Food Preservation

Resources

The term food preservation refers to any one of a number of techniques used to prevent food from spoiling. It includes methods such as canning, pickling, drying and freeze-drying, irradiation, pasteurization, smoking, and the addition of chemical additives. Food preservation has become an increasingly important component of the food industry as fewer people eat foods produced on their own lands, and as consumers expect to be able to purchase and consume foods that are out of season.

The vast majority of instances of food spoilage can be attributed to one of two major causes: (1) the attack by pathogens (disease-causing microorganisms) such as bacteria and molds, or (2) oxidation that causes the destruction of essential biochemical compounds and/or the destruction of plant and animal cells. The various methods that have been devised for preserving foods are all designed to reduce or eliminate one or the other (or both) of these causative agents.

For example, a simple and common method of preserving food is by heating it to some minimum temperature. This process prevents or retards spoilage because high temperatures kill or inactivate most kinds of pathogens. The addition of compounds known as BHA and BHT to foods also prevents spoilage in another different way. These compounds are known to act as antioxidants, preventing chemical reactions that cause the oxidation of food that results in its spoilage. Almost all techniques of preservation are designed to extend the life of food by acting in one of these two ways.

The search for methods of food preservation probably can be traced to the dawn of human civilization. People who lived through harsh winters found it necessary to find some means of insuring a food supply during seasons when no fresh fruits and vegetables were available. Evidence for the use of dehydration (drying) as a method of food preservation, for example, goes back at least 5,000 years. Among the most primitive forms of food preservation that are still in use today are such methods as smoking, drying, salting, freezing, and fermenting.

Early humans probably discovered by accident that certain foods exposed to smoke seem to last longer than those that are not. Meats, fish, fowl, and cheese were among such foods. It appears that compounds present in wood smoke have anti-microbial actions that prevent the growth of organisms that cause spoilage. today, the process of smoking has become a sophisticated method of food preservation with both hot and cold forms in use. Hot smoking is used primarily with fresh or frozen foods, while cold smoking is used most often with salted products. The most advantageous conditions for each kind of smoking—air velocity, relative humidity, length of exposure, and salt content, for example—are now generally understood and applied during the smoking process. For example, electrostatic precipitators can be employed to attract smoke particles and improve the penetration of the particles into meat or fish. So many alternative forms of preservation are now available that smoking no longer holds the position of importance it once did with ancient peoples. More frequently, the process is used to add interesting and distinctive flavors to foods.

Because most disease-causing organisms require a moist environment in which to survive and multiply, drying is a natural technique for preventing spoilage. Indeed, the act of simply leaving foods out in the sun and wind to dry out is probably one of the earliest forms of food preservation. Evidence for the drying of meats, fish, fruits, and vegetables go back to the earliest recorded human history. At some point, humans also learned that the drying process could be hastened and improved by various mechanical techniques. For example, the Arabs learned early on that apricots could be preserved almost indefinitely by macerating them, boiling them, and then leaving them to dry on broad sheets. The product of this technique, quamar-adeen, is still made by the same process in modern Muslim countries.

Today, a host of dehydrating techniques are known and used. The specific technique adopted depends on the properties of the food being preserved. For example, a traditional method for preserving rice is to allow it to dry naturally in the fields or on drying racks in barns for about two weeks. After this period of time, the native rice is threshed and then dried again by allowing it to sit on straw mats in the sun for about three days. Modern drying techniques make use of fans and heaters in controlled environments. Such methods avoid the uncertainties that arise from leaving crops in the field to dry under natural conditions. Controlled temperature air drying is especially popular for the preservation of grains such as maize, barley, and bulgur.

Vacuum drying is a form of preservation in which a food is placed in a large container from which air is removed. Water vapor pressure within the food is greater than that outside of it, and water evaporates more quickly from the food than in a normal atmosphere. Vacuum drying is biologically desirable since some enzymes that cause oxidation of foods become active during normal air drying. These enzymes do not appear to be as active under vacuum drying conditions, however. Two of the special advantages of vacuum drying are that the process is more efficient at removing water from a food product, and it takes place more quickly than air drying. In one study, for example, the drying time of a fish fillet was reduced from about 16 hours by air drying to six hours as a result of vacuum drying.

Coffee drinkers are familiar with the process of dehydration known as spray drying. In this process, a concentrated solution of coffee in water is sprayed though a disk with many small holes in it. The surface area of the original coffee grounds is increased many times, making dehydration of the dry product much more efficient. Freeze-drying is a method of preservation that makes use of the physical principle known as sublimation. Sublimation is the process by which a solid passes directly to the gaseous phase without first melting. Freeze-drying is a desirable way of preserving food because at low temperatures (commonly around 14°F to –13°F [–10°C to –25°C]) chemical reactions take place very slowly and pathogens have difficulty surviving. The food to be preserved by this method is first frozen and then placed into a vacuum chamber. Water in the food first freezes and then sublimes, leaving a moisture content in the final product of as low as 0.5%.

The precise mechanism by which salting preserves food is not entirely understood. It is known that salt binds with water molecules and thus acts as a dehydrating agent in foods. A high level of salinity may also impair the conditions under which pathogens can survive. In any case, the value of adding salt to foods for preservation has been well known for centuries. Sugar appears to have effects similar to those of salt in preventing spoilage of food. The use of either compound (and of certain other natural materials) is known as curing. A desirable side effect of using salt or sugar as a food preservative is, of course, the pleasant flavor each compound adds to the final product.

Curing can be accomplished in a variety of ways. Meats can be submerged in a salt solution known as brine, for example, or the salt can be rubbed on the meat by hand. The injection of salt solutions into meats has also become popular. Food scientists have now learned that a number of factors relating to the food product and to the preservative conditions affect the efficiency of curing. Some of the food factors include the type of food being preserved, the fat content, and the size of treated pieces. Preservative factors include brine temperature and concentration, and the presence of impurities.

Curing is used with certain fruits and vegetables, such as cabbage (in the making of sauerkraut), cucumbers (in the making of pickles), and olives. It is probably most popular, however, in the preservation of meats and fish. Honey-cured hams, bacon, and corned beef (“corn” is a term for a form of salt crystals) are common examples.

Freezing is an effective form of food preservation because the pathogens that cause food spoilage are killed or do not grow very rapidly at reduced temperatures. The process is less effective in food preservation than are thermal techniques such as boiling because pathogens are more likely to be able to survive cold temperatures than hot temperatures. In fact, one of the problems surrounding the use of freezing as a method of food preservation is the danger that pathogens deactivated (but not killed) by the process will once again become active when the frozen food thaws.

A number of factors are involved in the selection of the best approach to the freezing of foods, including the temperature to be used, the rate at which freezing is to take place, and the actual method used to freeze the food. Because of differences in cellular composition, foods actually begin to freeze at different temperatures ranging from about 31°F (–0.6°C) for some kinds of fish to 19°F (–7°C) for some kinds of fruits.

The rate at which food is frozen is also a factor, primarily because of aesthetic reasons. The more slowly food is frozen, the larger the ice crystals that are formed. Large ice crystals have the tendency to cause rupture of cells and the destruction of texture in meats, fish, vegetables, and fruits. In order to deal with this problem, the technique of quick-freezing has been developed. In quick-freezing, a food is cooled to or below its freezing point as quickly as possible. The product thus obtained, when thawed, tends to have a firm, more natural texture than is the case with most slow-frozen foods.

