crystallised form where the preserved material is cooked in sugar to the point of crystralisation and the resultant product is then stored dry. This method is used for the skins of citrus fruit (candied peel), angelica and ginger. A modification of this process produces glacÃ© fruit such as glacÃ© cherries where the fruit is preserved in sugar but is then extracted from the syrup and sold, the preservation being maintained by the sugar content of the fruit and the superficial coating of syrup. The use of sugar is often combined with alcohol for preservation of luxury products such as fruit in brandy or other spirits. These should not be confused with fruit flavoured spirits such as Cherry Brandy or Sloe gin
Pickling is a method of preserving food by placing it or cooking it in a substance that inhibits or kills bacteria and other micro-organisms. This material must also be fit for human consumption. Typical pickling agents include brinE (high in salt), vinegar, ethanol, and vegetable oilm, especially olive oil but also many other oils. Most pickling processes also involve heating or boiling so that the food being preserved becomes saturated with the pickling agent. Frequently pickled items include vegetables such as cabbage (to make sauerkraut and curtido), peppers, and some animal products such as corned beef and eggs. EDTA may also be added to chelate calcium. Calcium is essential for bacterial growth.
Â Canning and bottling
Canning involves cooking fruits or vegetables, sealing them in sterile cans or jars, and boiling the containers to kill or weaken any remaining bacteria as a form of pasteurization. Various foods have varying degrees of natural protection against spoilage and may require that the final step occur in a pressure cooker. High-acid fruits like strawberries require no preservatives to can and only a short boiling cycle, whereas marginal fruits such as tomatoes require longer boiling and addition of other acidic elements. Many vegetables require pressure canning. Food preserved by canning or bottling is at immediate risk of spoilage once the can or bottle has been opened.
Lack of quality control in the canning process may allow ingress of water or micro-organisms. Most such failures are rapidly detected as decomposition within the can causes gas production and the can will swell or burst. However, there have been examples of poor manufacture and poor hygiene allowing contamination of canned food by the obligate anaerobe, Clostridium botulinum which produces an acute toxin within the food leading to severe illness or deaTh. This organism produces no gas or obvious taste and remains undetected by taste or smell. Food contaminated in this way has included Corned beeFÂ and Tuna.
Food may be preserved by cooking in a material that solidifies to form a gel. Such materials include gelatine, agar, maize flour and arrowroot flour. Some foods naturally form a protein gel when cooked such as eels and elvers, and sipunculid worms which are a delicacy in the town of Xiamen in Fujian province of the People's Republic of China. Jellied eels are a delicacy in the East End of London where they are eaten with mashed potatoes. Potted meats in aspic, (the gel made from arrowroot flour) were a common way of serving meat off-cuts in the UK until the 1950s
Meat can be preserved by jugging, the process of stewing the meat (commonly game or fish) in a covered earthenware jug or casserole. The animal to be jugged is usually cut into pieces, placed into a tightly-sealed jug with brine or gravy, and stewed. Red wine and/or the animal's own blood is sometimes added to the cooking liquid. Jugging was a popular method of preserving meat up until the middle of the 20th century.
Irradiation is the treatment of food with x-rays or gamma radiation to kill bacteria and mold. It may be combined with vacuum packing to seal out microbes.
As with sunlight, exposure to the intense light from the lamps used for food irradiation is harmful to human skin. As with sunlight, the light from the lamps used for food irradiation does not make the food "radioactive." Food irradiation is effective against a wide variety of pathogens including bacteria, fungi, viruses and parasites.
A 1950s issue of Popular Mechanics details the impending arrival of "food irradiation". But the implications of irradiation are not fully understood, and the use of the technology is limited. Irradiation of potatoes, strawberries, and meat is common in many countries where refrigerated facilities and trucks are not. In 2002, the Food and Drug Administration permitted irradiation of meat and poultry to reduce the spread of E. coli and salmonella.
In the US and most of Europe, irradiation of spices is common, as the only alternative (treatment with gas) is potentially carcinogenic. The process is called "cold pasteurization" because it is feared that the label "irradiation" would hurt sales. Foods may also carry labels saying "Picowaved For Your Protection" as food processors may not want to openly label their foods as being irradiated.
It should be noted that although irradiation is effective at killing bacteria, fungi and other pathogens, there is still a danger that the food may contain some of their toxins.
Modified atmosphere is a way to preserve food operating on the atmosphere around it. Salad crops which are notoriously difficult to preserve are now being packaged in sealed bags with an atmosphere modified to reduce the oxygen (O2) concentration and increase the carbon dioxide (CO2) concentration. There is concern that although salad vegetables retain their appearance and texture in such conditions, this method of preservation may not retain nutrients, especially vitamins.
Grains May be preserved using carbon dioxide. A block of dry ice is placed in the bottom and the can is filled with grain. The can is then "burped" of excess gas. The carbon dioxideNTfrom the sublimation of the dry ice prevents insects, mold, and oxidation from damaging the grain. Grain stored in this way can remain edible for five years.
Nitrogen gas (N2) at concentrations of 98% or higher is also used effectively to kill insects in grain through hypoxia. However, carbon dioxide has an advantage in this respect as it kills organisms through both hypoxia and hypercarbia, requiring concentrations of only 80%, or so. This makes carbon dioxide preferable for fumigation in situations where an hermetic seal cannot be maintained.
Many root vegetables are very resistant to spoilage and require no other preservation other than storage in cool dark conditions, usually in field clamps.
Â Biological processes
Some foods, such as many traditional cheeses, will keep for a long time without use of any special procedures. The preservation occurs because of the presence in very high numbers of beneficial bacteria or fungi which use their own biological defences to prevent other organisms gaining a foot-hold.