II.

Processing and Preservation Methods

Food processing encompasses all the steps that food goes through from the time it is harvested to the time it arrives on supermarket shelves. At simplest, processing may involve only picking, sorting, and washing fruits and vegetables before they are sent to market. Some processing methods convert raw materials into a different form or change the nature of the product, as in the manufacture of sugar from sugar beets, oil from corn or olives, or cheese from milk. Processing may also involve an extremely complex set of techniques and ingredients to create ready-to-eat convenience foods.

Food preservation refers specifically to the processing techniques that are used to keep food from spoiling. Spoilage is any change that makes food unfit for consumption, and includes chemical and physical changes, such as bruising and browning; infestation by insects or other pests; or growth of microorganisms, such as bacteria, yeast, and molds.

Some food preservation techniques destroy enzymes, proteins that are present in all raw foods, which are responsible for the chemical and physical changes that naturally occur after harvesting. Food preservation techniques also help eliminate the moisture or temperature conditions that are favorable for the growth of microorganisms. As they multiply and grow, microorganisms are capable of causing food-borne illness. They also break down foods, producing unpleasant changes in taste, texture, and appearance—changes that we recognize as spoilage. Although people have known about spoilage and some preservation methods to prevent it for centuries, it was only in 1857 that French chemist Louis Pasteur demonstrated the role of microorganisms in the process.

A.

Curing

Curing is one of the oldest forms of food preservation. It is used to preserve meat and fish, yielding common products such as bacon, ham, frankfurters, and corned beef. Curing involves adding some combination of salt, sugar, spices, vinegar, or sodium nitrite to animal foods. Smoking, a flavoring technique and preservation method, is another ancient technique that is commonly used with curing. Smoking involves cooking meat or fish very slowly over a low wood fire. Curing and smoking preserve food by binding or removing water so that it is not available for the growth of microorganisms. These methods impart a distinctive color and flavor to food and, in some cases, eliminate the need for refrigeration. Some studies, however, show that curing agents such as sodium nitrite may combine with other chemicals to form cancer-causing nitrosamines. In addition, cured products tend to be very salty, and the sodium in salty foods has been linked to hypertension, also known as high blood pressure. Smoked meats and fish may contain toxic and even carcinogenic compounds that they absorb from wood smoke.

B.

Drying

Cultures throughout the world have used drying to preserve food, probably since prehistoric times when people learned that dried foods—for example, fruits left out in the sun—remain wholesome for long periods. In modern times, the dried foods industry greatly expanded after World War II (1939-1945) but remains restricted to certain foods, including milk, soup, eggs, fruits, yeast, some meats, and instant coffee, that are particularly suited to the process. Three basic methods of drying are used today: sun drying, a traditional method in which foods dry naturally in the sun; hot air drying, in which foods are exposed to a blast of hot air; and freeze-drying, in which frozen food is placed in a vacuum chamber to draw out the water.

Removing the water preserves food because microorganisms need water to grow and food enzymes cannot work without a watery environment. Removing the water also decreases the weight and volume of foods, thereby reducing transportation and storage costs. However, dried foods may be less convenient for consumers because most must be rehydrated before consumption. In addition, most dried foods only reabsorb about two-thirds of their original water content, leaving the rehydrated product with a tougher, chewier texture than the original. Some scientists and consumer groups have raised concerns about the sulfites commonly added to fruits before drying to prevent browning. These chemicals may cause severe allergic reactions in people with asthma or other people sensitive to the chemicals.

In freeze-drying, frozen food is placed in a special vacuum cabinet. There, water escapes from the food by sublimation, a process in which ice changes from a solid directly to a vapor without first becoming a liquid. Freeze-dried foods retain their original flavor, texture, and nutrients upon rehydration but must be packaged in moisture-proof, hermetically sealed containers. Freeze-drying is an expensive process used for such products as instant coffee, dried soup mixes, strawberries, mushrooms, and shrimp. Flash freezing is a process of supercooling foods to temperatures of -195°C (-320°F) through the use of liquid nitrogen. The process reduces cellular deterioration and increases retained moisture so that foods are tastier when they are unfrozen.

C.

Canning

Canning is used to preserve a wide variety of foods, including soups, sauces, fruits, vegetables, juices, meats, fish, and some dairy products. Canning preserves food by heating it in airtight, vacuum-sealed containers. The can is filled with food, and air is pumped out of the space remaining at the top of the can to form a vacuum. The container is sealed, heated in a cooker called a retort, and then cooled to prevent overcooking of the food inside. This process removes oxygen, destroys enzymes involved in food spoilage, and kills most microorganisms that may be present in the food.