About a half dozen methods for the freezing of foods have been developed. One, described as the plate, or contact, freezing technique, was invented by the American inventor Charles Birdseye in 1929. In this method, food to be frozen is placed on a refrigerated plate and cooled to a temperature less than its freezing point. Alternatively, the food may be placed between two parallel refrigerated plates and frozen. Another technique for freezing foods is by immersion in very cold liquids. At one time, sodium chloride brine solutions were widely used for this purpose. A 10% brine solution, for example, has a freezing point of about 21°F (–6°C), well within the desired freezing range for many foods. More recently, liquid nitrogen has been used for immersion freezing. The temperature of liquid nitrogen is about –320°F (–195.5°C), so that foods immersed in this substance freeze very quickly.

As with most methods of food preservation, freezing works better with some foods than with others. Fish, meat, poultry, and citrus fruit juices (such as frozen orange juice concentrate) are among the foods most commonly preserved by this method.

Fermentation is a naturally occurring chemical reaction by which a natural food is converted into another form by pathogens. It is a process in which food spoils, but results in the formation of an edible product. Perhaps the best example of such a food is cheese. Fresh milk does not remain in edible condition for a very long period of time. Its pH is such that harmful pathogens begin to grow in it very rapidly. Early humans discovered, however, that the spoilage of milk can be controlled in such a way as to produce a new product, cheese.

Bread is another food product made by the process of fermentation. Flour, water, sugar, milk, and other raw materials are mixed together with yeasts and then baked. The addition of yeasts brings about the fermentation of sugars present in the mixture, resulting in the formation of a product that will remain edible much longer than will the original raw materials used in the bread-making process.

Heating food is an effective way of preserving it because the great majority of harmful pathogens are killed at temperatures close to the boiling point of water. In this respect, heating foods is a form of food preservation comparable to that of freezing but much superior to it in its effectiveness. A preliminary step in many other forms of food preservation, especially forms that make use of packaging, is to heat the foods to temperatures sufficiently high to destroy pathogens.

In many cases, foods are actually cooked prior to their being packaged and stored. In other cases, cooking is neither appropriate nor necessary. The most familiar example of the latter situation is pasteurization. During the 1860s, the French bacteriologist Louis Pasteur (1822-1895) discovered that pathogens in foods could be destroyed by heating those foods to a certain minimum temperature. The process was particularly appealing for the preservation of milk since preserving milk by boiling is not a practical approach. Conventional methods of pasteurization called for the heating of milk to a temperature between 145 and 149°F (63-65°C) for a period of about 30 minutes, and then cooling it to room temperature. In a more recent revision of that process, milk can also be “flash-pasteurized” by raising its temperature to about 160°F (71°C) for a minimum of 15 seconds, with equally successful results. A process known as ultra-high-pasteurization uses even higher temperatures, of the order of 194-266° F (90-130° C), for periods of a second or more.

One of the most common methods for preserving foods today is to enclose them in a sterile container. The term “canning” refers to this method although the specific container can be glass, plastic, or some other material as well as a metal can, from which the procedure originally obtained its name. The basic principle behind canning is that a food is sterilized, usually by heating, and then placed within an airtight container. In the absence of air, no new pathogens can gain access to the sterilized food. In most canning operations, the food to be packaged is first prepared—cleaned, peeled, sliced, chopped, or treated in some other way—and then placed directly into the container. The container is then placed in hot water or some other environment where its temperature is raised above the boiling point of water for some period of time. This heating process achieves two goals at once. First, it kills the vast majority of pathogens that may be present in the container. Second, it forces out most of the air above the food in the container.

After heating has been completed, the top of the container is sealed. In home canning procedures, one way of sealing the (usually glass) container is to place a layer of melted paraffin directly on top of the food. As the paraffin cools, it forms a tight solid seal on top of the food. Instead of or in addition to the paraffin seal, the container is also sealed with a metal screw top containing a rubber gasket. The first glass jar designed for this type of home canning operation, the Mason jar, was patented in 1858.

The commercial packaging of foods frequently makes use of tin, aluminum, or other kinds of metallic cans. The technology for this kind of canning was first developed in the mid-1800s, when individual workers hand-sealed cans after foods had been cooked within them. At this stage, a single worker could seldom produce more than 100 “canisters” (from which the word “can” later came) of food a day. With the development of far more efficient canning machines in the late nineteenth century, the mass production of canned foods became a reality.

As with home canning, the process of preserving foods in metal cans is simple in concept. The foods are prepared and the empty cans are sterilized. The prepared foods are then added to the sterile metal can, the filled can is heated to a sterilizing temperature, and the cans are then sealed by a machine. Modern machines

KEY TERMS

Additive —A chemical compound that is added to foods to give them some desirable quality, such as preventing them from spoiling.

Antioxidant —A chemical compound that has the ability to prevent the oxidation of substances with which it is associated.

Curing —A term used for various methods of preserving foods, most commonly by treating them with salt or sugar.

Dehydration —The removal of water from a material.

Fermentation —A chemical reaction in which sugars are converted to organic acids.

Irradiation —The process by which some substance, such as a food, is exposed to some form of radiation, such as gamma rays or x rays.

Oxidation —A chemical reaction in which oxygen reacts with some other substance.

Pasteurization —A method for treating milk and other liquids by heating them to a high enough temperature for a long enough period of time to kill or inactivate any pathogens present in the liquid.

Pathogen —A disease causing microorganism such as a mold or a bacterium.

are capable of moving a minimum of 1,000 cans per minute through the sealing operation.

The majority of food preservation operations used today also employ some kind of chemical additive to reduce spoilage. Of the many dozens of chemical additives available, all are designed either to kill or retard the growth of pathogens or to prevent or retard chemical reactions that result in the oxidation of foods. Some familiar examples of the former class of food additives are sodium benzoate and benzoic acid; calcium, sodium propionate, and propionic acid; calcium, potassium, sodium sorbate, and sorbic acid; and sodium and potassium sulfite. Examples of the latter class of additives include calcium, sodium ascorbate, and ascorbic acid (vitamin C); butylated hydroxyanisole (BHA) and buty-lated hydroxytoluene (BHT); lecithin; and sodium and potassium sulfite and sulfur dioxide.

A special class of additives that reduce oxidation is known as the sequestrants. Sequestrants are compounds that “capture” metallic ions, such as those of copper, iron, and nickel, and remove them from contact with foods. The removal of these ions helps preserve foods because in their free state they increase the rate at which oxidation of foods takes place. Some examples of sequestrants used as food preservatives are ethylenediamine-tetraacetic acid (EDTA), citric acid, sorbitol, and tartaric acid.

Resources

BOOKS

Considine, Glenn D. Van Nostrand’s Scientific Encyclopedia. Glenn D Considine New York: Wiley-Interscience, 2002.

Jango-Cohen, Judith. The History of Food (Major Inventions Through History). Breckenridge, CO: Twenty-First Century Books, 2005.

Matthews, Karl R. Microbiology of Fresh Produce (Emerging Issues in Food safety). Washington DC: ASM Press, 2005.

McSwane, David, Richard Linton, and Nancy R. Rue. Essentials of Food Safety and Sanitation (4th edition). New York: Prentice-Hall, 2004.

PERIODICALS

Hwang, Deng Fwu. “Tetrodotoxin In Gastropods (Snails) Implicated In Food Poisoning.” Journal of Food Protection 65, no. 8 (2002): 1341-1344.

“Preventing Food Poisoning.” Professional Nurse 18, no. 4 (2002): 185-186.

