Food preservation is any technique used to slow the normal decay of food. There are many methods of food preservation. Some methods, such as drying and fermenting, were first used thousands of years ago. Other methods of preservation, such as canning, rely on technology and processes first developed in the 1800’s. Still other methods, such as irradiation, have developed even more recently.
Food preservation has long been necessary because the availability of food varies with the seasons. In the past, for example, many people were farmers and grew their own food. But many farm products can only be harvested during certain times of year. To ensure a steady supply of food throughout the year, people learned ways to preserve foods. In modern times, preservation methods enable the food industry to process, transport, and distribute food products around the world. In all developed countries, government agencies highly regulate the food industry to ensure that food is safely preserved.
How food spoils
All food will eventually spoil if not preserved. Some foods, such as nuts and grains, can be stored for months with almost no treatment. Other foods, such as milk and meat, stay fresh only one or two days without preservation. The time that a particular food can be maintained in edible condition is called its shelf life. Preservation can extend a food’s shelf life, but not indefinitely.
Food is subject to three predominant kinds of deterioration or spoilage. They are (1) biological, (2) chemical, and (3) physical deterioration.
Biological deterioration
results from harmful microorganisms that grow in food. Microorganisms include bacteria, molds, and yeasts. Harmful microorganisms exist everywhere in the environment. They grow on virtually any surface, including on animal skins and vegetable peels. But such protective coverings generally stop harmful microorganisms from getting inside healthy living tissue. Thus, most cases of biological deterioration occur because of _contamination—_that is, the introduction of microorganisms into healthy food through a break in skin or weakened tissue.
Food spoils when harmful microorganisms multiply in the food. Bacteria multiply rapidly, producing acids, gases, and other chemicals in the process. Some chemicals produced by bacteria may be poisonous to human beings. For example, the bacteria Clostridium botulinum produces a dangerous toxin that causes a type of food poisoning called botulism. Botulism is often fatal to humans. Yeast and mold, which are larger and more complex than bacteria, can also multiply rapidly in food.
Seedlike structures called spores are of special concern in food preservation. All molds—and some bacteria and yeast—produce spores. Under proper conditions, spores can germinate (activate) and become full-sized cells. Spores can survive high temperatures, poisonous chemicals, and other extreme conditions. Food preservation methods are designed to inactivate bacterial spores, which are especially resistant.
Microorganisms are among the top causes of food loss. But not all microorganisms cause disease or food spoilage. In fact, some microorganisms are actually used to make and preserve foods. For example, yeast is used to make bread. Bacteria and molds are important in cheese making. Bacteria also create acids used to preserve pickled foods.
Chemical deterioration
is caused by reactions that change the molecular arrangement of food. Such chemical reactions may not be harmful in every circumstance. For example, chemical reactions involving molecules called enzymes ripen fruit after harvest. But beyond a certain point, enzymatic reactions cause fruit to become overripe and spoil. The weakened tissues enable microorganisms to penetrate and cause further damage. Foods can also deteriorate from chemical reactions involving light or oxygen.
Physical deterioration
involves large-scale damage to food products, such as being crushed, broken apart, or melted. Storing food at improper temperatures causes physical deterioration. In addition, such pests as insects and rodents cause physical deterioration.
How foods are preserved
A number of methods are used to preserve food. Preservation methods make use of heat, cold, drying, acid, salt and sugar, smoke, added chemicals, controlled air, and high-energy radiation. Any of these methods will cause deterioration of the food if used in excess. For instance, high heat over a long period will kill all microorganisms. However, too much heat ruins food’s flavor and texture. Often, food is preserved with a number of less extreme methods used in combination.
Heat
is effective for food preservation because most microorganisms grow best in the temperature range of 40 to 140 °F (4 to 60 °C). Most bacteria are killed in the temperature range of 170 to 200 °F (77 to 93 °C) if they are held at such temperatures for a long enough time. Some heat-resistant spores may require temperatures greater than 212 °F (100 °C), the boiling temperature of water.
