Food preservation
Food preservation is, at its heart, a war against the invisible. Microorganisms grow, fats oxidize and turn rancid, and a cut apple browns through an enzymatic reaction before your eyes. Every method humans have devised exists to slow or stop that decay. Some of these techniques pre-date the dawn of agriculture itself. Others were born in research laboratories for commercial use. The earliest known practice of drying food dates back to 12,000 B.C., among inhabitants of the modern Asian and Middle Eastern regions. And yet one of the most important breakthroughs, canning, was used by the French Navy by 1806, decades before anyone understood why it worked. How did people keep grapes fresh for six months in mud vessels, or butter for centuries in a bog? Why does the World Health Organization call one preservation method carcinogenic and another wholesome? What turns a jar of strawberries into a thing that lasts, and a poorly canned mushroom into a deadly one? The answers run from peat bogs to electric fields, and they reveal how much of what we eat is shaped by the fight against rot.
Burial preserves food through a combination of factors: lack of light, lack of oxygen, cool temperatures, the right pH level, or desiccants in the soil. Cabbage was traditionally buried during autumn in northern US farms, and depending on the method, it stayed crispy or became sauerkraut. A similar process underlies the traditional production of kimchi.
In Odisha, India, rice can be stored by burying it underground, a method that keeps it through three to six months of the dry season. Meat buried on hot coals or ashes is preserved several ways at once. The heat kills pathogens, the dry ash desiccates, and the earth blocks oxygen and further contamination. Where the earth is very cold it acts like a refrigerator, and in areas of permafrost it becomes a freezer.
Butter has survived for centuries as bog butter in Irish peat bogs. Century eggs take a stranger route still, created by placing eggs in alkaline mud or another alkaline substance. The raised pH triggers what amounts to inorganic fermentation, preserving the eggs while breaking down complex, less flavorful proteins and fats into simpler, more flavorful ones. The same chemistry that locks food underground also reshapes its taste.
Curing works by addition rather than subtraction at first, adding salt to meat, fish, and vegetables to draw moisture out through osmosis. By raising the solute concentration and lowering the food's water potential, curing makes the food inhospitable to the microbes that cause spoilage. Salt also directly inhibits the growth of several common strains of bacteria.
Drying is the earliest form of curing, removing water through evaporation by air, sun, smoke, or wind, though electric food dehydrators now speed and standardize the process. Smoking improves on plain drying by depositing pyrolysis products onto the food, including the phenols syringol, guaiacol, and catechol, which act as antimicrobial agents. More recently, nitrites have been used to cure meat, giving it a characteristic pink color.
There is a cost hidden in the cure. In 2015, the International Agency for Research on Cancer of the World Health Organization classified processed meat, meaning meat that has undergone salting, curing, and smoking, as carcinogenic to humans. Sugaring offers a gentler version of the same osmotic principle. Sugar draws water from the microbes in a process called plasmolysis, leaving the cells dehydrated and dead, which is why the earliest cultures stored fruit in honey.
Canning was invented by the French confectioner Nicolas Appert, who found that cooking food, sealing it in sterilized cans or jars, and boiling the containers could kill or weaken remaining bacteria. By 1806 the French Navy used the process to preserve meat, fruit, vegetables, and even milk. Yet the reason it worked stayed a mystery until 1864, when Louis Pasteur uncovered the relationship between microorganisms, food spoilage, and illness.
Different foods demand different rigor. High-acid fruits like strawberries need no preservatives and only a short boiling cycle, while low-acid vegetables and meats require pressure canning. The danger lives in the failures. Poor manufacture and poor hygiene can let the obligate anaerobe Clostridium botulinum contaminate canned food, producing an acute toxin that gives off no gas, no taste, and no smell, leading to severe illness or death. Cooking denatures that toxin, but not every threat yields so easily. Cooked mushrooms handled poorly and then canned can support Staphylococcus aureus, whose toxin survives both canning and reheating.
Pasteur lent his name to a second method as well. Pasteurization was first applied to combat the souring of young local wines, then turned mainly to dairy. Milk is heated to about 70 C for fifteen to thirty seconds, then cooled quickly to 10 C, a method Pasteur invented in 1862.
Jellying preserves food by cooking it in a material that solidifies into a gel, using substances such as gelatin, agar, maize flour, and arrowroot flour. Some animal flesh forms a protein gel on its own when cooked. Eels and elvers and sipunculid worms are a delicacy in Xiamen, China, while jellied eels are eaten with mashed potatoes in the East End of London.
British cuisine carries a rich tradition of potted meats. Until the 1950s, meat off-cuts were preserved in aspic, a gel made from gelatin and clarified meat broth. Potted shrimps could be set under a layer of fat and served on toast, and calf's foot jelly was once prepared for invalids.
Fruit preserves are jellying's sweeter branch, cooking fruits, vegetables, and sugar together for storage in glass jam jars and Mason jars. Jams, jellies, and marmalades are all fruit preserves that differ by the fruit used and how it is prepared. Some are sweet, made from strawberry or apricot, and others savory, made from tomatoes or squash. Making jam shows how methods stack: boiling reduces moisture and kills bacteria, sugaring prevents regrowth, and sealing in an airtight jar prevents recontamination.
Fermentation turns the enemy into an ally, cultivating specific microorganisms to combat spoilage from less benign ones. These organisms produce acid or alcohol that eventually creates an environment toxic to themselves and other microbes, holding pathogens in check. Cheeses, wines, and beers all owe their existence to this microbial conversion of starch and sugars into alcohol.
