Food preservation

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, texture and flavor.

Preservation Processes

Method	Effect on microbial growth or survival
Refrigeration or chilling	Low temperature to retard growth
Freezing	Low temperature and reduction of water activity to prevent growth
Drying, curing and conserving	Reduction in water activity sufficient to delay or prevent growth
Vacuum and oxygen free modified atmosphere packaging	Low oxygen tension inhibits strict aerobes and delay growth of facultative anaerobes
Carbon dioxide enriched modified atmosphere packaging	Specific inhibition of some micro-organisms by carbon dioxide
Addition of weak acids	Reduction of the intracellular pH of micro-organisms
Lactic fermentation	Reduction of pH value in situ by microbial action and sometimes additional inhibition by the lactic and acetic acids formed and by other microbial products. (e.g. ethanol, bacteriocins)
Sugar preservation	Cooking in high sucrose concentration creating too high osmotic pressure for most microbial survival.
Ethanol preservation	Steeping or cooking in Ethanol produces toxic inhibition of microbes. Can be combined with sugar preservation
Emulsification	Compartmentalisation and nutrient limitation within the aqueous droplets in water-in-oil emulsion foods
Addition of preservatives such as nitrite or sulphite ions	Inhibition of specific groups of micro-organisms
Pasteurization and appertization	Delivery of heat sufficient to inactivate target micro-organisms to the desired extent
Food irradiation (Radurization, radicidation and radappertization)	Delivery of ionising radiation
Application of high hydrostatic pressure (Pascalization)	Pressure-inactivation of vegetative bacteria, yeasts and moulds

Preservation usually involves preventing the growth of bacteria, fungi and other micro-organisms, as well as retarding the oxidation of fats which cause rancidity. It also includes processes to inhibit natural aging and discolouration that can occur during food preparation such as the polyphenoloxidase reaction in apples which causes browning when apples are cut. 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:

Heating to kill or denature organisms (e.g. boiling)
Oxidation (e.g use of sulphur dioxide)
Toxic inhibition (e.g. smoking, use of carbon dioxide, vinegar, alcohol etc)
Dehydration (drying)
Osmotic inhibition ( e.g use of syrups)
Low temperature inactivation (e.g. freezing)
Many combinations of these methods
Chelation

Methods

Common methods of applying these processes include drying, freeze drying, freezing, vacuum-packing, canning, preserving in syrup, sugar crystalisation, food irradiation, adding preservatives or inert gases such as carbon dioxide.

Other methods that not only help to preserve food, but also add flavor, include pickling, salting, smoking , preserving in syrup or alcohol , sugar crystalisation and curing.

Drying

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 and this is especially valuable in the case of pig meat since this is difficult to keep without preservation. Many fruits can also be dried and the process is often applied to apples, pears, bananas, mangos, papaya, coconut etc. 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.

Freezing

Probably as old as drying, many Arctic communities would preserve food in holes or larders dug into the ice. There is a tradition in Scandinavia of preserving fish and especially herrings in this way.

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.

Vacuum Packing

Vacuum-packing stores food in a vacuum environment, usually in an air-tight bag or bottle. The vacuum environment strips bacteria of oxygen needed for survival, hence preventing the food from spoiling. Vacuum-packing is commonly used for storing nuts.

Curing

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 with fruit such as apples, pears, peaches, apricots, plums or in crystalised 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 glacé fruit such as glacé 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

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 (to make sauerkraut and curtido), peppers, and some animal products such as corned beef and eggs.

A less-common form of pickling uses sodium hydroxide (lye) to make the food too alkaline for bacterial growth. Lye will saponify fats in the food, which will change its flavor and texture. Lutefisk and hominy use lye in their preparation, as do some olive recipes.

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. 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 contanmination 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 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 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

Irradiation

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

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 concentration and increase the carbon dioxide 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.

Clamps

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

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.

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