Zurer, Pamela S. “Food Irradiation: A Technology at a Turning Point.” Chemical & Engineering News (May 5, 1986): 46-56.

Brian D. Hoyle

Food preservation

The term food preservation refers to any one of a number of techniques used to prevent food from spoiling. It includes methods such as canning, pickling, drying and freeze-drying, irradiation, pasteurization , smoking, and the addition of chemical additives. Food preservation has become an increasingly important component of the food industry as fewer people eat foods produced on their own lands, and as consumers expect to be able to purchase and consume foods that are out of season.

The vast majority of instances of food spoilage can be attributed to one of two major causes: (1) the attack by pathogens (disease-causing microorganisms ) such as bacteria and molds, or (2) oxidation that causes the destruction of essential biochemical compounds and/or the destruction of plant and animal cells. The various methods that have been devised for preserving foods are all designed to reduce or eliminate one or the other (or both) of these causative agents.

For example, a simple and common method of preserving food is by heating it to some minimum temperature. This process prevents or retards spoilage because high temperatures kill or inactivate most kinds of pathogens. The addition of compounds known as BHA and BHT to foods also prevents spoilage in another different way. These compounds are known to act as antioxidants, preventing chemical reactions that cause the oxidation of food that results in its spoilage. Almost all techniques of preservation are designed to extend the life of food by acting in one of these two ways.

The search for methods of food preservation probably can be traced to the dawn of human civilization. People who lived through harsh winters found it necessary to find some means of insuring a food supply during seasons when no fresh fruits and vegetables were available. Evidence for the use of dehydration (drying) as a method of food preservation, for example, goes back at least 5,000 years. Among the most primitive forms of food preservation that are still in use today are such methods as smoking, drying, salting, freezing, and fermenting.

Early humans probably discovered by accident that certain foods exposed to smoke seem to last longer than those that are not. Meats, fish, fowl, and cheese were among such foods. It appears that compounds present in wood smoke have antimicrobial actions that prevent the growth of organisms that cause spoilage. Today, the process of smoking has become a sophisticated method of food preservation with both hot and cold forms in use. Hot smoking is used primarily with fresh or frozen foods, while cold smoking is used most often with salted products. The most advantageous conditions for each kind of smoking—air velocity, relative humidity, length of exposure, and salt content, for example—are now generally understood and applied during the smoking process. For example, electrostatic precipitators can be employed to attract smoke particles and improve the penetration of the particles into meat or fish. So many alternative forms of preservation are now available that smoking no longer holds the position of importance it once did with ancient peoples. More frequently, the process is used to add interesting and distinctive flavors to foods.

Because most disease-causing organisms require a moist environment in which to survive and multiply, drying is a natural technique for preventing spoilage. Indeed, the act of simply leaving foods out in the sun and wind to dry out is probably one of the earliest forms of food preservation. Evidence for the drying of meats, fish, fruits, and vegetables go back to the earliest recorded human history. At some point, humans also learned that the drying process could be hastened and improved by various mechanical techniques. For example, the Arabs learned early on that apricots could be preserved almost indefinitely by macerating them, boiling them, and then leaving them to dry on broad sheets. The product of this technique, quamaradeen, is still made by the same process in modern Muslim countries.

Today, a host of dehydrating techniques are known and used. The specific technique adopted depends on the properties of the food being preserved. For example, a traditional method for preserving rice is to allow it to dry naturally in the fields or on drying racks in barns for about two weeks. After this period of time, the native rice is threshed and then dried again by allowing it to sit on straw mats in the sun for about three days. Modern drying techniques make use of fans and heaters in controlled environments. Such methods avoid the uncertainties that arise from leaving crops in the field to dry under natural conditions. Controlled temperature air drying is especially popular for the preservation of grains such as maize, barley, and bulgur.

Vacuum drying is a form of preservation in which a food is placed in a large container from which air is removed. Water vapor pressure within the food is greater than that outside of it, and water evaporates more quickly from the food than in a normal atmosphere. Vacuum drying is biologically desirable since some enzymes that cause oxidation of foods become active during normal air drying. These enzymes do not appear to be as active under vacuum drying conditions, however. Two of the special advantages of vacuum drying are that the process is more efficient at removing water from a food product, and it takes place more quickly than air drying. In one study, for example, the drying time of a fish fillet was reduced from about 16 hours by air drying to six hours as a result of vacuum drying.

Coffee drinkers are familiar with the process of dehydration known as spray drying. In this process, a concentrated solution of coffee in water is sprayed though a disk with many small holes in it. The surface area of the original coffee grounds is increased many times, making dehydration of the dry product much more efficient. Freeze-drying is a method of preservation that makes use of the physical principle known as sublimation. Sublimation is the process by which a solid passes directly to the gaseous phase without first melting. Freeze-drying is a desirable way of preserving food because at low temperatures (commonly around 14°F to –13°F [–10°C to –25°C]) chemical reactions take place very slowly and pathogens have difficulty surviving. The food to be preserved by this method is first frozen and then placed into a vacuum chamber. Water in the food first freezes and then sublimes, leaving a moisture content in the final product of as low as 0.5%.

The precise mechanism by which salting preserves food is not entirely understood. It is known that salt binds with water molecules and thus acts as a dehydrating agent in foods. A high level of salinity may also impair the conditions under which pathogens can survive. In any case, the value of adding salt to foods for preservation has been well known for centuries. Sugar appears to have effects similar to those of salt in preventing spoilage of food. The use of either compound (and of certain other natural materials) is known as curing. A desirable side effect of using salt or sugar as a food preservative is, of course, the pleasant flavor each compound adds to the final product.

Curing can be accomplished in a variety of ways. Meats can be submerged in a salt solution known as brine, for example, or the salt can be rubbed on the meat by hand. The injection of salt solutions into meats has also become popular. Food scientists have now learned that a number of factors relating to the food product and to the preservative conditions affect the efficiency of curing. Some of the food factors include the type of food being preserved, the fat content, and the size of treated pieces. Preservative factors include brine temperature and concentration, and the presence of impurities.

Curing is used with certain fruits and vegetables, such as cabbage (in the making of sauerkraut), cucumbers (in the making of pickles), and olives. It is probably most popular, however, in the preservation of meats and fish. Honey-cured hams, bacon, and corned beef ("corn" is a term for a form of salt crystals) are common examples.

Freezing is an effective form of food preservation because the pathogens that cause food spoilage are killed or do not grow very rapidly at reduced temperatures. The process is less effective in food preservation than are thermal techniques such as boiling because pathogens are more likely to be able to survive cold temperatures than hot temperatures. In fact, one of the problems surrounding the use of freezing as a method of food preservation is the danger that pathogens deactivated (but not killed) by the process will once again become active when the frozen food thaws.

A number of factors are involved in the selection of the best approach to the freezing of foods, including the temperature to be used, the rate at which freezing is to take place, and the actual method used to freeze the food. Because of differences in cellular composition, foods actually begin to freeze at different temperatures ranging from about 31°F (–0.6°C) for some kinds of fish to 19°F (–7°C) for some kinds of fruits.

The rate at which food is frozen is also a factor, primarily because of aesthetic reasons. The more slowly food is frozen, the larger the ice crystals that are formed. Large ice crystals have the tendency to cause rupture of cells and the destruction of texture in meats, fish, vegetables, and fruits. In order to deal with this problem, the technique of quick-freezing has been developed. In quick-freezing, a food is cooled to or below its freezing point as quickly as possible. The product thus obtained, when thawed, tends to have a firm, more natural texture than is the case with most slow-frozen foods.