Water itself was turned into beer and wine through fermentation. Boiling during brewing could kill bacteria that might make people sick, while barley and other ingredients infused the drink with nutrients and the microorganisms produced vitamins as they fermented. The common belief that premodern people avoided ordinary water is a myth. They avoided dirty or polluted water, inspecting, smelling, tasting, filtering, and boiling it when needed, and used clean water for drinking, diluting wine, and cooking.
Biopreservation formalizes this old partnership using natural or controlled microbiota and antimicrobials to extend shelf life. Lactic acid bacteria are especially useful, releasing lactic acid, acetic acid, hydrogen peroxide, and peptide bacteriocins as they compete for nutrients. Some lactic acid bacteria produce nisin, a particularly effective preservative whose bacteriocins now form an integral part of hurdle technology.
Irradiation exposes food to ionizing radiation, using beta particles or gamma rays emitted from radioactive sources such as cobalt-60 or cesium-137. It can kill bacteria, molds, and insect pests, slow the ripening and spoiling of fruit, and at higher doses induce sterility. Because the product is not heated, it is sometimes called cold pasteurization. Consumers often fear it, based on the misconception that irradiated food becomes radioactive, which it does not and cannot. Approximately 500,000 tons of food are irradiated each year worldwide in over 40 countries, mainly spices and condiments.
Pulsed electric field electroporation processes cells with brief pulses of a strong electric field that enlarge the pores of cell membranes, killing the cells and releasing their contents. This developing, low-temperature technology has reached the market in juice pasteurization in Europe and the US. Potato processors show great interest in it, with operational applications already running in the US and Canada.
High-pressure food preservation disables harmful microorganisms and spoilage enzymes while keeping the food's fresh appearance, flavor, texture, and nutrients. By 2005 it was widely sold for products ranging from orange juice to guacamole to deli meats. Nonthermal plasma takes a different tack, subjecting a food's surface to a flame of ionized gas molecules such as helium or nitrogen to kill micro-organisms there.
Hurdle technology rests on a simple idea: combine more than one approach so that a pathogen must overcome a series of obstacles to survive. The microbiologist Leistner defined it in 2000 as an intelligent combination of hurdles that secures microbial safety and stability along with the organoleptic and nutritional quality and the economic viability of food products. Organoleptic quality refers to a food's sensory properties, its look, taste, smell, and texture.
The hurdles themselves are the methods running through every preservation tradition. High temperature during processing and low temperature during storage, increased acidity and lowered water activity, reduced redox potential, and the presence of preservatives or biopreservatives. The intensity of each can be adjusted to the type of pathogen and its risk, meeting consumer preferences economically without sacrificing safety.
That balance is the larger story food preservation tells. Some traditional methods have a lower energy input and carbon footprint than modern ones, and reducing food waste cuts production costs while improving food security and environmental sustainability. In rural Afghanistan, grapes are sealed in disc-shaped vessels of mud and straw called kangina, which passively restrict gas exchange and water loss to keep late-harvested grapes alive for up to six months. A single clay vessel, doing quietly what an electric field does with a jolt.
Common questions
What is food preservation and how does it work?
Food preservation includes processes that make food more resistant to microorganism growth and slow the oxidation of fats, which slows decomposition and rancidification. It can also inhibit visual deterioration, such as the enzymatic browning of cut apples. Many processes combine more than one method, such as making jam through boiling, sugaring, and sealing.
Who invented canning and when was it used by the French Navy?
Canning was invented by the French confectioner Nicolas Appert. By 1806 the process was used by the French Navy to preserve meat, fruit, vegetables, and even milk. The reason it worked was not understood until 1864, when Louis Pasteur found the relationship between microorganisms, food spoilage, and illness.
Why is processed meat considered carcinogenic?
In 2015, the International Agency for Research on Cancer of the World Health Organization classified processed meat as carcinogenic to humans. Processed meat means meat that has undergone salting, curing, and smoking. Curing can also involve nitrites, which contribute a characteristic pink color to the meat.
What is hurdle technology in food preservation?
Hurdle technology ensures that pathogens in food are eliminated or controlled by combining more than one approach, treated as hurdles a pathogen must overcome. Leistner defined it in 2000 as an intelligent combination of hurdles that secures microbial safety, stability, organoleptic and nutritional quality, and economic viability. Examples include high temperature, low temperature, increased acidity, and lowered water activity.
How does food irradiation work and is irradiated food radioactive?
Food irradiation exposes food to ionizing radiation such as beta particles or gamma rays from sources like cobalt-60 or cesium-137, killing bacteria, molds, and insect pests. Irradiated food does not and cannot become radioactive. Approximately 500,000 tons of food are irradiated per year worldwide in over 40 countries, mainly spices and condiments.
What is kangina and how long can it preserve grapes?
Kangina are disc-shaped vessels made of mud and straw used in rural Afghanistan to preserve grapes. The vessels can preserve fresh grapes for up to six months by passively controlling their internal environments to restrict gas exchange and water loss. This prolongs the lives of late-harvested grapes stored within them.
How does pasteurization preserve milk?
Pasteurization is a process for preserving liquid food, invented by Louis Pasteur in 1862, and was originally applied to combat the souring of young local wines. Today it is mainly applied to dairy products. Milk is heated to about 70 C for 15 to 30 seconds to kill bacteria, then cooled quickly to 10 C and stored in sterilized bottles or pouches in cold places.