Heat treatments range in severity. Blanching is a mild heat treatment in which food is briefly boiled or steamed. Blanching deactivates enzymes and reduces the number of microorganisms. Fruits and vegetables are frequently blanched prior to freezing.
Pasteurization is a more severe heat treatment. In pasteurization, food is held at a constant high temperature—typically below the boiling point of water—for a certain period. Pasteurization kills many dangerous microorganisms. But pasteurization is usually combined with other preservation methods, such as refrigeration, because pasteurized products still contain many living microorganisms.
The most severe heat treatment commonly used in food preservation is commercial sterilization. In commercial sterilization, all microorganisms that might spoil food under typical storage and handling conditions have been destroyed and will not grow in the food. Unlike medical equipment, however, commercially sterilized food is not completely sterile.
Canning
is a common method of food preservation in developed countries. The process ensures commercial sterilization. In canning, foods are sealed in airtight containers and are heated to destroy microorganisms that may cause spoilage. A wide variety of foods are canned. They include fruits, vegetables, fish, meat, poultry, and soups.
Before food is canned, it is thoroughly cleaned. Many foods, such as fruits and vegetables, are cut, sliced, or peeled before canning. After the raw food is prepared, the canning process follows five basic steps: (1) filling, (2) exhausting, (3) sealing, (4) processing, and (5) cooling.
Filling.
Food is placed in containers, typically metal cans or glass jars. Machines can fill more than a thousand containers in a minute. The empty space above the food in the container is called headspace. Headspace is carefully controlled. Too little headspace may cause cans to bulge during heating. Too much headspace results in underweight cans and shorter shelf life.
Exhausting
involves the removal of air in the headspace to form a partial vacuum in the container. Exhausting removes oxygen and thus slows the growth of many microorganisms that cause decay.
Sealing.
Machines seal several hundred metal cans per minute. Glass containers are sealed at a somewhat slower rate. Sealed containers are airtight.
Processing.
In processing, containers are heated to a carefully controlled temperature for a certain time. The time and temperature vary with the product being canned and the size of the container. During processing, microorganisms that may cause spoilage are destroyed. Containers are heated in cookers called retorts.
Cooling
follows processing to prevent overcooking. Containers may be cooled by transferring them from the retort into cold water. They may also be sprayed with cold water. Some canned products are first cooled partially by water and then fully cooled by air.
One of the disadvantages of canning is that the heat required for sterilization changes the food’s texture, color, and flavor. In addition, some nutrients are lost in the canning process. However, canned foods are popular with consumers because of their low cost, convenience, variety, and relatively long shelf life.
Cold storage,
including refrigeration and freezing, is used to control microorganisms and enzyme activity. Refrigeration keeps food fresh at temperatures above 32 °F (0 °C). Storage at or near that temperature largely prevents the growth and activity of most microorganisms that cause food spoilage. It also decreases enzyme activity that causes changes in the color, flavor, and texture of foods. Foods requiring refrigeration include eggs, fish, fruits, meats, milk, and vegetables.
Frozen storage maintains the food in the frozen state. Freezing prevents microbial activity and slows down enzymatic deterioration. Though most foods contain large amounts of water, food freezes differently than pure water. The solutes (dissolved sugars and salts) in foods reduce the initial freezing point to lower than 32 °F (0 °C). Typically, foods begin to freeze at around 31 to 28 °F (–0.5 to –2 °C). Frozen foods are stored at or below 0 °F (–18 °C).
Vegetables are among the main foods preserved by freezing. Before freezing, vegetables are first blanched. Blanching prevents enzymes from changing the flavor and texture of vegetables during frozen storage. Other foods preserved by freezing include fish, juices, meat, and poultry. Before freezing, food may be cleaned, peeled, or prepared in other ways. Some foods, such as frozen entrees, are cooked before freezing.
