Most foods contain enzymes or natural chemicals, such as acids or alcohols, that cause them to begin to lose desirable characteristics almost immediately after harvest or preparation. In addition, a host of environmental factors, such as heat and the presence of microorganisms, acts to change foodstuffs in ways that may harm the food product. Food preservation traditionally has three goals: the preservation of nutritional characteristics, the preservation of appearance, and a prolongation of the time that the food can be stored. Traditional methods of preservation usually aim to exclude air, moisture, and microorganisms, or to provide environments in which organisms that might cause spoilage cannot survive.
Ancient peoples are known to have had dried fruits, vegetables, and meats. The pemmican of Native Americans and the jerky consumed by present-day campers and hikers are both prepared by drying. The drying process can include smoking, which may actually add antibacterial agents. The processes of dehydration and freeze-drying, used today to produce such foods as powdered coffee and soup, are variations of drying.
Among the earliest preservatives were sugar and salt (NaCl), which produced food environments of high osmotic pressure that denied bacteria the aqueous surroundings they needed to live and reproduce. Jams and jellies are preserved as solutions of high sugar content, and many meats (e.g., hams) and fish are still preserved by salting. Unlike other microorganisms, molds can often withstand the effects of high salt or sugar concentrations in foods. Fortunately, they seldom cause illness.
Early methods of air removal included the sealing of foods inside containers (such as jars), or the covering of food surfaces with hot paraffin . The invention of canning by Nicolas Appert enabled commercial preparations of foodstuffs. In response to a prize offered by Napoléon in 1795, Appert developed a method of canning and preserving fruits and vegetables in glass containers for sea voyages. His process was used commercially in 1910 by Peter Durand in England, using metal cans. During the earliest days of canning, some persons (including some Arctic explorers) probably died as a result of exposure to the lead that was once used to solder cans. Modern techniques of air removal include vacuum sealing and the use of plastic wrappings.
Along with cooking, which kills most bacteria, freezing stops or slows bacterial growth as well as changes in foods brought about by enzymes present in the foods. Unlike the slower freezing of foods in homes, quick freezing in commercial processing enables foods to retain more of their natural appearance and taste.
Chemical preservatives include free radical scavengers (also known as antioxidants), such as vitamin C and compounds such as BHA (butylated hydroxyanisole), and bacterial growth inhibitors, such as benzoic acid, sulfur dioxide, and sodium nitrite (NaNO2). Ethanol (CH3CH2OH) has long been used as a preservative, both of itself (as in wine), and of other foods (e.g., fruits stored in brandy). Some chemical preservatives may be harmful: Sulfur dioxide (often used to preserve wines) is irritating to the bronchial tubes of persons who have asthma, and nitrites have been implicated as carcinogens.
After Louis Pasteur proved that it was the presence of bacteria that caused food to spoil, there was a tendency to consider all microorganisms harmful. But in fact, microbial action is responsible for the production and preservation of some foods. The action of microbes is a part of the production of cheese and some flavoring agents. Sauerkraut is both processed
and preserved by lactobacilli, and yeast cells ferment sugars, producing alcohol, which may add zest to beverages as well as help to preserve them.
The irradiation of foods has the advantage of enabling food packaging and preparation in which there is less person-to-food contact, thus decreasing the possibility of contamination and decreasing the need for chemical preservatives, some of which may be harmful. The ionizing radiation that is used to irradiate foods, wherein the foods are exposed to bursts of high-intensity x rays or streams of electrons, disrupts bacterial DNA . Some persons have objected to the irradiation of foods because of an (unfounded) fear of radioactivity. As pathogens such as virulent strains of coliform bacteria have caused food poisoning, the irradiation of animal carcasses and, in particular, of hamburger during its preparation has become more desirable. Irradiation currently extends the shelf lives of foods such as strawberries. Irradiation does not make foods radioactive, but may cause changes in food color or texture.
see also Ascorbic Acid; Carcinogen; Kinetics; Pasteur, Louis.
Dan M. Sullivan
Rosenthal, Ionel (1992). Electromagnetic Radiation in Food Science. New York: Springer-Verlag.
Thorne, Stuart (1986). The History of Food Preservation. Totowa, NJ: Barnes & Noble Books.
U.S. Department of Agriculture (1994). Complete Guide to Home Canning. Washington, DC: U.S. Government Printing Office.
National Center for Home Food Preservation. Available from <http://www.uga.edu/nchfp/>.