About a half dozen methods for the freezing of foods have been developed. One, described as the plate, or contact, freezing technique, was invented by the American inventor Charles Birdseye in 1929. In this method, food to be frozen is placed on a refrigerated plate and cooled to a temperature less than its freezing point. Alternatively, the food may be placed between two parallel refrigerated plates and frozen. Another technique for freezing foods is by immersion in very cold liquids. At one time, sodium chloride brine solutions were widely used for this purpose. A 10% brine solution, for example, has a freezing point of about 21°F (–6°C), well within the desired freezing range for many foods. More recently, liquid nitrogen has been used for immersion freezing. The temperature of liquid nitrogen is about –320°F (–195.5°C), so that foods immersed in this substance freeze very quickly.

As with most methods of food preservation, freezing works better with some foods than with others. Fish, meat, poultry, and citrus fruit juices (such as frozen orange juice concentrate) are among the foods most commonly preserved by this method.

Fermentation is a naturally occurring chemical reaction by which a natural food is converted into another form by pathogens. It is a process in which food spoils, but results in the formation of an edible product. Perhaps the best example of such a food is cheese. Fresh milk does not remain in edible condition for a very long period of time. Its pH is such that harmful pathogens begin to grow in it very rapidly. Early humans discovered, however, that the spoilage of milk can be controlled in such a way as to produce a new product, cheese.

Bread is another food product made by the process of fermentation. Flour, water, sugar, milk, and other raw materials are mixed together with yeasts and then baked. The addition of yeasts brings about the fermentation of sugars present in the mixture, resulting in the formation of a product that will remain edible much longer than will the original raw materials used in the bread-making process.

Heating food is an effective way of preserving it because the great majority of harmful pathogens are killed at temperatures close to the boiling point of water. In this respect, heating foods is a form of food preservation comparable to that of freezing but much superior to it in its effectiveness. A preliminary step in many other forms of food preservation, especially forms that make use of packaging, is to heat the foods to temperatures sufficiently high to destroy pathogens.

In many cases, foods are actually cooked prior to their being packaged and stored. In other cases, cooking is neither appropriate nor necessary. The most familiar example of the latter situation is pasteurization. During the 1860s, the French bacteriologist Louis Pasteur discovered that pathogens in foods could be destroyed by heating those foods to a certain minimum temperature. The process was particularly appealing for the preservation of milk since preserving milk by boiling is not a practical approach. Conventional methods of pasteurization called for the heating of milk to a temperature between 145 and 149°F (63 and 65°C) for a period of about 30 minutes, and then cooling it to room temperature. In a more recent revision of that process, milk can also be "flash-pasteurized" by raising its temperature to about 160°F (71°C) for a minimum of 15 seconds, with equally successful results. A process known as ultra-high-pasteurization uses even higher temperatures, of the order of 194–266°F (90–130°C), for periods of a second or more.

One of the most common methods for preserving foods today is to enclose them in a sterile container. The term "canning" refers to this method although the specific container can be glass, plastic, or some other material as well as a metal can, from which the procedure originally obtained its name. The basic principle behind canning is that a food is sterilized, usually by heating, and then placed within an air-tight container. In the absence of air, no new pathogens can gain access to the sterilized food. In most canning operations, the food to be packaged is first prepared in some way—cleaned, peeled, sliced, chopped, or treated in some other way—and then placed directly into the container. The container is then placed in hot water or some other environment where its temperature is raised above the boiling point of water for some period of time. This heating process achieves two goals at once. First, it kills the vast majority of pathogens that may be present in the container. Second, it forces out most of the air above the food in the container.

After heating has been completed, the top of the container is sealed. In home canning procedures, one way of sealing the (usually glass) container is to place a layer of melted paraffin directly on top of the food. As the paraffin cools, it forms a tight solid seal on top of the food. Instead of or in addition to the paraffin seal, the container is also sealed with a metal screw top containing a rubber gasket. The first glass jar designed for this type of home canning operation, the Mason jar, was patented in 1858.

The commercial packaging of foods frequently makes use of tin, aluminum, or other kinds of metallic cans. The technology for this kind of canning was first developed in the mid-1800s, when individual workers hand-sealed cans after foods had been cooked within them. At this stage, a single worker could seldom produce more than 100 "canisters" (from which the word "can" later came) of food a day. With the development of far more efficient canning machines in the late nineteenth century, the mass production of canned foods became a reality.

As with home canning, the process of preserving foods in metal cans is simple in concept. The foods are prepared and the empty cans are sterilized. The prepared foods are then added to the sterile metal can, the filled can is heated to a sterilizing temperature, and the cans are then sealed by a machine. Modern machines are capable of moving a minimum of 1,000 cans per minute through the sealing operation.

The majority of food preservation operations used today also employ some kind of chemical additive to reduce spoilage. Of the many dozens of chemical additives available, all are designed either to kill or retard the growth of pathogens or to prevent or retard chemical reactions that result in the oxidation of foods. Some familiar examples of the former class of food additives are sodium benzoate and benzoic acid; calcium, sodium propionate, and propionic acid; calcium, potassium, sodium sorbate, and sorbic acid; and sodium and potassium sulfite. Examples of the latter class of additives include calcium, sodium ascorbate, and ascorbic acid (vitamin C); butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT); lecithin; and sodium and potassium sulfite and sulfur dioxide.

A special class of additives that reduce oxidation is known as the sequestrants. Sequestrants are compounds that "capture" metallic ions, such as those of copper, iron, and nickel, and remove them from contact with foods. The removal of these ions helps preserve foods because in their free state they increase the rate at which oxidation of foods takes place. Some examples of sequestrants used as food preservatives are ethylenediamine-tetraacetic acid (EDTA), citric acid, sorbitol, and tartaric acid.

The lethal effects of radiation on pathogens has been known for many years. Since the 1950s, research in the United States has been directed at the use of this technique for preserving certain kinds of food. The radiation used for food preservation is normally gamma radiation from radioactive isotopes or machine-generated x rays or electron beams. One of the first applications of radiation for food preservation was in the treatment of various kinds of herbs and spices, an application approved by the U.S. Food and Drug Administration (FDA) in 1983. In 1985, the FDA extended its approval to the use of radiation for the treatment of pork as a means of destroying the pathogens that cause trichinosis. Experts predict that the ease and efficiency of food preservation by means of radiation will develop considerably in the future. That future is somewhat clouded, however, by fears expressed by some scientists and members of the general public about the dangers that irradiated foods may have for humans. In addition to a generalized concern about the possibilities of being exposed to additional levels of radiation in irradiated foods (not a possibility), critics have raised questions about the creation of new and possibly harmful compounds in food that has been exposed to radiation.

See also Biotechnology; Botulism; Food safety; History of microbiology; History of public health; Salmonella food poisoning; Winemaking

Food preservation

The term food preservation refers to any one of a number of techniques used to prevent food from spoiling. It includes methods such as canning, pickling, drying and freeze-drying, irradiation, pasteurization, smoking, and the addition of chemical additives. Food preservation has become an increasingly important component of the food industry as fewer people eat foods produced on their own lands, and as consumers expect to be able to purchase and consume foods that are "out of season."

Scientific principles

The vast majority of instances of food spoilage can be attributed to one of two major causes: (1) the attack by pathogens (disease-causing microorganisms ) such as bacteria and molds, or (2) oxidation that causes the destruction of essential biochemical compounds and/or the destruction of plant and animal cells. The various methods that have been devised for preserving foods are all designed to reduce or eliminate one or the other (or both) of these causative agents.

