Many processes designed to preserve food involve more than one food preservation method. Preserving fruit by turning it into jam, for example, involves boiling (to reduce the fruit's moisture content and to kill bacteria, etc.), sugaring (to prevent their re-growth) and sealing within an airtight jar (to prevent recontamination).
Different food preservation methods have different impacts on the quality of the food and food systems. Some traditional methods of preserving food have been shown to have a lowerenergy input andcarbon footprint compared to modern methods.[3][2] Some methods of food preservation are also known to createcarcinogens.
Traditional techniques
Some techniques of food preservation pre-date thedawn of agriculture. Others were discovered more recently.
Boiling liquids can kill any existing microbes. Milk and water are often boiled to kill any harmful microbes that may be present in them.
Burial
Burial of food can preserve it due to a variety of factors: lack of light, lack of oxygen, cool temperatures, pH level, ordesiccants in the soil. Burial may be combined with other methods such as salting or fermentation. Most foods can be preserved in soil that is very dry and salty (thus a desiccant) such as sand, or soil that is frozen.
Manyroot vegetables are very resistant to spoilage and require no other preservation than storage in cool dark conditions, for example by burial in the ground, such as in astorage clamp (not to be confused with aroot cellar). Cabbage was traditionally buried during autumn in northern US farms for preservation. Some methods keep it crispy while other methods producesauerkraut. A similar process is used in the traditional production ofkimchi.
Sometimes meat is buried under conditions that 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, or, in areas ofpermafrost, a freezer.
InOdisha, India, it is practical to store rice by burying it underground. This method helps to store for three to six months during the dry season.
Butter and similar substances have been preserved asbog butter in Irishpeat bogs for centuries.Century eggs are traditionally 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.
Canning involves cooking food, sealing it in sterilized cans or jars, andboiling the containers to kill or weaken any remaining bacteria as a form ofsterilization. It was invented by the French confectionerNicolas Appert.[4] By 1806, this process was used by the French Navy to preserve meat, fruit, vegetables, and even milk. Although Appert had discovered a new way of preservation, it was not understood until 1864 whenLouis Pasteur found the relationship between microorganisms, food spoilage, and illness.[5]
Foods have varying degrees of natural protection against spoilage and may require that the final step occurs in apressure cooker. High-acid fruits likestrawberries require no preservatives to can and only a short boiling cycle, whereas marginal vegetables such ascarrots require longer boiling and the 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 cause gas production and the can will swell or burst. However, there have been examples of poor manufacture (underprocessing) and poorhygiene allowing contamination of canned food by the obligateanaerobeClostridium 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, however. Cookedmushrooms, when handled poorly and then canned, can support the growth ofStaphylococcus aureus, which produces a toxin that is not destroyed by canning or subsequent reheating.
Meat can be preserved by salting it, cooking it at or near 100 °C (212 °F) in some kind of fat (such aslard ortallow), and then storing it immersed in the fat. These preparations were popular in Europe before refrigerators became ubiquitous. They are still popular in France, where the term originates.[6][7] The preparation will keep longer if stored in a cold cellar or buried in cold ground.
Cooling preserves food by slowing down the growth and reproduction of microorganisms and the action of enzymes that causes the food to rot. The introduction of commercial and domestic refrigerators drastically improved the diets of many in theWestern world by allowing food such as fresh fruit, salads and dairy products to be stored safely for longer periods, particularly during warm weather.
Bag of Prague powder#1, also known as "curing salt" or "pink salt". It is typically a combination of salt and sodium nitrite, with the pink color added to distinguish it from ordinary salt.
Curing is any of various food preservation and flavoring processes of foods such as meat, fish and vegetables, by the addition ofsalt, with the aim of drawing moisture out of the food by the process ofosmosis. Because curing increases thesolute concentration in the food and hence decreases itswater potential, the food becomes inhospitable for themicrobial growth that causesfood spoilage.Smoking andsalting techniques improve on the drying process and add antimicrobial agents that aid in preservation. Smoke deposits a number of pyrolysis products onto the food, including thephenolssyringol,guaiacol andcatechol.[8] Salt accelerates the drying process usingosmosis and also inhibits the growth of several common strains of bacteria. More recentlynitrites have been used to cure meat, contributing a characteristic pink colour.[9]
The earliest form of curing is drying, which has been in use since ancient times, the earliest known practice dates back to 12,000 B.C. by inhabitants of the modern Asian and Middle Eastern regions.[13] Water is traditionally removed throughevaporation by using methods such as air drying, sun drying, smoking or wind drying, although today electricfood dehydrators can be used to speed the drying process and ensure more consistent results.[14]
Some foods, such as manycheeses,wines, andbeers, are prepared by fermentation. This involves cultivating specificmicroorganisms to combat spoilage from other, less benign organisms. These microorganisms keep pathogens in check by producing acid or alcohol, which eventually creates an environment toxic for themselves and other microorganisms.
