1.  To increase shelf life by acting as a barrier to moisture, oxygen, carbon dioxide, volatile aromas, and other compounds whose loss would lead to deterioration.
2. To impart improved handling characteristics, such as the ability to bend more easily without breaking.
3. To improve appearance through increased gloss and color.
4. To serve as a vehicle for added ingredients such as flavors, antioxidants, antimicrobials, etc.

Although commonly used for vegetables and fruits (cucumbers, tomatoes, peppers, eggplants, pumpkins, summer squash, apples, bananas, guavas, mangoes, papaya, melons, nectarines, and citrus fruits), edible coatings are also used for candy, cheese, nuts, dried fruit (prevents stickiness, especially in raisins and dates), eggs in their shell (as moisture and bacterial barrier), and processed meats (especially sausages), poultry, and fish.

Edible coatings can be produced from carbohydrate or protein materials, but the most common edible coatings are lipid-based (beeswax, candelilla wax, carnuba wax, rice bran wax), oils (parafin oil, mineral oil, vegetable oils), and petroleum-based waxes (paraffin, polyethylene wax).

Pickling uses vinegar to preserve foods, because the acidity of the vinegar keeps many microorganisms in check. In the Middle East, vinegar was used as early as 1000 B.C. to preserve such foods as fruits, onions, and walnuts. The food was simply covered with vinegar, boiled, and sealed in a container. It was then allowed to stand for at least three weeks to give the vinegar time enough to penetrate all parts of the food. Most people associate “pickles” with pickles made from cucumbers, but pickled foods include beets, cauliflower pieces, green tomatoes, green beans, chilies, bell peppers, and sliced Jerusalem artichokes. These foods are preserved in vinegar and salt, with spices often added to enhance their flavor. Pickled foods set aside for long-term storage are canned for safety reasons.

Fermentation has been used both for the production and preservation of various foods for thousands of years. Laborers building the Great Wall of China in the third century B.C. were fed fermented vegetables as part of their rations. Vegetables are still commonly fermented throughout Asia, but in North America, the foods most often preserved by fermentation are cucumbers, olives, and cabbage.

Carbohydrates are required for the fermentation process. Although vegetables are not usually considered sugary foods, they contain enough carbohydrates and natural bacteria for fermentation to occur. Although only the plants’ natural carbohydrates and microorganisms are needed for fermentation, salt or vinegar (acid) may be added to regulate bacterial growth.

Cucumbers can be manufactured into sweet, sour, dill, kosher dill, and other pickles by one of two methods: the longer process of fermentation, which yields brined pickles (with acid produced from the bacteria); and pickling, where acid is added in the form of vinegar, resulting in quick pickles. During the process of fermentation, a 10 percent salt solution serves as the liquid in which the cucumbers are submerged and allowed to ferment for several weeks. During this fermentation period, bacteria normally found on vegetables break down the sugar in the cucumbers. The salt penetrates the cucumbers, and the brine concentration is increased to 15 percent, except in the processing of dill pickles. Once fermented, pickles (whole or sliced) are placed in warm water, packed in glass jars, covered with a combination of vinegar, sugar, spices, and garlic, and pasteurized. It is this canning process, rather than fermentation, that preserved the pickles.

Prior to drying, some foods, such as apricots and peaches, are dipped in a sulfite solution or exposed to sulfur dioxide gas to preserve their natural color and prevent spoilage. Although sulfur protects against enzymatic browning and the loss of vitamin A and C, it destroys thiamin (vitamin B1) and may cause headaches or allergic reactions, including coma and death, in certain sensitive individuals.

Plums are briefly dipped in lye or very hot water to “check” their skins before they are dried into prunes. This process cracks the skin, thereby improving skin texture and shortening the drying time by exposing more surface area to drying. Some vegetables, such as potatoes or carrots, are blanched prior to drying to prevent enzymatic browning.