For example, a simple and common method of preserving food is by heating it to some minimum temperature . This process prevents or retards spoilage because high temperatures kill or inactivate most kinds of pathogens. The addition of compounds known as BHA and BHT to foods also prevents spoilage in another different way. These compounds are known to act as antioxidants , preventing chemical reactions that cause the oxidation of food that results in its spoilage. Almost all techniques of preservation are designed to extend the life of food by acting in one of these two ways.

Historical methods of preservation

The search for methods of food preservation probably can be traced to the dawn of human civilization. Certainly people who lived through harsh winters found it necessary to find some means of insuring a food supply during seasons when no fresh fruits and vegetables were available. Evidence for the use of dehydration (drying) as a method of food preservation, for example, goes back at least 5,000 years. Among the most primitive forms of food preservation that are still in use today are such methods as smoking, drying, salting, freezing, and fermenting.

Smoking

Early humans probably discovered by accident that certain foods exposed to smoke seem to last longer than those that are not. Meats, fish , fowl, and cheese were among such foods. It appears that compounds present in wood smoke have anti-microbial actions that prevent the growth of organisms that cause spoilage.

Today, the process of smoking has become a sophisticated method of food preservation with both hot and cold forms in use. Hot smoking is used primarily with fresh or frozen foods, while cold smoking is used most often with salted products. The most advantageous conditions for each kind of smoking—air velocity , relative humidity , length of exposure, and salt content, for example–are now generally understood and applied during the smoking process. For example, electrostatic precipitators can be employed to attract smoke particles and improve the penetration of the particles into meat or fish.

So many alternative forms of preservation are now available that smoking no longer holds the position of importance it once did with ancient peoples. More frequently the process is used to add interesting and distinctive flavors to foods.

Drying

Since most disease-causing organisms require a moist environment in which to survive and multiply, drying is a natural technique for preventing spoilage. Indeed, the act of simply leaving foods out in the sun and wind to dry out is probably one of the earliest forms of food preservation. Evidence for the drying of meats, fish, fruits, and vegetables go back to the earliest recorded human history.

At some point, humans also learned that the drying process could be hastened and improved by various mechanical techniques. For example, the Arabs learned early on that apricots could be preserved almost indefinitely by macerating them, boiling them, and then leaving them to dry on broad sheets. The product of this technique, quamaradeen, is still made by the same process in modern Muslim countries.

Today, a host of dehydrating techniques are known and used. The specific technique adopted depends on the properties of the food being preserved. For example, a traditional method for preserving rice is to allow it to dry naturally in the fields or on drying racks in barns for about two weeks. After this period of time, the native rice is threshed and then dried again by allowing it to sit on straw mats in the sun for about three days.

Modern drying techniques make use of fans and heaters in controlled environments. Such methods avoid the uncertainties that arise from leaving crops in the field to dry under natural conditions. Controlled temperature air drying is especially popular for the preservation of grains such as maize, barley , and bulgur.

Vacuum drying is a form of preservation in which a food is placed in a large container from which air is removed. Water vapor pressure within the food is greater than that outside of it, and water evaporates more quickly from the food than in a normal atmosphere. Vacuum drying is biologically desirable since some enzymes that cause oxidation of foods become active during normal air drying. These enzymes do not appear to be as active under vacuum drying conditions, however.

Two of the special advantages of vacuum drying is that the process is more efficient at removing water from a food product, and it takes place more quickly than air drying. In one study, for example, the drying time of a fish fillet was reduced from about 16 hours by air drying to six hours as a result of vacuum drying.

Coffee drinkers are familiar with the process of dehydration known as spray drying. In this process, a concentratedsolution of coffee in water is sprayed though a disk with many small holes in it. The surface area of the original coffee grounds is increased many times, making dehydration of the dry product much more efficient.

Freeze-drying is a method of preservation that makes use of the physical principle known as sublimation. Sublimation is the process by which a solid passes directly to the gaseous phase without first melting. Freeze-drying is a desirable way of preserving food since it takes place at very low temperatures (commonly around 14°F to -13°F [-10°C to -25°C]) at which chemical reactions take place very slowly and pathogens survive only poorly. The food to be preserved by this method is first frozen and then placed into a vacuum chamber. Water in the food first freezes and then sublimes, leaving a moisture content in the final product of as low as 0.5%.

Salting

The precise mechanism by which salting preserves food is not entirely understood. It is known that salt binds with water molecules and thus acts as a dehydrating agent in foods. A high level of salinity may also impair the conditions under which pathogens can survive. In any case, the value of adding salt to foods for preservation has been well known for centuries.

Sugar appears to have effects similar to those of salt in preventing spoilage of food. The use of either compound (and of certain other natural materials) is known as curing. A desirable side effect of using salt or sugar as a food preservative is, of course, the pleasant flavor each compound adds to the final product.

Curing can be accomplished in a variety of ways. Meats can be submerged in a salt solution known as brine, for example, or the salt can be rubbed on the meat by hand. The injection of salt solutions into meats has also become popular. Food scientists have now learned that a number of factors relating to the food product and to the preservative conditions affect the efficiency of curing. Some of the food factors include the type of food being preserved, the fat content, and the size of treated pieces. Preservative factors include brine temperature and concentration and the presence of impurities.

Curing is used with certain fruits and vegetables, such as cabbage (in the making of sauerkraut), cucumbers (in the making of pickles), and olives. It is probably most popular, however, in the preservation of meats and fish. Honey-cured hams, bacon, and corned beef ("corn" is a term for a form of salt crystals) are common examples.

Freezing

Freezing is an effective form of food preservation because the pathogens that cause food spoilage are killed or do not grow very rapidly at reduced temperatures. The process is less effective in food preservation than are thermal techniques such as boiling because pathogens are more likely to be able to survive cold temperatures than hot temperatures. In fact, one of the problems surrounding the use of freezing as a method of food preservation is the danger that pathogens deactivated (but not killed) by the process will once again become active when the frozen food thaws.

A number of factors are involved in the selection of the best approach to the freezing of foods, including the temperature to be used, the rate at which freezing is to take place, and the actual method used to freeze the food. Because of differences in cellular composition, foods actually begin to freeze at different temperatures ranging from about 31°F (-0.6°C) for some kinds of fish to 19°F (-7°C) for some kinds of fruits.

The rate at which food is frozen is also a factor, primarily because of aesthetic reasons. The more slowly food is frozen, the larger the ice crystals that are formed. Large ice crystals have the tendency to cause rupture of cells and the destruction of texture in meats, fish, vegetables, and fruits. In order to deal with this problem, the technique of quick-freezing has been developed. In quick-freezing, a food is cooled to or below its freezing point as quickly as possible. The product thus obtained, when thawed, tends to have a firm, more natural texture than is the case with most slow-frozen foods.

About a half dozen methods for the freezing of foods have been developed. One, described as the plate, or contact, freezing technique, was invented by the American inventor Charles Birdseye in 1929. In this method, food to be frozen is placed on a refrigerated plate and cooled to a temperature less than its freezing point. Or, the food may be placed between two parallel refrigerated plates and frozen.

Another technique for freezing foods is by immersion in very cold liquids. At one time, sodium chloride brine solutions were widely used for this purpose. A 10% brine solution, for example, has a freezing point of about 21°F (-6°C), well within the desired freezing range for many foods. More recently, liquid nitrogen has been used for immersion freezing. The temperature of liquid nitrogen is about -320°F (-195.5°C), so that foods immersed in this substance freeze very quickly.