Methods of fermentation include, but are not limited to, starter microorganisms, salt, hops, controlled (usually cool) temperatures and controlled (usually low) levels of oxygen. These methods are used to create the specific controlled conditions that will support the desirable organisms that produce food fit for human consumption. Fermentation is the microbial conversion of starch and sugars into alcohol. Not only can fermentation produce alcohol, but it can also be a valuable preservation technique. Fermentation can also make foods more nutritious and palatable.
Water was also turned into alcoholic beverages through fermentation. When water is used to make beer, the boiling during the brewing process may kill bacteria that could make people sick. The barley and other ingredients also infuse the drink with nutrients, and the microorganisms can also produce vitamins as they ferment.[5] However, the common belief that premodern people avoided drinking ordinary water is a myth. While people avoided drinking dirty or polluted water, they also avoided using it for the production of beer and wine. Water was visually inspected, smelled, tasted, filtered, and boiled if necessary. It was used for drinking as well as for diluting wine, cooking, and many other processes.[15]
Freezing is also one of the most commonly used processes, both commercially and domestically, for preserving a very wide range of foods, including prepared foods that 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.
Heating
Heating to temperatures which are sufficient to kill microorganisms inside the food is a method used withperpetual stews.
Food may be preserved by cooking in a material that solidifies to form a gel. Such materials includegelatin,agar,maize flour, andarrowroot flour.
Some animal flesh forms a protein gel when cooked.Eels and elvers, andsipunculid worms, are a delicacy inXiamen, China, as arejellied eels in theEast End of London, where they are eaten with mashed potatoes.British cuisine has a rich tradition ofpotted meats. Meat off-cuts were, until the 1950s, preserved inaspic, a gel made from gelatin and clarified meat broth. Another form of preservation is setting the cooked food in a container and covering it with a layer of fat. Potted chicken liver can be prepared in this way, and so canpotted shrimps, to be served on toast.Calf's foot jelly used to be prepared for invalids.
Jellying is one of the steps in producing traditionalpâtés. Manyjugged meats (see below) are also jellied.
Another type of jellying isfruit preserves, which are preparations of cooked fruits, vegetables and sugar, often stored in glass jam jars andMason jars. Many varieties of fruit preserves are made globally, including sweet fruit preserves, such as those made from strawberry or apricot, and savory preserves, such as those made from tomatoes or squash. The ingredients used and how they are prepared determine the type of preserves;jams,jellies, andmarmalades are all examples of different styles of fruit preserves that vary based upon the fruit used. In English, the wordpreserves, in plural form, is used to describe all types of jams and jellies.
Meat can be preserved by jugging. Jugging is the process ofstewing the meat (commonlygame orfish) in a coveredearthenware jug orcasserole. The animal to be jugged is usually cut into pieces, placed into a tightly sealed jug with brine orgravy, 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.
In ruralAfghanistan, grapes are preserved in disc-shaped vessels made of mud and straw, calledkangina. The vessels, which can preserve fresh grapes for up to 6 months, passively control their internal environments to restrict gas exchange and water loss, prolonging the lives of late-harvested grapes stored within them.[16]
Sodium hydroxide (lye) makes food tooalkaline for bacterial growth. Lye willsaponify fats in the food, which will change its flavor and texture.Lutefisk uses lye in its preparation, as do some olive recipes. Modern recipes forcentury eggs also call for lye.
Pickling is a method of preserving food in an edible, antimicrobial liquid. Pickling can be broadly classified into two categories: chemical pickling and fermentation pickling.
In chemical pickling, the food is placed in an edible liquid that inhibits or kills bacteria and other microorganisms. Typical pickling agents includebrine (high in salt),vinegar,alcohol, andvegetable oil. 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 includecucumbers,peppers,corned beef,herring, andeggs, as well as mixed vegetables such aspiccalilli.
The earliest cultures have usedsugar as a preservative, and it was commonplace to store fruit in honey. "Sugar tends to draw water from the microbes (plasmolysis). This process leaves the microbial cells dehydrated, thus killing them. In this way, the food will remain safe from microbial spoilage."[8]
In northern climates without sufficient sun to dry foods,preserves are made by heating the fruit with sugar.[5] Sugar is used to preserve fruits, either in anantimicrobial syrup with fruit such as apples, pears, peaches, apricots, and plums, 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. The latter method is used for the skins ofcitrus fruit (candied peel),angelica, andginger.