Canned foods are popular because they are already partially prepared and cooked, can be stored without refrigeration for long periods, and are generally low in cost. However, because of the high temperatures required for sterilization, canning affects the color, texture, flavor, and nutrient content of foods. Fat-soluble vitamins and minerals are barely affected by heat processing, but water-soluble vitamins, especially thiamine, riboflavin, and vitamin C, can leach into canning or cooking water that may later be thrown away during preparation. Up to half of the original content of water-soluble vitamins in a canned product can be lost in this way. Rapid, high-temperature processes generally conserve nutrient content best, as every 4.4° C (18° F) rise in processing temperature yields approximately a ten-fold increase in microbial destruction, with little additional nutrient loss.

D.

Additives

Food additives are chemicals that are added to food in small amounts. Direct additives are added deliberately during processing to make food look and taste better, maintain or improve nutritive value, maintain freshness, and help in processing or preparation. Some additives help preserve food by preventing or slowing chemical changes and the growth of microorganisms in food. As many as 3,000 substances are approved by the Food and Drug Administration (FDA) for use as direct additives. An additional 10,000 substances are present in foods as indirect additives. These substances enter food incidentally during handling or from processing equipment or packaging.

Food additives have been used for thousands of years. The salts and other chemicals used in curing are additives, and before the advent of canning and mechanical refrigeration, chemical additives were the only means of preservation available. Additives were not limited to use as preservatives, however. People in ancient Rome added certain chemicals to wine and cooked vegetables to improve the color of these foods. Other examples of additives that have been used since ancient times include yeast and baking powder used as leavening in baked goods.

Advances in the knowledge of chemistry have greatly expanded the number of additives that are used in foods. Such recent additions to the ranks of food additives include artificial sweeteners, such as aspartame and saccharin; fat replacements, such as Simplesse; and colors, such as FD&C yellow No. 5, which is used in beverages, ice cream, cereals, and other foods.

The development of new chemical additives has also played an enormous role in the growth of convenience foods. Additives that help ensure the quality of convenience foods include anti-caking agents, such as calcium silicate and magnesium stearate, to prevent lumps in dry mixes; humectants, such as glycerol, propylene glycol, and sorbitol, to help retain moisture in breads and cakes; emulsifiers, such as egg yolk, lecithin, and monoglycerides, which bind oil and water to improve the uniformity and smoothness of foods; and stabilizers and thickeners, such as guar gum, carrageenan, and gelatin.

As the use of food additives has grown, so has public concern about the type and amount of these additives and their potential to cause cancer or other illnesses in human beings. Some studies have suggested that saccharin, nitrites, and other additives may cause cancer, but these results remain controversial. At the same time, some additives may actually provide a health benefit. For example, the vitamins used to fortify foods such as bread and milk are additives.

E.

Freezing and Refrigeration

Low-temperature storage as a preservation method probably began when prehistoric humans stored meat and other foods in ice caves. However, mechanical refrigeration and large-scale freezing are relatively recent innovations. Mechanical refrigeration was pioneered by American inventor John Gorrie in 1842, but a mechanical refrigeration system suitable for widespread commercial use was not developed until the 1870s. American inventor Clarence Birdseye developed procedures, equipment, and packaging for quick-freezing in the 1920s and in 1953 frozen TV dinners were introduced by C. A. Swanson and Sons.

Storage at low temperature slows many of the enzymatic reactions involved in spoilage and reduces the growth rate of microorganisms (though it does not kill them). To minimize microbial growth, refrigerators should be kept at 0° to 4° C (32° to 40° F) and freezers at or below 0° C (32° F). Refrigeration is advantageous because it does not cause chemical or physical changes to food. Freezing allows foods to be stored for longer periods than refrigeration because it inhibits enzyme activity and microbial growth to a greater degree. The greatest disadvantage of freezing is that the water in food expands and forms ice crystals. The ice crystal formation disrupts the structure of plant and animal cells, giving frozen food a softer texture after thawing. Newer technologies in which freezing occurs more rapidly help minimize this problem: Faster freezing, such as flash freezing, means that smaller ice crystals form, resulting in less damage to cells.

Foods that should be refrigerated include meats, fish, eggs, milk, some fruits, and some vegetables. Many of these foods can also be frozen. Frozen produce is often high in quality and can rival the flavor of fresh. In many cases, produce frozen and stored under proper conditions contains more nutrients than produce picked unripened and allowed to mature during transportation. Briefly cooking vegetables in boiling water before freezing, a process known as blanching, inactivates enzymes altogether and reduces discoloration and nutrient loss.

F.