As with most methods of food preservation, freezing works better with some foods than with others. Fish, meat, poultry, and citrus fruit juices (such as frozen orange juice concentrate) are among the foods most commonly preserved by this method.

Fermentation

Fermentation is a naturally occurring chemical reaction by which a natural food is converted into another form by pathogens. It is a process in which food "goes bad," but results in the formation of an edible product. Perhaps the best example of such a food is cheese. Fresh milk does not remain in edible condition for a very long period of time. Its pH is such that harmful pathogens begin to grow in it very rapidly. Early humans discovered, however, that the spoilage of milk can be controlled in such a way as to produce a new product, cheese.

Bread is another food product made by the process of fermentation. Flour, water, sugar, milk, and other raw materials are mixed together with yeasts and then baked. The addition of yeasts brings about the fermentation of sugars present in the mixture, resulting in the formation of a product that will remain edible much longer than will the original raw materials used in the bread-making process.

Thermal processes

The term "thermal" refers to processes involving heat . Heating food is an effective way of preserving it because the great majority of harmful pathogens are killed at temperatures close to the boiling point of water. In this respect, heating foods is a form of food preservation comparable to that of freezing but much superior to it in its effectiveness. A preliminary step in many other forms of food preservation, especially forms that make use of packaging, is to heat the foods to temperatures sufficiently high to destroy pathogens.

In many cases, foods are actually cooked prior to their being packaged and stored. In other cases, cooking is neither appropriate nor necessary. The most familiar example of the latter situation is pasteurization. During the 1860s, the French bacteriologist Louis Pasteur discovered that pathogens in foods can be destroyed by heating those foods to a certain minimum temperature. The process was particularly appealing for the preservation of milk since preserving milk by boiling is not a practical approach. Conventional methods of pasteurization called for the heating of milk to a temperature between 145 and 149°F (63 and 65°C) for a period of about 30 minutes, and then cooling it to room temperature. In a more recent revision of that process, milk can also be "flash-pasteurized" by raising its temperature to about 160°F (71°C) for a minimum of 15 seconds, with equally successful results. A process known as ultra-high-pasteurization uses even higher temperatures—of the order of 194 to 266°F (90 to 130°C)—for periods of a second or more.

Packaging

One of the most common methods for preserving foods today is to enclose them in a sterile container. The term "canning" refers to this method although the specific container can be glass , plastic, or some other material as well as a metal can, from which the procedure originally obtained its name.

The basic principle behind canning is that a food is sterilized, usually by heating, and then placed within an air-tight container. In the absence of air, no new pathogens can gain access to the sterilized food.

In most canning operations, the food to be packaged is first prepared in some way—cleaned, peeled, sliced, chopped, or treated in some other way—and then placed directly into the container. The container is then placed in hot water or some other environment where its temperature is raised above the boiling point of water for some period of time. This heating process achieves two goals at once. First, it kills the vast majority of pathogens that may be present in the container. Second, it forces out most of the air above the food in the container.

After heating has been completed, the top of the container is sealed. In home canning procedures, one way of sealing the (usually glass) container is to place a layer of melted paraffin directly on top of the food. As the paraffin cools, it forms a tight solid seal on top of the food. Instead of or in addition to the paraffin seal, the container is also sealed with a metal screw top containing a rubber gasket. The first glass jar designed for this type of home canning operation, the Mason jar, was patented in 1858.

The commercial packaging of foods frequently makes use of tin, aluminum , or other kinds of metallic cans. The technology for this kind of canning was first developed in the mid-1800s, when individual workers hand-sealed cans after foods had been cooked within them. At this stage, a single worker could seldom produce more than 100 "canisters" (from which the word "can" later came) of food a day. With the development of far more efficient canning machines in the late nineteenth century, the mass production of canned foods became a reality.

As with home canning, the process of preserving foods in metal cans is very simple in concept. The foods are prepared and the empty cans sterilized. The prepared foods are then added to the sterile metal can, the filled can is heated to a sterilizing temperature, and the cans are then sealed by a machine. Modern machines are capable of moving a minimum of 1,000 cans per minute through the sealing operation.

Chemical additives

The majority of food preservation operations used today also employ some kind of chemical additive to reduce spoilage. Of the many dozens of chemical additives available, all are designed either to kill or retard the growth of pathogens or to prevent or retard chemical reactions that result in the oxidation of foods.

Some familiar examples of the former class of food additives are sodium benzoate and benzoic acid ; calcium , sodium propionate, and propionic acid; calcium, potassium, sodium sorbate, and sorbic acid; and sodium and potassium sulfite. Examples of the latter class of additives include calcium, sodium ascorbate, and ascorbic acid (vitamin C); butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT); lecithin ; and sodium and potassium sulfite and sulfur dioxide .

A special class of additives that reduce oxidation is known as the sequestrants. Sequestrants are compounds that "capture" metallic ions, such as those of copper , iron , and nickel, and remove them from contact with foods. The removal of these ions helps preserve foods because in their free state they increase the rate at which oxidation of foods takes place. Some examples of sequestrants used as food preservatives are ethylenediamine-tetraacetic acid (EDTA), citric acid , sorbitol, and tartaric acid .

Irradiation

The lethal effects of radiation on pathogens has been known for many years. Since the 1950s, research in the United States has been directed at the use of this technique for preserving certain kinds of food. The radiation used for food preservation is normally gamma radiation from radioactive isotopes or machine-generated x rays or electron beams. One of the first applications of radiation for food preservation was in the treatment of various kinds of herbs and spices, an application approved by the U.S. Food and Drug Administration (FDA) in 1983. In 1985, the FDA extended its approval to the use of radiation for the treatment of pork as a means of destroying the pathogens that cause trichinosis . Experts predict that the ease and efficiency of food preservation by means of radiation will develop considerably in the future.

That future is somewhat clouded, however, by fears expressed by some scientists and members of the general public about the dangers that irradiated foods may have for humans. In addition to a generalized concern about the possibilities of being exposed to additional levels of radiation in irradiated foods (not a possibility), critics have raised questions about the creation of new and possibly harmful compounds in food that has been exposed to radiation.

Resources

books

Considine, Glenn D. Van Nostrand's Scientific Encyclopedia. New York: Wiley-Interscience, 2002.

Francis, Frederick. Wiley Encyclopedia of Food Science andTechnology. New York: Wiley, 1999.

periodicals

Hwang, Deng Fwu. "Tetrodotoxin In Gastropods (Snails) Implicated In Food Poisoning." Journal of Food Protection 65, no. 8 (2002): 1341-1344.

"Preventing Food Poisoning." Professional Nurse 18, no. 4 (2002): 185-186.

Zurer, Pamela S. "Food Irradiation: A Technology at a Turning Point." Chemical & Engineering News (May 5, 1986): 46-56.

David E. Newton

KEY TERMS

Additive

—A chemical compound that is added to foods to give them some desirable quality, such as preventing them from spoiling.

Antioxidant

—A chemical compound that has the ability to prevent the oxidation of substances with which it is associated.

Curing

—A term used for various methods of preserving foods, most commonly by treating them with salt or sugar.

Dehydration

—The removal of water from a material.

Fermentation

—A chemical reaction in which sugars are converted to organic acids.

Irradiation

—The process by which some substance, such as a food, is exposed to some form of radiation, such as gamma rays or x rays.

