Pasteurization and Irradiation

Summary

Pasteurization and irradiation partially sterilize food to make it safe to eat without substantially altering its nutritional content, structure, and taste. Pasteurization uses mild heat treatment, whereas irradiation uses ionizing radiation. Both reduce the levels of pathogenic (disease-causing) and spoilage microorganisms to a level that renders the food safe to eat, provided it is stored appropriately for no longer than the prescribed time. Irradiation can also be used on fresh fruits and vegetables to kill insects and to retard biological processes, such as ripening. Irradiation will thoroughly sterilize food, packing material, and disposable medical items at high levels.

Definition and Basic Principles

Pasteurization uses mild heat to treat food, whereas irradiation, sometimes called radiation pasteurization or cold pasteurization, uses ionizing radiation. The primary purpose of each is to destroy microorganisms that would be pathogenic to human consumers without significantly changing the food's attributes. In addition, these processes can be used to eliminate microorganisms or enzymes that spoil food, leading to a longer shelf life and less waste. Irradiation can also be used on fresh fruits and vegetables to kill insects and delay germination, ripening, or sprouting.

Pasteurization, used primarily with liquid foods, such as milk, fruit juices, and beer, refers to heat treatments that do not exceed 100 degrees Celsius (C). In contrast, heat sterilization (such as canning) uses temperatures of 100 degrees C or higher. High-temperature short-time (HTST) pasteurization generally destroys undesirable organisms while minimizing deleterious effects on the food. In milk, comparable killing of microorganisms can be achieved by conventional pasteurization at 63 degrees C for thirty minutes or by HTST at 72 degrees C for fifteen seconds or 88 degrees C for one second. Sterilization of milk at ultrahigh temperature (UHT) typically requires 138 degrees C for two seconds.

For irradiation, the ionizing radiation used is gamma rays (generated from the decay of radioisotopes cobalt 60 or cesium 137), X rays, and electrons, the latter two generated by machines for such purposes. The operators of equipment involving ionizing radiation need to be protected from its effects. The ability subsequently to pasteurize or irradiate food should not compensate for best practices to minimize contamination of food before treatment. Moreover, treated food also needs to be protected from subsequent contamination.

Background and History

Thermal and nonthermal processes have long been used to ensure the safety and storage of food. Cooking and smoking food were practiced in prehistoric times and likely permitted a survival advantage. Subsequently, drying, salting, and pickling were also used.

Pasteurization was developed in 1862 by French scientists Louis Pasteur and Claude Bernard, initially for to preserve beer and wine. The pasteurization of milk became widespread in the early 1900s.

X-rays were discovered by German physicist Wilhelm Röntgen in 1895, and radiation emitted from uranium and other radioactive elements was discovered by French physicists Henri Becquerel, Marie Curie, and Pierre Curie shortly after that. Patents were issued for food preservation using ionizing radiation in 1905 and for using X-rays to destroy Trichinella in pork in 1921.

In the twenty-first century, food irradiation was widely used in Asia, less often in America and Eastern Europe, and rarely in Western Europe, where consumer resistance was high. The Food and Drug Administration (FDA) allows irradiation for herbs and spices, fresh fruits and vegetables, wheat, flour, pork, poultry, and some seafood in the United States (US). It is also used for root crops (such as potatoes and garlic, to inhibit sprouting) and grains. According to FDA guidelines, such food must carry the label “Treated by irradiation” and the international Radura logo.

How It Works

Pasteurization. Heat kills pathogenic and spoilage microorganisms by disrupting their cellular structure and metabolism. In pasteurization, sufficient heat is applied to the food being treated to kill undesirable organisms but without damaging the food. In this balance, not all undesirable organisms are destroyed, but they are reduced to such a level that the product, if stored appropriately, will be safe for consumption until its use-by date.

Although some liquid foods, such as beer and fruit juices, may be pasteurized after filling containers (with warm water or steam applied to raise the temperature appropriately), most are pasteurized in a vat process or a continuous-flow process and then packaged. The vat (or batch) process involves heating in a well-agitated tank for the required time and temperature. The vat process is suitable for relatively small-scale operations.

The continuous flow process became possible when plate heat exchangers were developed in the late 1920s and has been enhanced by the development of concentric-tube heat exchangers. It is particularly well-suited to HTST and large-scale operations. In a typical system, the liquid to be pasteurized flows in a continuous tube from a cooled holding tank, through a preheater, through the heater (which heats the fluid to the required temperature), through a holding tube (whose size coupled with the flow rate determines the length of time that the liquid is held at the specified temperature), and then cooled down for storage. In practice, the preheater acts as a precooler, extracting heat from the heated liquid before it is further cooled, permitting 85 to 90 percent of the heat to be reclaimed. The temperature in the holding tube must be monitored to ensure that the desired temperature has been maintained and if not, the flow must be automatically diverted back to the starting tank.

