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Preservation Glossary

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Food Preservation

Food preservation is the process of treating and handling foods in such a way as to stop or greatly slow down spoilage to prevent food-borne illness while maintaining nutritional value, density, texture and flavor.

Preservation involves preventing the growth of bacteria, fungi, and other micro-organisms as well as retarding the oxidation of fats that cause rancidity.  It also includes processes to inhibit natural aging and discoloration that occurs during food preparation, such as apples browning when sliced  Some preservation methods require the food to be sealed after treatment to prevent re-contamination with microbes; others, such as drying, allow food to be stored without any special containment for long periods.

Preservation processes include:
1)  Heating to kill or denature organisms (e.g. boiling)
2)  Oxidation (e.g. use of sulphur dioxide)
3)  Toxic inhibition (e.g. smoking, use of CO/2, vinegar, alcohol etc)
4)  Dehydration (e.g. drying)
5)  Osmotic inhibition (e.g. use of syrups)
6)  Low temperature inactivation (e.g. freezing)
7)  Combinations of these methods

Common methods of applying these processes include drying, spray drying, freeze drying, refrigeration, freezing, vacuum-packing, canning, preserving in syrup, sugar crystallization, food irradiation, adding preservatives or inert gases such as carbon dioxide.  Other methods that preserve food, and add flavor, include pickling, salting, smoking, preserving in alcohol, sugar crystallization and curing.

Drying:  One of the oldest food preservation methods is drying that sufficiently reduces water activity to delay or prevent bacterial growth.  Most types of meat can be dried.  In addition, many fruits can be dried.  Drying is also the normal means of preservation for cereal grains such as wheat, maize, oats, barley, rice, millet, and rye.

Smoking:  Meat, fish, and some other foods may be both preserved and flavored with smoke, which is typically infused in a smokehouse.  The combination of heat to dry the food without cooking it, and the addition of the aromatic hydrocarbons from the smoke preserves the food.

Refrigeration & Freezing:  Refrigeration and freezing are two of the most commonly used processes commercially and domestically for preserving a very wide range of foodstuffs including prepared foodstuffs, 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.

Vacuum Packing:  Vacuum-packing stores food in a vacuum environment, usually in an airtight bag or bottle.  The vacuum environment strips bacteria of oxygen needed for survival, hence preventing the food from spoiling.  Home vacuum packing is available in bags, canisters, Mason jars, and bottles using the FoodSaver Home Vacuum Packing System.

Salt:  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:  Sugar is used to preserve fruits, either in syrup or in crystallized form where the preserved material is cooked in sugar to the point of crystallization 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 glacÚ fruit such as glacÚ 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.

Pickling:  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 oil, 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, 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.

Lye:  Sodium hydroxide (lye) makes food too alkaline for bacterial growth.  Lye will saponify fats in the food, which will change its flavor and texture.

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 causing gas production and the selling or bursting of the can.  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 beef and tuna.

Jellying: Food may be preserved by cooking in a material that solidifies to a gel. Such materials include gelatin, agar, maize flour and arrowroot flour.  Some foods naturally form a protein gel when cooked.

Jugging: Meat can be preserved by jugging, the process of stewing the meat 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: 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.  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. One should note 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:  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 (O/2) concentration and increase the carbon dioxide (CO/2) 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 dioxide from 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:  (N/2) 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 a hermetic seal is not maintainable.

Cellars:  Many root vegetables are very resistant to spoilage and require no other preservation other than storage in cool dark conditions, usually in cellars.

Biological Processes:  Some foods, such as traditional cheeses, keep for a long time without any special procedures.  The preservation occurs due to the presence in very high numbers of beneficial bacteria or fungi, which use their own biological defenses to prevent other organisms from gaining a foothold.