Curing Foods

One of the earliest methods of preservation was to rub the surface of meats and fish with salt. High concentrations of salt bind to the water in the food, making it unavailable to microorganisms. The earliest recorded use of salt as a preservative dates back to 3000 B.C., when salt was used to cure fish. Native Americans used salt to preserve some of their foods, the Hopi people traveled long distances to the salt mines within the Grand Canyon to obtain salt for this purpose. Corned beef is a cured meat, the word “corn” referring to the Latin word for “grain”, in this case, “grains” of salt. Today, the most commonly cured meats include ham, sausage, hot dogs, bacon, and bologna. These and other cured foods, however, should be treated as fresh foods, because they do not contain enough salt to store them on the shelf.

Cured meats sometimes undergo the optional treatment of smoking for added flavor. Meats are placed in smokers where they are exposed to the smoke of burning wood. The type of wood selected (sawdust, mesquite, hickory, oak, and various combinations) determines the resulting flavor. There are health concerns with smoked foods, however, as they have been linked to cancer in laboratory animals.

Since sun drying takes a long time and exposes foods to the weather and to the action of insects, most foods are now dried by various commercial processes, although raisins are still sun dried. Among the commercial types of drying, the three most important are conventional (heat), vacuum (pulls the water out), and freeze-drying (ice crystals vaporize).

Conventional Drying

Conventional drying uses heat to evaporate the water. In one method, the food is spread on a slatted floor or on shelves within kilns or drying rooms. A blower then passes hot air from a heater over and through the food. In tunnel drying, food is placed on trays or “carts”, which are moved through a tunnel of carefully controlled hot air. Liquids can be dried by either spray drying or drum drying. In the former, a fine spray of the liquid is dried very quickly in mid-air. Spray drying is used to produce such foods as nonfat dried milk and some types of instant coffee. Drum drying occurs when liquid is poured over the very hot surface of a drum dryer, an apparatus resembling a large barrel. The dried food can then be peeled off like tissue paper, ground into flakes, and packaged. Some mashed-potato flakes and quick-cooking hot cereals are dried in this way.

Vacuum Drying

Vacuum drying dehydrates foods to very low moisture levels (1 to 3 percent) through the use of a vacuum. Milk, tomato paste, orange juice, and coffee are often concentrated by vacuum drying. The food is placed in a chamber, and the surrounding pressure is reduced below atmospheric pressure, which lowers water’s boiling point. The water is more easily boiled off at this lower boiling temperature. This drying method preserves flavor and color while increasing shelf life considerably, but it has two drawbacks: it is expensive, and it requires that the dehydrated food be stored in airtight containers to prevent rehydration by drawing moisture from the air.

Freeze-Drying

Freeze-drying consists of first freezing the food and then placing it in a vacuum, where the ice sublimates to a vapor. This process of sublimination is the most effective method for drying foods, because it does not subject the food to high heat, which alters a food’s flavor, color, and structure. The heat used is low enough to prevent melting of the ice, while warm enough to assist in evaporation of the water. As a result, freeze-dried products yield the highest quality, store indefinitely, and can be reconstituted easily. The process, however, is more costly than conventional drying.

Drying is the preservation of food by the process of removing its water, which effectively inhibits the growth of microorganisms. Bacteria and molds need approximately a 15 percent moisture level to survive, while yeast needs at least a 20 percent moisture content. Once dried, the food can be eaten as it, or, unlike microorganisms, be rehydrated (have water added), which changes its size, color, flavor, and texture.

Sun Drying

The drying process relies on some form of heat to evaporate the water, and the hotter the environment surrounding the food, the faster the rate of evaporation. The sun provided the heat for evaporation for many centuries, and continues to do so in various countries around the world. Many early cultures subsisted throughout the year on naturally dry foods such as nuts, grains and dried legumes; the discovery that fruits, vegetables, and meats could be dried in the sun was a natural extension of these practice.