Oxidation

—A chemical reaction in which oxygen reacts with some other substance.

Pasteurization

—A method for treating milk and other liquids by heating them to a high enough temperature for a long enough period of time to kill or inactivate any pathogens present in the liquid.

Pathogen

—A diseasecausing microorganism such as a mold or a bacterium.

Food preservation

Food preservation refers to any one of a number of techniques used to prevent food from spoiling. All foods begin to spoil as soon as they are harvested or slaughtered. Some spoiling is caused by such microorganisms as bacteria and mold. Other spoilage results from chemical changes within the food itself due to natural processes such as enzyme action or oxidation.

Ancient methods

Ages-old food preservation techniques include drying, smoking, cooling, freezing, fermenting, salting, pickling, and canning.

Drying and smoking. One of the most ancient methods of food preservation is sun- or air-drying. Drying works because it removes much of the food's water. Without adequate water, microorganisms cannot multiply and chemical activities greatly slow down. Dried meat was one of the earliest staple foods of hunters and nomads (people who constantly moved about). Once fire was discovered, prehistoric cave dwellers heat-dried meat and fish, which probably led to the development of smoking as another way to preserve these foods. The Phoenicians of the Middle East air-dried fish. Ancient Egyptians stockpiled dried grains. Native North Americans produced a nutritious food called pemmican by grinding together dried meat, dried fruit, and fat.

Cooling and freezing. Early northern societies quickly learned that coolness as well as freezing helped preserve foods. Microbe growth and chemical changes slow down at low temperatures and completely stop when water is frozen. Pre-Columbian natives in Peru and Bolivia freeze-dried potatoes, while the early Japanese and Koreans freeze-dried their fish. Water evaporating through earthenware jars was used as a coolant in 2500 b.c. by Egyptians and East Indians. Ancient Chinese, Greeks, and Romans stored ice and mountain snow in cellars or icehouses to keep food cool.

Fermenting. Fermentation was particularly useful for people in southern climates, where cooling and freezing were not practical. When a food ferments, it produces acids that prevent the growth of organisms that cause spoilage. Grapes, rice, and barley were fermented into wine and beer by early people. Fermentation also was used to produce cheese and yogurt from milk.

Salting and pickling. Salting, which also inhibits bacteria growth, was a preferred method of preserving fish as early as 3500 b.c. in the Mediterranean world, and also was practiced in ancient China.

Substances besides salt were found to slow food spoilage. The Chinese began using spices as preservatives around 2700 b.c. Ancient Egyptians used mustard seeds to keep fruit juice from spoiling. Jars of fruit preserved with honey have been found in the ruins of Pompeii, Italy. Melted fat—as Native North Americans discovered with pemmican—preserved meat by sealing out air. Pickling—preserving foods in an acid substance like vinegar—also was used during ancient times.

Early canning. By the Middle Ages (400–1450), all of these ancient methods of preserving foods were widely practiced throughout Europe and Asia, often in combination. Salted fish became the staple food of poor people during this time—particularly salted herring, introduced in 1283 by Willem Beukelszoon of Holland. As the modern era approached, the Dutch navy in the mid-1700s developed a way of preserving beef in iron cans by packing it in hot fat and then sealing the cans. By the late 1700s, the Dutch also were preserving cooked, smoked salmon by packing it with butter or olive oil in sealed cans.

Modern methods

Modern methods of food preservation include canning, mechanical refrigeration and freezing, the addition of chemicals, and irradiation.

Words to Know

Additive: A chemical compound that is added to foods to give them some desirable quality, such as preventing them from spoiling.

Antioxidant: A chemical compound that has the ability to prevent the oxidation of substances with which it is associated.

Dehydration: The removal of water from a material.

Fermentation: A chemical reaction in which sugars are converted to organic acids.

Irradiation: The process by which some substance, such as a food, is exposed to some form of radiation, such as gamma rays or X rays.

Oxidation: A chemical reaction in which oxygen reacts with some other substance.

With the Industrial Revolution (1760–1870), populations became concentrated in ever-growing cities and towns. Thus other methods were needed to preserve food reliably for transportation over long distances and for longer shelf life.

The crucial development was the invention of sophisticated canning techniques during the 1790s by the Frenchman Nicolas François Appert (1750–1841), who operated the world's first commercial cannery in 1804. Appert's method, which first used bottles, was greatly improved by the 1810 invention of the tin can in England. Used at first for Arctic expeditions and by the military, canned foods came into widespread use among the general population by the mid-1800s.

Packaged frozen foods. The 1851 invention of a commercial icemaking machine by American John Gorrie (1803–1855) led to the development of large-scale commercial refrigeration of foods for shipping and storage. Clarence Birdseye (1886–1956) introduced tasty quick-frozen foods in 1925. Shortages of canned goods after World War II (1939–45) helped boost the popularity of frozen foods.

Dehydrated foods. Modern methods of drying foods began in France in 1795 with a hot-air vegetable dehydrator. Dried eggs were widely sold in the United States after 1895, but dried food was not produced in volume in the United States until it was used by soldiers during World War I (1914–18). World War II led to the development of dried skim milk, potato flakes, instant coffee, and soup mixes. After the war, freeze-drying was applied to items such as coffee and orange juice, and the technique continues to be applied to other foodstuffs today.

Chemicals, sterile packages, and irradiation. Chemicals are now commonly added to food to prevent spoilage. They include benzoic acid, sorbic acid, and sulfur dioxide. Antioxidants such as BHA (butylated hydroxyanisole) and ascorbic acid (vitamin C) prevent compounds in food from combining with oxygen to produce inedible changes. The use of chemical additives has not been without controversy. The spread of often unnecessary and sometimes harmful chemical additives to food during the late 1800s led to governmental regulation in both England and the United States.

Aseptic packaging is a relatively new way to keep food from spoiling. A food product is sterilized and then sealed in a sterilized container. Aseptic packages, including plastic, aluminum foil, and paper, are lighter and cheaper than the traditional metal and glass containers used for canning. Aseptically processed foods are also sterilized much more quickly, so their flavor is better. Aseptic packaging became commercially available in 1981. However, controversy has developed about the amount of disposable containers produced by this method.

Food irradiation uses low doses of radiation to kill microorganisms in food and to extend the amount of time in which food can be sold and eaten safely. Strawberries that are treated with irradiation can last for up to two weeks, compared to less than a week for untreated berries. The process, which remains controversial, is a relative newcomer among food-preserving techniques. The U.S. government first approved its use on fresh fruits and vegetables in 1986 and for poultry in 1990.

FOOD PRESERVATION

FOOD PRESERVATION, protecting food from deterioration and decay so that it will be available for future consumption.

Natural Processes

Human beings learned to gather naturally preserved foods and to assist nature in the preserving process about 10,000 years ago, before the dawn of agriculture and animal husbandry. Human beings in the Stone Age stored nuts and seeds for winter use and discovered that meat and fish could be preserved by drying in the sun. After the discovery of fire, cooking made food more appetizing and was an aid to preservation, since heating killed some of the microorganisms and enzymes that caused spoilage. Smoking meat and fish as a means of preservation grew out of cooking. After farming developed—the Neolithic Period, or New Stone Age—human beings had more dependable surpluses for preservation. Native Americans subsisted on dry corn and beans that they had stored for winter use; Plains Indians cut buffalo meat into thin strips and dried it in the sun on wooden frames.

