The role of food preservation in food safety
Since the earliest times humankind has searched for ways to make the food supply safer and to make food last longer. Without the use of some preservation technique, the natural microorganisms that are present everywhere in the environment will grow and multiply in foods.
Preservation aims either to destroy or inhibit the growth of harmful microorganisms in food by making an environment unsuitable for them.
The earliest recorded instances of food preservation date back to ancient Egypt and the drying of grains and subsequent storage in sealed silos. The stored grain could be kept for several years to insure against famine in case the Nile River flooded. People in many parts of the world developed techniques for drying and smoking foods as far back as 6000 B.C.E.
Microorganisms need water to carry out their metabolic processes. Many preservation techniques that are familiar to us, such as drying, smoking, and salting, seek to reduce available water in a product. Freezing foods and making foods more acidic through fermentation and pickling also inhibit microorganism growth. Salting was so important in Roman life that Roman soldiers received “salarium,” or salt, as payment. This is the origin of today’s term “salary.”
Large-scale deployment of armed forces led to the need for more advanced methods of food preservation to keep food safe for troops in the field. Napoleon’s realization that armies do indeed travel on their stomach caused him to offer a prize for an improved food preservation method. In response to this need, Nicolas Appert, a French candy maker, developed a process by which he placed food in bottles, sealed the bottles, and then heated them for hours in boiling water. When Appert published his method in 1810 he had no knowledge of bacteria. It took another 50 years and Louis Pasteur to elucidate the relationship between microorganisms and the spoilage of food. What Appert developed is essentially the process for canning food. Over the years, food scientists have made many improvements in the canning process, but the basic idea of using high heat remains the same.
Most homes have a device that is extremely useful for keeping foods safe—the refrigerator. Although refrigeration was developed in the early 1800s, refrigerators were not readily available for home use until the 1930s. Most pathogenic bacteria do not grow at all, or grow very slowly, at refrigerated temperatures. However, spoilage bacteria, those that cause food to smell or taste bad, can grow in the refrigerator. While spoilage bacteria cause foods to become of unacceptable quality, they do not cause illness. Spoilage bacteria serve a good purpose in that they prevent
people from eating food that may contain harmful bacteria.
Freezing keeps food safe by slowing the movement of molecules, causing microbes to enter a
dormant stage. Freezing preserves food for extended periods of time because it prevents the growth of microorganisms that cause both food spoilage and foodborne illness; so frozen food is theoretically safe forever. The quality of frozen food, however, diminishes quickly with time. For example, when air reaches the surface of food, it causes dry grayish-brown leathery spots to appear.
A method of making food safer that is well-known and accepted today is pasteurization. Pasteurization is the process of heating foods to a temperature for a designated period of time to destroy disease-causing and/or food spoilage bacteria. The amount of time the product is heated depends on the temperature; higher temperatures require less time. It is different from sterilization in that some spoilage bacteria survive. Pasteurization takes its name from its inventor, Louis Pasteur. The most familiar pasteurized product is milk. Before milk was routinely pasteurized it spread tuberculosis, brucellosis, typhoid fever, diphtheria, and scarlet fever. Milk is not the only pasteurized food product available in grocery stores.
A newer form of pasteurization is called ultrahigh temperature (UHT) pasteurization. The amount of bacteria killed with a heat method such as pasteurization depends on how high the temperature is, and how long the food is held at that temperature. For instance, heating to a lower temperature requires that the food be held at the low temperature for a longer time. But using very high heat means that the food can be kept at that heat for a shorter time. UHTpasteurization takes advantage
of this principle by using a very high temperature for a very short time. This provides an almost sterile product with an increased shelf life, but without significant changes in color, flavor, or texture of the food. Many countries use UHT for processing milk, which allows consumers to purchase cartons of milk that do not need refrigeration.
Preservation techniques that limit the availability of water, such as drying, salting, and smoking, and those that use heat, such as canning and pasteurization, dramatically alter the nature of the food itself. These processes degrade the color, flavor, texture, and nutrients in food. Today’s consumers want their food to appear fresh and natural, as close to just-picked or just-slaughtered as possible. They don’t want preservatives and other chemicals added to their foods, and at the same time they want convenience.
A number of new techniques are in use or in development that try to meet this demand for food with fresher, more natural qualities. There are several methods of applying electricity instead of heat to pasteurize food; these techniques are referred to as cold pasteurization. Irradiation, ohmic heating, and high-intensity pulsed electric fields are some of these technologies. One of the earliest examples of applying electricity to foods is ohmic heating. In this process, a continuous electric current, which generates heat, is passed through the food. This method of processing is useful for viscous liquids and foods containing particles.
High-intensity pulsed electric fields (PEF) is another emerging nonthermal technique. Unlike ohmic heating, PEF does not cause an increase in the temperature of food. PEF involves applying a short burst of high voltage to a food placed between two electrodes, which destroys bacterial cell membranes. This process has the potential to be used on juices, cream soups, milk, and egg products—all products in which heat produces undesirable changes.