Ancient Egyptian hieroglyphics show fish being dried in the sun, and tribes along the Nile buried fruits and meats just under the surface of the hot dessert sands to dry before storing them in earthenware containers. More than 3,000 years ago, the Incas were sun-drying foods available in abundance in summer to provide nourishment throughout their harsh winters. The Bible records that grapes and figs were dried, and Marco Polo observed that the Mongols consumed a sun-dried milk product to fuel them in their military conquests. The firtst food exported from the American colonies to England was sun-dried fish.

The Vietnam War spurred the refinement of the process of freeze-drying, which allowed for the development of complete, lightweight foods that could be carried into the field easily and transformed into ready-to-eat meals by adding water.

Because of newer preservation techniques and advances in refrigeration and transportation, people now enjoy a wide variety of foods, including out-of-season and exotic foods from all parts of the globe, that were not available to our ancestors.

Unlike chemical changes, physical changes in foods do not result in the formation of new compounds. A few of the common physical changes occurring in foods as they spoil are evaporation, drip loss, and separation. Water evaporates out of improperly stored foods, creating an unattractive, dried-out appearance and possible undesirable flavor changes. Water can also be lost (syneresis) out of foods such as gelatins, yoghurt, and sour cream as they age. Separation of water and oil occurs in such foods as non-homogenized milk, mayonnaise, salad dressings, and high-moisture cheeses when they are stored too long or are frozen and then thawed. This separation is the reason why sandwiches spread with mayonnaise or made with high-moisture cheeses do not freeze well.

Chemical changes, or reactions, can also contribute to the deterioration of foods. Enzymes play a significant role in catalyzing these reactions and can be categorized depending on the substance they act on (substrate) or their mode of action. Proteases, also called proteolytic enzymes, split proteins into smaller compounds. Fish have many more active proteases than meats, which is one of the reasons they deteriorate so quickly. Lobsters are also prone to proteolytic breakdown, which begins to occur the minute they expire, unless lobsters are kept alive to the very last second. The proteases cause the lower abdomen to partially liquefy and the tail meat to become crumbly when cooked.

Lipids are broken down by enzymes called lipases, which degrade the triglycerides of fat into glycerol and fatty acids. Further degradation leads to rancidity, or off-odors and tastes. Enzymes that decompose carbohydrates are carbohydrases, each named after the particular sugar on which it acts. For example, sucrase breaks sucrose down into glucose and frutose. Yet another group of enzymes serves to oxidize compounds. Some of the more common oxidases include ascorbic acid oxidase, peroxidase, tyrosinase, and polyphenolase. The latter two enzymes are involved in enzymatic browning, which leads to unappetizing brown discoloration in some fruits and vegetables. Hydrolysis may also contribute to the deterioration of foods.

Microorganisms, including bacteria, yeasts, and molds, are the prime biological factor involved in food spoilage. These tiny organisms need food to survive just as people do, so food is a natural target. The most common foods spoiled by bacteria include meat, eggs, milk, and opened canned goods.

The naturally occurring yeasts in the air are found almost everywhere, but normally pose no threat. The moisture in foods, however, can encourage their growth to a point that becomes unacceptable. Yeasts prefer high-sugar foods such as fruits, vegetables, and fruit preserves, and can cause unwanted fermentation of fruits and fruit juices in the presence of the proper amounts of exygen, moisture, and acidity (PH). Any method that keeps the moisture content low will be successful in their control.

These microorganisms, like yeasts, prefer high-sugar foods, but are particularly drawn to cheese and bread. The appearance of their bloom on foods indicates that spoilage has begun. Molds are easily spread through the air, are very resistant to drying, and can be difficult to control by the means used for bacteria and yeasts. Commercial food enterprises sometimes use vacuum pumps to remove oxygen from containers, because molds cannot grow in its absence.

There are many weapons in the arsenal with which to fight food spoilage resulting from microbial action, including boiling, refrigeration, drying, and curing with high concentrations of sugar or salt.