Salt was used for flavoring before it was learned that meat soaked in salt brine or rubbed with salt would keep for weeks or months. Brining, later called "pickling," became a favorite way to keep fruits and vegetables for winter use. Sugar was used as a preservative in ancient times, and making jam and marmalade was widely practiced. While spices were thought to preserve food, they mainly served to cover up unpleasant flavors.

Fermentation, the natural process of chemical change in food, was observed, probably by chance, and used thousands of years ago. Fermented fruit juices resulted in wine, a safe beverage in areas of uncertain water supply. Brewing came later. Another product of fermentation, vinegar, was useful for pickling meats, fish, fruits, and vegetables. The Chinese, and later the Germans and other Europeans, fermented cabbage or sauerkraut. About three thousand years ago milk, which does not keep well, was first fermented into cheese. About the same time, Egyptians developed raised sourdough bread, another result of fermentation. In some areas and during some parts of the year, people preserved food by freezing it, but thousands of years passed before freezing became available through man-made processes in all parts of the world throughout the year.

Processes Created by Humans

Canning. Until the nineteenth century human beings were dependent on the natural processes of drying, cooking,

salting, pickling, fermenting, and freezing for food preservation. These had been only slightly modified over the ages. Then, in the early 1800s a French chemist and confectioner, Nicolas Appert, developed canning, for which he was awarded a prize by his government in 1809. Although a theoretical understanding of the benefits of canning did not come until Louis Pasteur observed the relationships between microorganisms and food spoilage some fifty years later, Appert's ideas were still valid. He placed wholesome food in clean metal containers, which were then sealed and boiled long enough to kill the spoilage-causing microorganisms.

Canning spread rapidly. In 1810 an Englishman, Peter Durand, patented a can of iron coated with tin. Today's cans are primarily steel, with a thin coating of tin and, usually, an enamel lining. Commercial canning began in the United States with pickles and ketchup in Boston in 1819 and seafood in Baltimore in 1820. The cooking in boiling water took five or six hours in the early days, but this was sharply reduced in 1860 when Isaac Solomon added calcium chloride to the water, raising its boiling point. The introduction of the pressure cooker, or retort, in 1874 was an even more important step, permitting much more rapid processing. Commercial canners then turned to machines that would do many of the tasks formerly done by hand, such as shelling peas, cutting corn from the cob, and cleaning salmon.

After 1900, enthusiasts of domestic science, agents of agricultural extensions, and others encouraged home canning of all types of food, mainly in glass jars, as a means of utilizing home garden products, providing better diets, and reducing the cost of living on farms. By the early 2000s the decline of the family farm, the low cost of commercially canned foods, and the widespread use of freezers had made home canning rare.

Drying. A sizable dried-fruit industry flourished in the United States long before the arrival of mechanical refrigeration. In colonial times great quantities of apples were dried in the sun and by artificial means. Prior to 1795 drying and the use of salt and sugar were the principal methods of preserving foods. In 1854, it was estimated, Maine could furnish the nation's supply of dried apples. The perfection of fruit evaporators in 1870–1875 increased exports of dried-fruit products. Thirty million pounds of dried apples were exported in 1880. Of nearly a half-billion pounds of dried apples exported in 1909, 83 percent came from California. Later, new drying processes and machinery enlarged outputs for domestic and foreign markets. Meanwhile, refrigeration and canning developed vastly to aid drying in preserving fruit, vegetables, meat, and other foods for human consumption. New methods of preserving foods in their fresh state reduced the need for dried foods, which became high-priced delicacies, served as appetizers or candied.

Refrigeration and freezing. As a means of commercial food preservation, refrigeration preceded freezing. In 1803 Thomas Moore, a Maryland dairy farmer who lived about twenty miles from Washington, D.C., began transporting butter to the new capital city in an icebox of his own design, getting a premium price for his product. Moore patented his refrigerator and published a pamphlet describing it. By the 1840s, American families were beginning to use iceboxes for food storage and preservation. One of the first recorded refrigerated rail shipments was a load of butter, packed in ice and shipped from Ogdensburg, N.Y., to New York City in 1851. In 1868 William Davis patented a refrigerator car with metal tanks along the sides that were filled with ice from the top.

Beginning in the 1830s, various systems of mechanical refrigeration were patented in the United States. Eventually both freight cars and trucks with mechanical refrigeration were developed. In the home the mechanical refrigerator began to replace the icebox in the 1920s. Mechanical refrigeration made possible another major advance in food preservation—freezing. This process decentralized storage and improved the taste and nutritive value of storable foods. In 1912 Clarence Birdseye, a graduate of Amherst College, was in Labrador and noticed that freshly caught fish pulled through the ice quickly froze solid. When thawed, the fish might revive because quick freezing prevented large ice crystals from forming and thus avoided the breakage of cell walls. The physical character of the tissue, and incidentally its taste, remained the same. After much experimentation, Birdseye invented a machine for quick-freezing food products. The machine froze by conduction—that is, by pressing the food directly between very cold metal plates. In 1923 Birdseye established a frozen seafood company that was eventually successful.

Frozen concentrated orange juice, based on a process developed in the U.S. Department of Agriculture, became widely used after World War II. The freezing process also permitted the marketing of precooked food, sold ready to heat and serve. As frozen foods became more prevalent, deep-freeze compartments were included in many home refrigerators. Central frozen-food lockers became popular in many small towns and were widely used to preserve meat. After World War II frozen foods became even more popular and many families began purchasing separate deep-freeze units. These could be used for freezing home-produced foods or for storing commercially prepared products. By 1973 one household in three had its own deep-freeze unit; by the end of the century

virtually all full-size domestic refrigerators included freezers.

Quick freezing led to the development of another means for preserving food—freeze drying. World War II supplied a strong impetus to the development of improved methods of drying food. In general the problem was to dry quickly without heat damage. Spray drying was particularly helpful in improving the quality of dried eggs and powdered milk. Other methods of drying produced potatoes, soup mixes, fruit juices, and other items that could be conveniently shipped and stored before being reconstituted for consumption. Freeze drying developed after World War II. In this process the product is frozen and the moisture is then removed as a vapor by sublimation. The resulting food, after reconstitution, retains much of its original flavor, color, and texture. By the 1970s freeze drying was widely used for coffee, soup mixes, and other dehydrated convenience foods. Some meat was freeze dried, and other developments kept meats edible for prolonged periods of time. Antibiotics introduced into chilling tanks, for example, prolonged the freshness of poultry.

Irradiation. The late–twentieth century saw the emergence of irradiation, or radurization, a pasteurization method in which food is exposed to low levels of high-energy ionizing radiation in an effort to kill microbial contaminants. If properly refrigerated and packaged, irradiated meat, fruit, and vegetable products enjoyed a significantly extended storage life. However, because of inherent concerns about radiation, and the tendency of irradiated foods to lose some of their nutritional value, irradiation was used only sparingly. Scientists, consumer

groups, and the food industry continued to debate its effectiveness.

BIBLIOGRAPHY

Anderson, Oscar E. Refrigeration in America. Port Washington, N.Y.: Kennikat Press, 1972.

David, Elizabeth. Harvest of the Cold Months: The Social History of Ice and Ices. Edited by Jill Norman. New York: Viking, 1994.

Woolrich, Willis R. The Men Who Created Cold: A History of Refrigeration. New York: Exposition Press, 1967.

LouisPelzer

Wayne D.Rasmussen/a. r.

See alsoCanning Industry ; Food and Cuisines ; Pure Food and Drug Movement ; Refrigeration ; World War II .