Modern industrial techniques
Techniques of food preservation were developed in research laboratories for commercial applications.
Aseptic processing works by placing sterilized food (typically by heat, seeultra-high temperature processing) into sterlized packaging material under sterile conditions. The result is a sealed, sterile food product similar to canned food, but depending on the technique used, damage to food quality is typically reduced compared to canned food. A greater variety of packaging materials can be used as well.
Besides UHT, aseptic processing may be used in conjunction with any of the microbe-reduction technologies listed below. With pasteurization and "high pressure pasteurization", the food may not be completely sterilized (instead achieving a specifiedlog reduction), but the use of sterile packaging and environments is retained.
Pasteurization is a process for preservation of liquid food. It was originally applied to combat the souring of young local wines. Today, the process is mainly applied to dairy products. In this method, milk is heated at about 70 °C (158 °F) for 15–30 seconds to kill the bacteria present in it and cooling it quickly to 10 °C (50 °F) to prevent the remaining bacteria from growing. The milk is then stored in sterilized bottles or pouches in cold places. This method was invented byLouis Pasteur, aFrench chemist, in 1862.
Vacuum-packing stores food in a vacuum environment, usually in an air-tight bag or bottle. Thevacuum environment strips bacteria of oxygen needed for survival. Vacuum-packing is commonly used for storingnuts to reduce loss of flavor from oxidization. A major drawback to vacuum packaging, at the consumer level, is that vacuum sealing can deform contents and rob certain foods, such as cheese, of its flavor.
Freeze drying, also known as lyophilization or cryodesiccation, is a low temperaturedehydration process[17] that involves freezing the product and lowering pressure, thereby removing the ice bysublimation.[18] This is in contrast to dehydration by most conventional methods that evaporate water using heat.[19]
Because of the low temperature used in processing,[17] the rehydrated product retains many of its original qualities. When solid objects like strawberries are freeze dried the original shape of the product is maintained.[20] If the product to be dried is a liquid, as often seen in pharmaceutical applications, the properties of the final product are optimized by the combination ofexcipients (i.e., inactive ingredients). Primary applications of freeze drying include biological (e.g., bacteria and yeasts), biomedical (e.g., surgical transplants), food processing (e.g., coffee), and preservation.[17]
Irradiation of food[23] is the exposure of food toionizing radiation. Multiple types of ionizing radiation can be used, includingbeta particles (high-energyelectrons) andgamma rays (emitted from radioactive sources such ascobalt-60 orcesium-137). Irradiation can kill bacteria, molds, and insect pests, reduce the ripening and spoiling of fruits, and at higher doses induce sterility. The technology may be compared topasteurization; it is sometimes called "cold pasteurization", as the product is not heated. Irradiation may allow lower-quality or contaminated foods to be rendered marketable.
National and international expert bodies have declared food irradiation as "wholesome"; organizations of theUnited Nations, such as theWorld Health Organization andFood and Agriculture Organization, endorse food irradiation.[24][25] Consumers may have a negative view of irradiated food based on the misconception that such food is radioactive;[26] in fact, irradiated food does not and cannot become radioactive. Activists have also opposed food irradiation for other reasons, for example, arguing that irradiation can be used to sterilize contaminated food without resolving the underlying cause of the contamination.[27] International legislation on whether food may be irradiated or not varies worldwide from no regulation to a full ban.[28]
Approximately 500,000 tons of food items are irradiated per year worldwide in over 40 countries. These are mainlyspices andcondiments, with an increasing segment of fresh fruit irradiated for fruit fly quarantine.[29][30]
Pulsed electric field (PEF) electroporation is a method for processing cells by means of brief pulses of a strong electric field. PEF holds potential as a type of low-temperature alternative pasteurization process for sterilizing food products. In PEF processing, a substance is placed between two electrodes, then the pulsed electric field is applied. The electric field enlarges the pores of the cell membranes, which kills the cells and releases their contents. PEF for food processing is a developing technology still being researched. There have been limited industrial applications of PEF processing for the pasteurization of fruit juices. To date, several PEF treated juices are available on the market in Europe. Furthermore, for several years a juice pasteurization application in the US has used PEF. For cell disintegration purposes especially potato processors show great interest in PEF technology as an efficient alternative for their preheaters. Potato applications are already operational in the US and Canada. There are also commercial PEF potato applications in various countries in Europe, as well as in Australia, India, and China.
