Food irradiation is a technology for controlling
spoilage and eliminating food-borne pathogens, such as
salmonella. The result is similar to conventional pasteurization
and is often called "cold pasteurization" or "irradiation
pasteurization." Like pasteurization, irradiation kills
bacteria and other pathogens, that could otherwise result in
spoilage or food poisoning. The fundamental difference between
the two methods is the source of the energy they rely on to
destroy the microbes. While conventional pasteurization relies
on heat, irradiation relies on the energy of ionizing radiation.
The FDA emphasizes that no preservation method is a substitute
for safe food handling procedures.
How is food irradiated?
Bulk or packaged food passes through a radiation chamber
on a conveyor belt. The food does not come into contact
with radioactive materials, but instead passes through a
radiation beam, like a large flashlight. The type of food
and the specific purpose of the irradiation determine the
amount of radiation, or dose, necessary to process a particular
product. The speed of the belt helps control the radiation
dose delivered to the food by controlling the exposure time.
The actual dose is measured by dosimeters within the food
containers.
Cobalt-60 is the most commonly used radionuclide for food
irradiation. However, there are also large cesium-137 irradiators
and the Army has also used spent fuel rods for irradiation.
What are the sources of radiation used?
The food irradiation process uses three types of ionizing
radiation sources:
Cobalt-60 emits ionizing radiation in the form of intense
gamma rays.
"Gamma facilities" store it in stainless steel
capsules (like "pencils" of cobalt), in underwater
tanks. Cobalt-60 has several advantages:
- up to 95% of its emitted energy is available for use
- penetrates deeply
- yields substantial uniformity of the dose in the food
product
- decays to non-radioactive nickel
- considered to pose low risk to the environment.
However, its 5.3-year half-life offers disadvantages:
- cobalt-60 "pencils" require frequent replenishment
- treatment of the food is relatively slow.
Cesium-137 is a gamma source that is also used for irradiation.
Cesium-137 has a less penetrating gamma beam and a longer
half-life, making it more suitable under certain circumstances.
Electron Beam Facilities
Electron beam facilities generate e-beams with an electron
beam linear accelerator. (It works on the same principle
as a television tube.) The electrons are concentrated and
accelerated to 99% of the speed of light and energies of
up to 10 MeV.
Because e-beams are generated electrically, they offer
certain advantages:
- they can be turned on only as needed
- they do not require replenishment of the source as does
cobalt-60
- there is no radioactive waste
E-beam technology also has disadvantages:
- shallow depth of penetration
- e-beams must be converted to x-rays to penetrate large
items such as carcasses
- high electric power consumption
- complexity, and potentially high maintenance
X-Ray Facilities
X-ray facilities use an electron beam accelerator to target
electrons on a metal plate. Although some energy is absorbed,
the rest is converted to X-rays. Like gamma rays, X-rays
are penetrating, and can be used on food boxes 15 inches
thick or more. This allows food to be processed in a shipping
container.
X-rays offer the advantage of high penetration, but share
the other e-beam technology disadvantages.
What is the radiation dose to the food?
Radiation doses vary for different foodstuffs. For the
vast majority of foods, the limit is less than 10 kiloGray.
The U.S. Food and Drug Administration (FDA) sets radiation
dose limits for specific food types:
| Food Type |
Dose (kiloGrays) |
 |
| fruit |
1 |
|
| poultry |
3 |
|
| spices, seasonings |
30 |
The dose limit for spices and seasons is higher, because
they are consumed in very small quantities.
How does irradiation kill bacteria?
When ionizing radiation strikes bacteria and other microbes,
its high energy breaks chemical bonds in molecules that
are vital for cell growth and integrity. As a result, the
microbes die, or can no longer multiply causing illness
or spoilage.
Breaking chemical bonds with radiation is known as radiolysis.
References
- Center for Consumer Research at the University of California-Davis
- Iowa State University Food Safety Research Project
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