Food preservation is the process of treating and handling food in a way that preserves its edibility and nutrition value. The main effort is to stop or greatly slow down spoilage to prevent foodborne illness (e.g. salting, cooling, cooking). However some methods utilise benign bacteria, yeasts or fungi to add specific qualities and to preserve food (e.g. cheese, wine). While maintaining or creating nutritional value, texture and flavour is important in preserving its value as food; this is a culturally dependent determinant as what qualifies as food fit for humans in one culture may not qualify in another culture.
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 ageing and discolouration that can occur during food preparation such as the enzymatic browning 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.
Common methods of applying these processes include drying, spray drying, freeze drying, freezing,vacuum-packing, canning, preserving in syrup, sugar crystallisation, food irradiation, adding preservatives or inert gases such as carbon dioxide. Other methods that not only help to preserve food, but also add flavour, include pickling, salting, smoking, preserving in syrup or alcohol, sugar crystallisation and curing.
|Method||Effect on microbial growth or survival|
|Refrigeration||Low temperature to retard growth|
|Freezing||Low temperature and reduction of water activity to prevent microbial growth|
|Drying, curing and conserving||Reduction in water activity sufficient to delay or prevent microbial 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|
|Pulsed electric field processing (PEF treatment)||Short bursts of electricity for microbial inactivation|
Preservation processes include:
One of the oldest methods of food preservation is by drying, which reduces water activity sufficiently to delay or prevent bacterial growth. Most types of meat can be dried. This is especially valuable in the case of pork, 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, mangoes, papaya, apricot, 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 flavored through the use of smoke, typically 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.
Freezing is also one of the most commonly used processes commercially and domestically for preserving a very wide range of food 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 stores food in a vacuum environment, usually in an air-tight bag or bottle. The vacuum environment strips bacteria of oxygen needed for survival, preventing the food from spoiling. Vacuum-packing is commonly used for storing nuts.
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 crystallized form where the preserved material is cooked in sugar to the point of crystallisation 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 flavored spirits such as Cherry Brandy or Sloe gin.
Pickling is a method of preserving food in an edible anti-microbial liquid. Pickling can be broadly categorized as chemical pickling (for example, brining) and fermentation pickling (for example, making sauerkraut).
In chemical pickling, the food is placed it in an edible liquid that inhibits or kills bacteria and other micro-organisms. Typical pickling agents include brine (high in salt), vinegar, alcohol, and vegetable oil, especially olive oil but also many other oils. Many chemical pickling processes also involve heating or boiling so that the food being preserved becomes saturated with the pickling agent. Common chemically pickled foods include cucumbers, peppers, corned beef, herring, and eggs, as well mixed vegetables such as piccalilli, chow-chow, giardiniera, and achar.
In fermentation pickling, the food itself produces the preservation agent, typically by a process that produces lactic acid. Fermented pickles include sauerkraut, nukazuke, kimchi, surströmming, and curtido. Some chemically pickled cucumbers are also fermented.
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 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, (a gel made from gelatine and clarified meat broth) were a common way of serving meat off-cuts in the UK until the 1950s. Many jugged meats are also jellied.
Fruit preserved by jellying is known as jelly, marmalade, or fruit preserves. In this case, the jellying agent is usually pectin, either added during cooking or arising naturally from the fruit. Most preserved fruit is also sugared.
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 of food is the exposure of food to ionizing radiation; either high-energy electrons or X-rays from accelerators, or by gamma rays (emitted from radioactive sources as Cobalt-60 or Caesium-137). The treatment has a range of effects, including killing bacteria, molds and insect pests, reducing the ripening and spoiling of fruits, and at higher doses inducing sterility. The technology may be compared to pasteurization; it is sometimes called 'cold pasteurization', as the product is not heated. Irradiation is not effective against viruses or prions, it cannot eliminate toxins already formed by microorganisms, and is only useful for food of high initial quality.
The radiation process is unrelated to nuclear energy, but it may use the radiation emitted from radioactive nuclides produced in nuclear reactors. Ionizing radiation is hazardous to life; for this reason irradiation facilities have a heavily shielded irradiation room where the process takes place. Radiation safety procedures ensure that neither the workers in such facility nor the environment receive any radiation dose from the facility. Irradiated food does not become radioactive, and national and international expert bodies have declared food irradiation as wholesome. However, the wholesomeness of consuming such food is disputed by opponents and consumer organizations. National and international expert bodies have declared food irradiation as 'wholesome'; UN-organizations as WHO and FAO are endorsing to utilize food irradiation. International legislature on whether food may be irradiated or not varies worldwide from no regulation to full banning.
It is estimated that about 500,000 tons of food items are irradiated per year world-wide in over 40 countries. These are mainly spices and condiments with an increasing segment of fresh fruit irradiated for fruit fly quarantine.
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 (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, for example by burial in the ground, such as in a storage clamp.
Century eggs are created by placing eggs in alkaline mud (or other alkaline substance) resulting in their "inorganic" fermentation through raised pH instead of spoiling. The fermentation preserves them and breaks down some of the complex, less flavorful proteins and fats into simpler more flavorful ones.
Most foods can be preserved in soil that is very dry and salty (thus a desiccant), or soil that is frozen.
Cabbage was traditionally buried in the fall in northern farms in the USA for preservation. Some methods keep it crispy while other methods produce sauerkraut. A similar process is used in the traditional production of kimchi.
Sometimes meat is buried under conditions which cause preservation. If buried on hot coals or ashes, the heat can kill pathogens, the dry ash can desiccate, and the earth can block oxygen and further contamination. If buried where the earth is very cold, the earth acts like a refrigerator. Fish (e.g. Gravlax) has been buried to preserve by fermentation.