Various preserved foods
Canadian World War I poster encouraging people to preserve food for the winter.
is the process of treating and handling
food to stop or greatly slow down spoilage (loss of quality, edibility or nutritive value) caused or accelerated by micro-organisms. Some methods, however, use benign bacteria, yeasts or fungi to add specific qualities and to preserve food (e.g., cheese, wine). Maintaining or creating
nutritional value, texture and
flavour is important in preserving its value as food. This is culturally dependent, 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 regarding 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 recontamination 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, and 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.
Preservation processes
Preservation processes include:
- Heating to kill or denature micro-organisms (e.g. boiling)
- Oxidation (e.g. use of sulphur dioxide)
- Toxic inhibition (e.g. smoking, use of carbon dioxide, vinegar, alcohol etc)
- Osmotic inhibition ( e.g. use of syrups)
- Low temperature inactivation (e.g. freezing)
- Ultra high water pressure (e.g. fresherized, a kind of “cold” pasteurization, the pressure kills naturally occurring pathogens, which cause food deterioration and affect food safety.)
- Many combinations of these methods
Drying
One of the oldest methods of food preservation is by drying, which reduces
water activity sufficiently to prevent or delay
bacterial growth. Drying also reduces weight, making food more portable. Most types of meat can be dried; a good example is beef
jerky. 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.
Freezing
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
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, slowing spoiling. Vacuum-packing is commonly used for storing
nuts to reduce loss of flavor from oxidation.
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 and contribute the characteristic pink color, as well as inhibition of
Clostridium botulinum.
Sugar
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
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 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.
In commercial pickles, a preservative like
sodium benzoate or
EDTA may also be added to enhance
shelf life.
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.
Lutefisk uses lye in its preparation, as do some olive recipes. Modern recipes for
century eggs also call for lye. Masa harina and
hominy use lye in their preparation, but not for preservation.
Canning and bottling
Preserved food
involves cooking food, sealing it in sterile cans or jars, and
boiling the containers to kill or weaken any remaining bacteria as a form of
sterilization, inventor
Nicolas Appert . 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. Low acid foods, such as vegetables and meats 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 (underprocessing) 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. Its toxin is denatured by cooking, though. Cooked mushrooms, handled poorly and then canned, can support the growth of
Staphylococcus aureus, which produces a toxin that is not destroyed by canning or subsequent reheating.
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 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 in jars. Heating, packaging and acid and sugar provide the preservation.
Potting
Spam is a canned and preserved meat product.
A traditional British way of preserving meat (particularly
shrimp) is by setting it in a pot and sealing it with a layer of fat. Also common is potted chicken liver; compare
pâté.
Jugging
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
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 use food irradiation. International legislation 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 worldwide in over 40 countries. These are mainly
spices and
condiments with an increasing segment of fresh fruit irradiated for fruit fly quarantine.
Modified atmosphere
is a way to preserve food by 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 an
hermetic seal cannot be maintained.
Burial in the ground
Burial of food can preserve it due to a variety of factors: lack of light, lack of oxygen, cool temperatures, pH level, or
desiccants in the soil. Burial may be combined with other methods such as salting or fermentation.
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.
Controlled use of micro-organism
Some foods, such as many
cheeses,
wines, and
beers will keep for a long time because their production uses specific micro-organisms that combat spoilage from other less benign organisms. These micro-organisms keep pathogens in check by creating an environment toxic for themselves and other micro-organisms by producing acid or alcohol. Starter micro-organisms, salt, hops, controlled (usually cool) temperatures, controlled (usually low) levels of oxygen and/or other methods are used to create the specific controlled conditions that will support the desirable organisms that produce food fit for human consumption.
High pressure food preservation
High pressure food preservation refers to
high pressure used for food preservation. "Pressed inside a vessel exerting 70,000 pounds per square inch or more, food can be processed so that it retains its fresh appearance, flavour, texture and nutrients while disabling harmful microorganisms and slowing spoilage." By 2001, adequate commercial equipment was developed so that by 2005 the process was being used for products ranging from
orange juice to
guacamole to
deli meats and widely sold.
See also