Food irradiation is another technology to make food safe that, like canning. All irradiation is energy moving through space in invisible waves. The length of the wave determines the nature of the energy. As the wavelength gets shorter, the energy of the wave increases. Microwaves have a relatively long wavelength, so they have lower energy that is strong enough to move molecules and cause heat through friction, but not strong enough to structurally change atoms in the molecules.
Ionizing radiation has a shorter wavelength and therefore higher energy—enough energy to change atoms by knocking electrons from them to form ions, but not enough energy to split atoms and cause exposed objects to become radioactive.
Food irradiation exposes foods to very high-energy, short-length invisible waves. Depending on the dose, irradiation performs different functions. Low doses delay ripening and sprouting in fresh fruits and vegetables, and control insects and parasites in foods. Medium doses extend the shelf life of foods and reduce both spoilage and pathogenic microorganisms by damaging the genetic material of bacteria so they can no longer survive or multiply. High doses disinfect certain food ingredients, such as spices, and sterilize meat, poultry, seafood, and pre-pared foods.
Although 40 countries currently permit irradiation of food, its usage has been slow to catch on in the United States. This is mainly due to high start-up costs and to fears about consumer acceptance of the process. When the technology was new, many thought irradiation meant the food would be radioactive. As consumers have become more educated, they have realized that is not the case. Opponents of irradiation argue that irradiating foods produces chemical changes in the food. Proponents of the technology answer that these same chemical changes occur when food is cooked. Recent outbreaks of E. coli O157:H7, Listeria, and other bacteria are creating a demand for irradiated products. Irradiated foods also retain their texture, color, and taste better than do foods that are preserved by heat treatments.
Applying high pressure uniformly throughout a food product is another method of non-thermal food preservation. This inactivates microorganisms, spores, and un-desirable enzymes, and increases the shelf life of foods. Japan is a leader in this technology. Although this method was initially studied at the end of the nineteenth century, consumer demand has caused renewed interest in commercializing the process.
Jams made by high-pressure processing retain the taste and color of fresh fruit, unlike conventionally cooked jams. High-pressure processing is currently used in yogurts, salad dressings, and citrus juices. It has the potential to be used for minimally processed meat and fish products, convenience foods with long shelf lives and fresh and natural colors, and frozen foods with improved quality. The major draw-back to this method is that it is costly to implement. Future usage will depend on how much the consumer is willing to pay for more natural food.
Modified atmosphere packaging (MAP) is a process in which oxygen is removed from a food package and other gases, usually carbon dioxide and/or nitrogen, are added. Vacuum-packaging, in which oxygen is removed but no other gases are added is also a type of MAP. The role of oxygen in food is that of a spoiler, one that causes degradation and spoilage of foods. Thus eliminating or reducing the amount of oxygen in a package prolongs the shelf life of the product.
However, from a food safety standpoint, there are some dangers with MAP products. Because lack of oxygen suppresses most spoilage bacteria in MAP products, the odors that would normally warn consumers that a food is spoiled are not present. There are some pathogenic bacteria, notably C. botulinum, that do not need oxygen to survive. Without the competition for food and water from spoilage bacteria, these bacteria can thrive. Manufacturers combat this by adding other gases such as carbon dioxide, which lowers the pH; decreasing the available water in a food; adding salt; and keeping the temperature low. It is important for consumers to understand that these types of products must be kept at the proper temperatures to keep them safe.
Ultraviolet (UV) radiation is the major bacteria-destroying factor in sunlight. Scientists are using UVlight to kill pathogenic microorganisms. The most prevalent use of this technology is to kill pathogens in water systems. It is environmentally friendly, safe, more cost-effective than chlorination, and doesn’t affect the taste of water as chlorination does. One problem with this technique is that it doesn’t penetrate sub-stances very deeply, so action is limited to the surface. At high doses products develop off flavors and odors, but at low doses it can extend the shelf life of foods without damaging quality. It is used in dairy plants, in meat and vegetable processing
plants, in the ice cream industry, and to sterilize packaging materials. As consumers have become chemical- and preservative-phobic, food preservation using natural antimicrobials has evolved. This concept involves a more natural and milder alternative to making food safer. By their very nature of being milder, natural antimicrobials by themselves are not sufficient to control pathogens. However, when used in combination with other food preservation methods they can improve the safety of foods without the use of traditional chemical preservatives such as sorbate or benzoate, which consumers no longer consider natural and healthy. Nature contains many antimicrobial compounds. Those used in food processing are derived from either plants or microorganisms. Spices and herbs have long been used to inhibit yeasts, bacteria, and molds. However, the spices and herbs themselves are less effective than the active ingredients such as essential oils, organic acids, and phenols found in them. Scientists are working to more actively exploit these active ingredients rather than using the whole spice or herb. As part of their life cycle, microorganisms produce compounds that affect the growth of other microorganisms around them. Many of these compounds inhibit microbial growth to increase the competitive
edge of the producing organism. Lactic acid bacteria are the most important of these natural antimicrobials. Lactic acid bacteria have been used for centuries in fermentation, cheeses, and sausages.