Some common commercial equipment freezes food with cold air. Such freezers ensure that cold air is constantly moving around the food product. Moving air carries heat away from food much more quickly than does still air. Air-blast freezers use a fan to create a strong wind. This wind, combined with temperatures as low as –40 °F (–40 °C), causes rapid freezing of foods. In many air-blast freezers, packages of food travel slowly through the unit on a conveyor belt until frozen.
Other commercial equipment freezes food through the use of solid surfaces or liquids. In plate freezers, food is frozen through contact with cold surfaces. Such freezers are cabinets with shelves that contain refrigerants. Food is placed on or in between the shelves. The cabinet is then closed until the food freezes. Cryogenic freezers use liquid nitrogen, liquid carbon dioxide, or solid carbon dioxide to freeze foods. These substances form at extremely low temperatures and rapidly freeze food. Food products that take several hours to freeze in an air-blast freezer may require only minutes in a cryogenic freezer.
Drying
is the removal of water from food. Although there are a number of ways to remove water, usually “drying” means thermal drying. Thermal drying uses heat to remove moisture from food. The microorganisms that cause food spoilage require moisture to survive. After much of the water is removed from foods, microorganisms cannot grow or cause food to spoil. Hundreds of foods are dried, including eggs, milk, mushrooms, peas, raisins, and soups.
Some thermal drying methods make use of hot air. In sun drying or solar drying, food is spread out in thin layers in hot, sunny weather. The sun’s heat dries the food. Fruits and grains are often dried in this manner. Convection air drying uses moving hot air to dry food. A fan may blow air around food that is put in a sealed cabinet. Alternately, the food may move on conveyor belts through hot air. Spray drying involves spraying droplets of liquid foods or slurries (mixtures of liquids and finely ground solid particles) into a large, heated chamber. Hot air is also blown into the chamber. The hot air dries the food droplets to form powders. Milk is often dried by this method.
Freeze-drying is typically done in a low air environment. Under partial vacuum conditions, ice can change directly from a solid to a gas, without first becoming a liquid. This process, called sublimation, is the basis of freeze-drying. As the ice vaporizes, the food maintains its shape, but becomes a porous (spongelike) dry solid. Freeze-drying is a more expensive process than air drying methods, so it is used for fewer types of foods. These foods are often heat-sensitive, such as coffee and tea, or highly valued and perishable items, such as mushrooms, shrimp, and strawberries.
Acid
can preserve foods by creating an environment in which spoilage microorganisms will not grow. The acid may be present in the food naturally, such as in berries, apples, and citrus fruits. Otherwise, food makers may add acids to the product. Such added acids include citric acid and acetic acid. Some pickled vegetables are preserved by acetic acid added as vinegar.
Acid may also be produced in foods by acid-making bacteria. This process is called fermentation. In contrast to other preservation methods, fermentation encourages the growth of certain kinds of bacteria. The bacteria most often used in fermented foods produce acetic acid or lactic acid. These two groups of bacteria are used in the production of many foods, including kimchi, salami, sauerkraut, sour cream, vinegar, and many cheeses.
Salt and sugar
are solutes that dissolve in water. The solutes draw water out of bacteria, yeast, and mold present in food. The microorganisms become dehydrated and cannot grow or multiply. The high sugar content in fruit jams, for example, enables them to last much longer than fresh fruits. Meats and fish are preserved with salt in a process called curing. Often curing also involves chemicals called nitrates and nitrites, as well as sugar and spices for flavoring.
In general, yeasts and molds are more tolerant of high solute conditions than are bacteria. For this reason, yeasts and molds may grow on such foods as jam.
Smoking
foods can preserve them through both heat and chemicals in the smoke. For example, wood smoke contains small amounts of formaldehyde and other chemicals that preserve food. Smoked products, such as many types of ham and bacon, are often cured first. The smoking is primarily done for flavor, rather than for preservation.
Additives
include chemicals added to food to prevent spoilage. There are many chemicals that can kill microorganisms or inhibit their growth. But most of these chemicals are not permitted as food additives. When a chemical is permitted for use, it is added at low levels to control microorganisms and to maintain quality. Additives generally require approval by the government before they can be used. Common preservatives include sodium benzoate, sorbic acid, and sulfur dioxide.