Modifying atmosphere is a way to preserve food by operating on the atmosphere around it. It is often used to package:
Fresh fruits and vegetables, especially salds crops, which contain living cells that respire even while refrigerated. Reducing oxygen (O2) concentration and increasing thecarbon dioxide (CO2) concentration slows down their respiration, conserves stored energy, and therefore increases shelf life.[31] High humidity is also used to reduce water loss.[32]
Red meat, which needs highO2 to reduce oxidation ofmyoglobin and maintain an attractive bright red color of the meat.[33]
Other meat and fish, which uses higherCO2 to reduce oxidation and slow down some microbes.[34]
This process subjects the surface of food to a "flame" of ionized gas molecules, such as helium or nitrogen. This causes micro-organisms to die off on the surface.[35]
High pressure can be used to disable harmful microorganisms and spoilage enzymes while retaining the food's fresh appearance, flavor, texture and nutrients. By 2005, the process was being used for products ranging fromorange juice toguacamole todeli meats and widely sold.[36] Depending on temperature and pressure settings, HP processing can achieve either pasteurization-equivalentlog reduction or go all the way to achieve sterilization of all microbes.[37]
Biopreservation
3D stick model ofnisin. Somelactic acid bacteria manufacture nisin. It is a particularly effective preservative.
Biopreservation is the use of natural or controlledmicrobiota orantimicrobials as a way of preserving food and extending itsshelf life.[38] Beneficial bacteria or thefermentation products produced by these bacteria are used in biopreservation to control spoilage and renderpathogens inactive in food.[39] It is a benign ecological approach which is gaining increasing attention.[38]
Lactic acid bacteria (LAB) have antagonistic properties that make them particularly useful as biopreservatives. When LABs compete for nutrients, theirmetabolites often include active antimicrobials such as lactic acid, acetic acid, hydrogen peroxide, andpeptidebacteriocins. Some LABs produce the antimicrobialnisin, which is a particularly effective preservative.[40][41]
LAB bacteriocins are used in the present day as an integral part ofhurdle technology. Using them in combination with other preservative techniques can effectively control spoilage bacteria and other pathogens, and can inhibit the activities of a wide spectrum of organisms, including inherently resistantGram-negative bacteria.[38]
Hurdle technology is a method of ensuring thatpathogens infood products can be eliminated or controlled by combining more than one approach. These approaches can be thought of as "hurdles" the pathogen has to overcome if it is to remain active in the food. The right combination of hurdles can ensure all pathogens are eliminated or rendered harmless in the final product.[42]
Hurdle technology has been defined by Leistner (2000) as an intelligent combination of hurdles that secures themicrobial safety and stability as well as theorganoleptic and nutritional quality and the economic viability offood products.[43] The organoleptic quality of the food refers to its sensory properties, that is its look, taste, smell, and texture.
Examples of hurdles in a food system are high temperature during processing, low temperature during storage, increasing theacidity, lowering thewater activity orredox potential, and the presence ofpreservatives orbiopreservatives. According to the type of pathogens and how risky they are, the intensity of the hurdles can be adjusted individually to meet consumer preferences in an economical way, without sacrificing the safety of the product.[42]
Principal hurdles used for food preservation (after Leistner, 1995)[44][45]
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^Ratti, C (2001). "Hot air and freeze-drying of high-value foods: a review".Journal of Food Engineering.49 (4):311–319.doi:10.1016/s0260-8774(00)00228-4.
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^Food Irradation – A technique for preserving and improving the safety of food, WHO, Geneva, 1991
^World Health Organization. Wholesomeness of irradiated food. Geneva, Technical Report Series No. 659, 1981
^World Health Organization. High-Dose Irradiation: Wholesomeness of Food Irradiated With Doses Above 10 kGy. Report of a Joint FAO/IAEA/WHO Study Group. Geneva, Switzerland: World Health Organization; 1999. WHO Technical Report Series No. 890
^Conley, S.T., What do consumers think about irradiated foods, FSIS Food Safety Review (Fall 1992), 11–15
^Hauter, W. & Worth, M.,Zapped! Irradiation and the Death of Food, Food & Water Watch Press, Washington, DC, 2008
^C.M. Deeley, M. Gao, R. Hunter, D.A.E. Ehlermann, The development of food irradiation in the Asia Pacific, the Americas and Europe; tutorial presented to the International Meeting on Radiation Processing, Kuala Lumpur, 2006.[1]
^Brody, A.L., Zhuang, H., Han, J.H (2011).Modified atmosphere packaging for fresh-cut fruits and vegetables. West Sussex, UK: Blackwell Publishing Ltd. pp. 57–67.ISBN978-0-8138-1274-8.{{cite book}}: CS1 maint: multiple names: authors list (link)
