Food Preservation: The Case for Irradiation

Current preservation techniques can kill bacteria on or in many foods such as milk and eggs, but their use is difficult on raw produce. Meanwhile, the headlines urge us to eat more fruits and vegetables -- often raw.

But how safe are raw foods? We've all heard of foodborne illness caused by contaminated fruits and vegetables, not to mention tainted meat.

Irradiation is slowly gaining consumer acceptance as a way to make foods safer. The Centers for Disease Control and Prevention (CDC) says that like pasteurizing milk and pressure cooking canned foods, treating food with ionizing radiation can kill most bacteria and parasites that would otherwise cause foodborne disease. Similar technology is used to sterilize medical devices so they can be used in surgery or implanted without risk of infection. Irradiation could be used on a variety of foods to eliminate insect pests and replace toxic chemicals that now are routinely used for fumigation of many foods. It can also slow the growth of molds, inhibit sprouting and prolong shelf life.

Specific radiation

Three different irradiation technologies exist, using different kinds of rays: gamma rays, electron beams and X-rays.

  • Gamma rays have been used routinely for more than 30 years to sterilize medical, dental and household products, and for radiation treatment of cancer. Gamma rays can penetrate foods to a depth of several feet. They do not make anything around them radioactive.

  • Electron beams (E-beams) are propelled out of an electron gun that is a larger version of the part of a TV tube that propels electrons into the TV screen, making it light up. They have been used as sterilizers for more than 15 years, according to the CDC. No radioactivity is involved. The electrons can penetrate food only a little over an inch, so the food treated must be no thicker than that. Two opposing beams can treat food twice as thick.

  • The newest technology is X-ray irradiation. The X-ray machine is a more powerful version of the machines used in many hospitals and dental offices to take X-ray pictures. X-rays can pass through thick foods. They require heavy shielding for safety.

When food is irradiated, the high-energy ray is absorbed as it passes through food. The ray gives up its energy, causing the food to be slightly warmed. Irradiation damages the DNA in any microbe, fungus or parasite present, killing the organism. Disease-causing organisms differ in their sensitivity to irradiation. Irradiation works very well to eliminate parasites and bacteria from food, but it will not work to eliminate viruses or prions. Parasites and insects have large amounts of DNA and so are rapidly killed by extremely low doses of irradiation. Bacteria have smaller amounts of DNA, so it takes more irradiation to kill bacteria and bacterial spores, the dormant state of some bacteria.  Viruses are the smallest pathogens that have DNA or RNA, and they are relatively resistant to irradiation at the levels approved for foods. Prions, such as the one that causes “mad cow” disease (bovine spongiform encephalopathy, or BSE), have no DNA, so they also are not affected by irradiation at the levels approved for foods.

A variety of foods have been approved for irradiation in the United States. Irradiation of meats must be approved by both the U.S. Department of Agriculture (USDA) and the U.S. Food and Drug Administration (FDA). Irradiation of fresh or frozen meat and meat products, including hamburger, fruits and vegetables, has been approved. 

Approval Year

Food

Purpose

1963

Wheat flour

Control of mold

1964

White potatoes

Inhibit sprouting

1986

Pork

Kill Trichina parasites

1986

Fruit and vegetables

Insect control, increase shelf life

1986

Herbs and spices

Sterilization

1990 - FDA

Poultry

Bacterial pathogen reduction

1992 - USDA

Poultry

Bacterial pathogen reduction

1997 - FDA

Meat

Bacterial pathogen reduction

1999 - USDA (pending)

Meat

Bacterial pathogen reduction

CDC says these are foods that can be made safer by irradiation:

  • Raw meat and poultry. Irradiation can kill bacteria, such as E. coli O157:H7, Salmonella and Campylobacter, commonly found on these foods.

  • Ready-to-eat meats like hot dogs and deli meats. Listeria is commonly found on these meats.

  • Fresh produce. Irradiation can eliminate parasites like Cyclospora and bacteria like Shigella and Salmonella.

  • Animal feed. Irradiation kills bacteria like Salmonella, preventing the contaminated feed from infecting animals.

Slow-growing market

Worldwide, a fraction of the food supply is irradiated to keep it safe. But the market for irradiated foods is growing slowly.

One reason is that the food industry and consumers have been wary of embracing a process that sounds dangerous, says food biologist Donald W. Schaffner, Ph.D., of Rutgers University. "'Irradiation' brings to mind nuclear power plants and radioactive contamination."

"Obviously, if you were to [use] huge doses of radiation, much larger than needed, you could make a food radioactive," says Dr. Schaffner. "But it would be just like putting something in the oven for too long -- it would catch fire." At the minimal levels of radiation commonly used to process food, he says, there's no possibility of the food becoming radioactive.

Surveys have shown that once consumers understand what's involved in the irradiation process -- and they see how much fresher their produce stays -- they prefer irradiated foods to non-irradiated foods.

By killing most pathogens, irradiation makes food safer to eat. In the United States alone, bacteria and other foodborne pathogens claim thousands of lives each year, and cause millions of cases of diarrhea.

But just because a food has been irradiated, Dr. Schaffner says, "doesn't mean you can take a carefree approach to handling food." In fact, critics argue that irradiation gives consumers a false sense of security.

Food safety important

Irradiation should accompany other food safety steps, such as good sanitation, healthy animals and proper cooking, experts say. If food isn't handled carefully during cooking or preparation, bacteria and other pathogens can still contaminate it.

Consumers have found another plus to irradiation: freshness. "Besides killing microbes," Dr. Schaffner says, "irradiation also inactivates certain enzymes in foods. Those enzymes cause the foods to ripen and rot. Irradiation helps food stay fresh longer."

On the down side, irradiation won't work on all foods. For instance, it wilts leafy vegetables and sprouts. And it doesn't kill all viruses. Some viruses found in raw shellfish appear radiation-resistant, for instance. Although hepatitis A can be killed with irradiation, the virus requires higher doses than those currently in use.

Irradiated foods can cost more. But recent studies by Kansas State University found that consumers were willing to pay up to 10 percent more for meat that had been irradiated -- because they knew the meat was safer.

Currently, every food that undergoes this process must be labeled with a special radura symbol, which indicates that it has been irradiated, and must carry the words "treated by irradiation" or "treated with radiation." The FDA recently ruled that the symbol and words can appear in small type on the back of the label, with the ingredients list.

How irradiation changes food

Irradiation changes the makeup of food, experts say -- but so do other food-processing methods.

"When you heat food, it causes changes. When you microwave food, it causes changes. When you fry food, it causes changes," says Dr. Schaffner. What happens to foods during irradiation is no different.

What's important, experts say, is that none of the chemical changes in irradiated food has been found harmful. The changes to foods caused by irradiation are so minimal that it is not easy to determine whether or not a food has been irradiated. Nutritional value is not changed. The taste of some foods may be slightly different, just as pasteurized milk tastes differently from unpasteurized milk. Living cells, such as seeds, will be damaged or killed just as microbes are. This can prolong the shelf life of potatoes, for example, by keeping them from sprouting. The levels of the vitamin thiamine are slightly reduced, but not enough to cause a vitamin deficiency.

Products such as glucose and carbon dioxide form during irradiation, but these are found in raw and processed foods. Experts say the nutritional changes are small compared with other food-processing methods, such as canning, drying and heat pasteurization. Irradiation doesn't significantly raise the temperature of food processed, reducing nutrient loss.

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