Controlled air,
used in controlled-atmosphere storage, changes the composition of air in order to preserve food. The normal composition of air is 21 percent oxygen and 78 percent nitrogen, and 1 percent other gases. In a controlled atmosphere, the gas mixture is much lower in oxygen and higher in carbon dioxide than normal. The rest of the air is typically made up of nitrogen, which has no effect on food. The exact mix of gases depends on the product being preserved.
Fresh fruits and vegetables are stored in controlled atmospheres. Harvested fruits and vegetables are living tissues that respire. Respiration is the reaction of oxygen and sugars to produce carbon dioxide, water, and heat. During storage, it is important to reduce the rate of respiration to maintain the quality of fruits and vegetables. Air with less oxygen and more carbon dioxide helps slow respiration. Lower air temperatures also help reduce the rate of respiration.
Irradiation
makes use of ionizing radiation to kill or deactivate microorganisms in food. Ionizing radiation includes such high-energy forms of electromagnetic radiation as X rays and gamma rays. It also includes beams of electrons. Ionizing radiation can change the structure of atoms by stripping away the electrons that normally surround an atom’s nucleus (core). These changes can destroy genetic material in cells, killing bacteria and other organisms that grow in food. Low doses of such radiation cause little or no chemical change in the food itself.
Food irradiation is used in dozens of countries. Commonly irradiated foods include red meat, poultry, various grains, spices, and many kinds of fruits and vegetables. However, the specific foods allowed to be treated by radiation vary by country. Consumer safety concerns have led many countries to require labeling on irradiated food.
History
Prehistoric people probably dried grains, nuts, fruits, roots, and other plant products in the sun. People who lived in cold climates likely kept food outside their caves or huts in the winter to prevent spoilage. In warmer climates, people probably stored foods in caves and other cooler places.
Early preservation.
After fire was discovered, people probably dried fish and meat over a fire. Drying by fire may have led to the development of smoking as a method of preservation. Salt curing and fermentation are two other early methods of preservation. Ancient people salted meat or fish to prevent spoiling. Nomadic peoples in Asia used fermentation to make cheese. Fermentation of fruit juice to make wine also dates back to ancient times.
Modern food preservation
began in the 1700’s. Lazzaro Spallanzani, an Italian naturalist, sealed broth in glass flasks and heated them for an hour or more. The broth remained sterile as long as the flasks remained sealed.
By the early 1800’s, Nicolas Appert, a candy maker from Paris, had worked out a canning process in which food was packed in glass jars. The jars were tightly sealed with corks and then heated in boiling water. Appert also published the first book on canning, which gave specific canning methods for more than 50 foods. Although Appert had made a major contribution to canning, he did not understand why his process worked. This understanding came 50 years later when the French chemist Louis Pasteur discovered that heat kills harmful microorganisms (see Pasteur, Louis).
Cold storage had long been used to preserve foods. But special techniques were required to keep food cold in hot weather. At first, people cut ice from ponds and lakes during the winter, storing it in insulated buildings called ice houses. The insulation slowed its melting in the summer. In 1851, the first commercial machine for making ice was patented by John Gorrie, an American physician. Although Gorrie’s business was a commercial failure, the large-scale use of refrigeration became important over the next several decades. By the early 1900’s, there were about 2,000 ice plants in the United States.
Advances in refrigeration made possible frozen foods. In the mid-1920’s, Clarence Birdseye, an American inventor, developed the first modern quick-freezing process. He used moving, chilled metal belts to quick-freeze fish.
Food was not dried in great volume in the United States until World War I (1914-1918), when dried food became important to feed soldiers. World War II (1939-1945) created further demand for dried food.
Scientists began experimenting with food irradiation in the early 1900’s. In 1953, the United States formed the National Food Irradiation Program. Since that time, the FDA has approved a number of irradiation methods for various food products. In 2000, for example, the FDA approved irradiating fresh eggs to control salmonella bacteria.