Controlled Atmosphere Storage

Fruits and vegetables are sometimes stored in sealed warehouses where temperature and humidity are closely controlled, and perhaps most importantly, the composition of gases in the atmosphere is altered to minimize spoilage. Usually, the concentration of oxygen is reduced, the concentration of carbon dioxide is increased, and ethylene, a gas naturally produced by plants that accelerates ripening, is removed from the atmosphere. This controlled environment helps slow the enzymatic reactions that eventually lead to decomposition and decay, and may increase the time that produce can be stored by several months. Ripening rooms, in which ethylene gas is added to the atmosphere, also help produce higher quality fruits and vegetables. This technology enables produce to be picked before it is ripe, for easier handling, and then ripened quickly and uniformly under controlled conditions.

G.

Aseptic Packaging

Aseptic packaging is now commonly used for packaging milk and juice. Like canning, aseptic packaging involves heat sterilization of food, but unlike canning, the package and food are sterilized separately. Food can be sterilized more rapidly and at lower temperatures in aseptic packaging than in canning, allowing the food to retain more nutrients and better flavor. Containers are sterilized with hydrogen peroxide rather than with heat, permitting the use of plastic bags and foil-lined cartons, which would be destroyed by heat sterilization. These containers cost less than the metal and glass containers used in canning and also weigh less, reducing transport costs. Aseptically packaged foods will keep without refrigeration for long periods of time, perhaps even years. They are growing in popularity because of their low cost, good taste and nutrition, and convenience.

H.

Irradiation

Irradiation is a process in which food is passed through a chamber where it is exposed to gamma rays or X rays. These high-energy rays are strong enough to break chemical bonds, destroy cell walls and cell membranes, and break down deoxyribonucleic acid (DNA), the substance that carries genetic information in all cells. Irradiation kills most bacteria, molds, and insects that may contaminate food. Irradiation also delays the ripening of fruits and sprouting of vegetables, permitting produce to be stored for longer periods of time. Because irradiation involves minimal heating, it has very little effect on the taste, texture, and nutritive value of food.

The FDA first approved irradiation for use on wheat and wheat flour in 1963, and later approved its use on white potatoes, spices, pork, some fresh produce (onions, tomatoes, mushrooms, and strawberries), and poultry. In 1997, in response to several food-borne illness outbreaks and increasing public concern over the safety of the food supply, irradiation was approved for use on poultry products. In 1999, irradiation was approved to curb pathogens in raw meats including ground beef, steaks, and pork chops. Irradiation is also used to preserve some meals eaten by astronauts during long-term space missions. Some consumer groups have raised concerns that irradiation might cause the formation of toxic compounds in food. Because of these and other concerns, only a limited amount of irradiated food has been sold in the United States.

I.

Fermentation

Fermentation is a chemical reaction carried out by many types of microorganisms to obtain energy. In fermentation, microorganisms break down complex organic compounds into simpler substances. Although chemical changes and microbial growth usually mean food spoilage, in some cases fermentation is desirable and microorganisms are actually added to foods. For example, in the production of beer, wine, and other alcoholic beverages, yeasts convert sugar into ethyl alcohol and carbon dioxide. In the making of yogurt and cheese, bacteria convert lactose, a sugar found in milk, to lactic acid. Alcohol, acids, and other compounds produced in fermentation act as preservatives, inhibiting further microbial growth. In addition to its use with alcoholic beverages, cheese, and yogurt, fermentation is used to produce yeast bread, soy sauce, cucumber pickles, sauerkraut, and other products.

J.

Pasteurization

Pasteurization involves heating foods to a certain temperature for a specific time to kill harmful microorganisms. Milk, wine, beer, and fruit juices are all routinely pasteurized. Milk, for example, is usually heated to 63° C (145° F) for 30 minutes. Ultra-High Temperature (UHT) pasteurization, a relatively new technique, is used to sterilize foods for aseptic packaging. In UHT pasteurization, foods are heated to 138° C (280° F) for 2 to 4 seconds, allowing the food to retain more nutrients and better flavor.

K.

Genetic Engineering

Genetic engineering is aimed at improving the food supply even before harvest or slaughter by improving yields, increasing disease resistance, and enhancing the nutritional qualities of various foods. Broadly speaking, genetic engineering refers to any deliberate alteration of an organism’s DNA. Genetic manipulation has been practiced for thousands of years, ever since humans began selectively breeding plants and animals to create more nutritious, better tasting foods. In the past two decades, genetic engineering has become increasingly powerful as scientific advances have enabled the direct alteration of genetic material through the use of recombinant DNA. Genes have been cut and pasted from one species to another, yielding, for example, disease-resistant squash and rice, frost-resistant potatoes and strawberries, and tomatoes that ripen—and therefore spoil—more slowly. However, genetic engineering is controversial, as some critics argue that its possible environmental impact has not been sufficiently studied.