In both vat and continuous-flow processes, the system must be thoroughly cleaned between uses, but a single use of the latter may last for many hours.

Irradiation.Radiation kills pathogenic and spoilage organisms, as well as insects. It also retards germination, ripening, and sprouting. It does so by disrupting cell structure, cell metabolism, and, most importantly, DNA molecules, preventing further growth and reproduction. Irradiation exerts its effects by direct action of the radiation or indirectly, principally via the radiolysis of water. This leads to the generation of highly reactive chemical species, such as hydroxyl radicals and hydrogen peroxide. Smaller organisms are more resistant than larger ones, for instance, viruses compared with bacteria. Spores of species such as Clostridia that cause botulism are more resistant than vegetative cells. Gram-negative bacteria, including primary food pathogens Escherichia coli and Salmonella, are more sensitive than gram-positive bacteria. Gamma rays are more effective than x-rays, which, in turn, are more effective than electrons. These differences relate to the penetrating power of the radiation, with electron irradiation only suitable for the treatment of surfaces or thin packages.

In the process of irradiation, the product is brought in line with the radiation source for the requisite period. Electrons or X-rays are generated by machines for these purposes and can be turned on and off as required. Gamma rays result from radioactive decay so they cannot be turned on or off; when not needed, the sources of radioactivity are stored in a large water tank that absorbs the radiation. Irradiation with electrons and X-rays is well suited to a conveyor-belt system that brings the product into the radiation beam. With gamma rays, the use of an overhead rail system is preferred. The packages of product to be irradiated are suspended from that system and moved so that the package can be bombarded from various sides and angles to ensure uniformity of treatment. In all cases, a dosimeter (or dose meter) must be periodically included to ensure that the material has received the required dosage.

In food irradiation, sufficient radiation is applied to destroy undesirable organisms (including insects) or inhibit a biological process without adversely affecting the nutritional value and sensory characteristics of the food. As with pasteurization, organisms may not be completely eliminated but are reduced to a safe level, provided the food is stored appropriately for no longer than the permitted time. Radiation can penetrate packaging materials, reducing the risk of contamination after treatment. On the other hand, packaging materials can be affected by the ionizing radiation generating radiolysis that may migrate to the food and affect its taste. Careful choice of packaging material, as well as adhesives and printed material, must be made to avoid such problems. The fats in foods are susceptible to breakdown, forming products with unacceptable taste, but this effect is minimized by irradiating foods high in fat while frozen. Irradiation in the absence of oxygen minimizes the generation of byproducts that can affect the color and taste of the food.

Applications and Products

Pasteurized Products. Pasteurization is typically applied to liquids, such as milk, which is the best-known example. Most milk consumed around the world is pasteurized or heat sterilized. Before the development of milk pasteurization, more than 25 percent of food-borne diseases were attributed to milk and milk products. Many microorganisms, including pathogenic ones, survive well in milk. Combined with aseptic packaging technology, pasteurization makes milk less prone to spreading disease and less perishable. Fruit juices and beers may be flash pasteurized (HTST) to minimize spoilage. A few wines are pasteurized; wines with less than 14 percent alcohol are sometimes pasteurized (or ultra-filtered) to stop any further fermentation. Flash pasteurization is also used to make wines acceptable to strict Orthodox Jews. Liquid eggs can be similarly pasteurized; those in the shell can also be pasteurized in a series of warm-water baths.

Nonliquid pasteurized products include cheese, almonds, smokeless tobacco, crabmeat, bread, and ready-to-eat meals. Pasteurized cheese is made from pasteurized milk, so the liquid is treated in this process, although the cheese is subjected to heat treatments as well. Almonds can be pasteurized with a steam treatment, designed to kill any microorganisms on the outside of the nut; pasteurization of almonds can also refer to their treatment with propylene oxide, but that treatment is more appropriately called chemical fumigation. Smokeless (or chewing) tobacco is pasteurized by heating to 85 degrees C. Crabmeat labeled as pasteurized is heated to 113 degrees C for one minute in sealed cans or plastic containers. Because this temperature is higher than 100 degrees C, it is not properly termed pasteurization. Nevertheless, this treatment does not kill all pathogens present; it merely reduces them to a safe level, and the product must be stored at refrigerated temperatures for no longer than prescribed to ensure its safety. Bread and ready-to-eat meals are usually pasteurized by microwaves, which generate heat in the product being treated.

