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Unformatted text preview: disinfection of drinking water; it is more effective but more expensive
than chlorination. Irradiation
UV light Owing to its poor penetrating ability, UV light is only useful for surface sterilization. γ -irradiation Exposure to γ-rays, known as food irradiation, effectively sterilizes many kinds of
food. Radiation dosage is usually measured in a unit called the gray (Gy), which is the
amount of energy transferred to food, microbe, or other substance being irradiated. To
get an idea of the radiation doses used consider the following: a single chest X-ray
delivers roughly half a milligray [1 mGy = 0.001 Gy]). How it works Irradiation involves placing the food product near a γ-ray source such as Co60 .
(Obviously the Co60 is not added to food, because this would make the food
radioactive). When microbes present in food are irradiated, water and other intracellular
molecules absorb the energy and create transient reactive chemicals that damage DNA.
One reactive chemical created by γ-irradiation is the hydroxyl radical. Because OH
radicals are so reactive, they are very short-lived. Thus after the treatment, no OH
radicals remain in the food product. Effectiveness The size of the DNA "target" is a major factor in radiation efficacy. Parasites and
insect pests, which have large amounts of DNA, are rapidly killed by extremely low
doses of radiation, typically with D-values of less than 0.1 kGy (in this instance, the
D-value is the dose of radiation projected to kill 90% of the organisms). It takes more
radiation to kill bacteria (D-values in the range of 0.3-0.7 kGy) because they have
much less DNA per cell unit (less target per cell). It takes even more radiation to kill a
bacterial spore (D-values on the order of 2.8 kGy) because they contain little water,
the source of most hydroxyl radicals that damage DNA. Because of their small
genomes, irradiation is practically useless for destroying viruses in food. Irradiation
does not inactivate exotoxins already present in food.
To “kill” Salmonella in freshly slaughtered chicken, a dose of 4.5 kilograys (kGy)
is required to reduce the population by 12 logs (12 D-value). It takes a higher
dose of irradiation to kill microbes in frozen turkeys; this is true of frozen foods in
general. Deinococcus radiodurans,
a strange berry
As seen above, microbes differ greatly in their sensitivity to irradiation. This is due to
several factors including the size of their genome, the rate at which they can repair
damaged DNA, and other factors.
Deinococcus radiodurans is a member of a of an evolutionarily distinct
group of Bacteria (Figure 1-11) It has been nicknamed "Conan the bacterium" and
designated as a poster microbe for extremophiles, those prokaryotes that can handle FOOD-301 the most extreme environmental conditions. D. radiodurans was discovered in 1956
in a can of meat that spoiled despite having been “sterilized” by irradiation. This
microbe has the greatest ability to survive radiation of any known organism. It can
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- Fall '12