Irradiated Products. The extent of irradiation of a food will vary according to the desired end point and nature of the substance being irradiated. The absorbed doses are expressed in units of gray (Gy) and kilogray (kGy). Low doses (less than one kGy) will inhibit sprouting of the potatoes, garlic, onions, and other root foods, disinfect insects on fruits, grains, and dry foods, delay the ripening of fresh fruits and vegetables, and inactivate parasites on pork and fresh fish. Medium doses (one to ten kGy) will extend the shelf life of strawberries, mushrooms, fresh fish, and meat (if stored at between 0 and 4 degrees C), destroy parasites in meats, control molds on fresh fruit, and destroy pathogenic and spoilage microorganisms in spices, raw or frozen poultry, meat, and shrimp. High doses (greater than 10 kGy) will sterilize herbs, spices, meat, poultry, seafood, food additives (such as enzymes and natural gums), packaging materials (such as wine corks), disposable medical items (such as syringes, tubing, and gloves), and hospital food (especially for immune-compromised patients). The only exception to a maximum of thirty kGy in the United States is for sterilizing frozen packaged meats for National Aeronautics and Space Administration (NASA) space flights. When in space, American astronauts have been eating irradiated foods, such as beef, pork, smoked turkey, and corned beef, since the beginning of the space program. Interestingly, milk and milk products are not good irradiation prospects because they generate undesirable flavors.

In irradiation, gamma rays are used more often than electrons or X-rays, because of their greater penetrating power. Irradiation of fresh fruits and vegetables to destroy any mature or immature insects obviates any need to quarantine these products. In the past, such disinfection was done with methyl bromide, but its use is being phased out because it is an ozone-depleting chemical. The costs of food irradiation include high capital costs and modest operating costs. Irradiation facilities must protect workers from the radiation used; this involves thick walls and a fail-safe design that prevents accidental radiation exposure to employees when in operation. Several methods are available to determine if a food has been irradiated, but biological methods that enumerate dead and live microorganisms of the species of concern are particularly useful because they not only show how many survived but the total burden before irradiation, providing a check on good handling practices before treatment.

Careers and Course Work

Pasteurization and irradiation are integral to the food industry, ensuring food safety from its source on a farm to its consumption by people. To design or oversee operations involving these processes, one should possess undergraduate or graduate degrees in food science or food engineering; knowledge of the structure and value of food must be coupled with knowledge of engineering processes. In addition, one should possess a strong knowledge of microbiology and chemistry. One should understand the medical consequences of ingesting pathogenic microorganisms and, for irradiation, the fundamentals of nuclear reactions and nuclear safety. Mathematics is an essential foundation for careers in these fields.

Bachelor of Science majors appropriate for positions in these fields would be food science or food engineering, with coursework in mathematics, chemistry, microbiology, nutrition, food chemistry, food and industrial microbiology, and food processing. Such degrees prepare one for oversight positions in the food industry and for subsequent advanced degrees. Appropriate Master of Science and Doctoral degrees in food science or food engineering focus on research in the area of interest, with coursework to supplement that research. Such advanced degrees prepare one for positions in applied or basic research into pasteurization or irradiation processes in the food industry, government laboratories, or universities.

Social Context and Future Prospects

Pasteurization is a well-accepted technology. Nevertheless, proponents of raw milk contend that pasteurization is unnecessary if milk is kept clean from the udder of the cow to the consumer and that it destroys some desirable components in milk. Replacing hand milking with milking machines and direct transfer of raw milk to a cooling tank have made contamination from microorganisms from the environment less likely but do not eliminate the risk. Milk is a good medium for growing many microorganisms, including pathogens. Concerning components of milk that are destroyed in pasteurization, the only substantial loss is some enzymes, such as alkaline phosphatase. However, when eating, enzymes are inactivated and digested in the stomach and intestines of consumers, and they do not provide any demonstrable health benefits aside from being a source of amino acids. Public health and medical organizations promote the pasteurization of milk in the interest of food safety. Still, in the beginning decades of the twenty-first century, raw milk experienced a surge in popularity thanks to the claims made on the Internet and social media about its health benefits. 

Irradiation is not as accepted, mainly because of public concerns about nuclear radiation. Gamma irradiation does not increase radioactivity in food over what occurs naturally. Although it produces radiolytic products in foods, animal testing indicates that irradiated food is safe, and high doses have no adverse effects. Public health and medical organizations, such as the American Medical Association, attest to the safety of irradiation in protecting the food supply. Public education at the point of sale has effectively overcome resistance to irradiated foods.

Innovations continue in the pasteurization and irradiation processes, ensuring consumers that food safety is a priority. In pasteurization, high-pressure processing subjects foods to high pressure, destroying microorganisms while retaining nutritional value. Ultrasonic pasteurization and the use of pulsed electromagnetic fields kill bacteria and microbes. The US Department of Agrigulture developed a pasteurization process that uses Radio Frequency technology to kill food-born pathogens like salmonella. Irridation techniques expanded out of food safety and are used by the pharmaceutical and medical industries to sterilize products. 

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