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    Question ID:   11274         Current Version: 1
Question: How do you measure the amount of irradiation used?
Category: Food & Drink > Other
Keywords: D-value, Gray, Gy., measure, amount, dose, irradiation, used
Type: how
Rating:(0 ratings)    Views: 348    Discussions: 0   In Watch Lists: 1  

 
    Answer:
The dose of irradiation is usually measured in a unit called the Gray, abbreviated Gy.. This is a measure of the amount of energy transferred to food, microbe or other substance being irradiated. 10 kiloGrays, or 10,000 Grays, is the same as an older measure, the megaRad. A single chest X-ray has a dose of roughly a half of a milliGray (a thousandth of a Gray). To kill Salmonella., fresh chicken can be irradiated at up to 4.5 kiloGrays, which is about 7 million times more irradiation than a single chest X-ray. To measure the amount of irradiation something is exposed to, photographic film is exposed to the irradiation at the same time. The film fogs at a rate that is proportional to the irradiation level.

The killing effect of irradiation on microbes is measured in D-values. One D-value is the amount of irradiation needed to kill 90% of that organism. For example, it takes 0.3 kiloGrays to kill 90% of E. coli O157, so the D-value of E. coli is 0.3 kGy. These numbers can be added exponentially. It takes two D (or 0.6 kGy in the case of E. coli) to kill 99% of the organisms present, 3 D (or 0.9 kGy) to kill 99.9% and so on. Thus, once you know the D-value for an organism, and how many organisms might possibly be present in a food, the technician can estimate how much irradiation it will take to kill all of them. For example, if you think that a thousand E. coli O157 could be present in a food, then you want to be able to treat with at least 4 D, or 4 x 0.3 kGy, or 1.2 kGy. The D-values are different for each organism, and need to be measured for each organism. They can even vary by temperature, and by the specific food.

The energy of e-beams and of x-rays is measured in the amount of energy developed by the electron gun, and is measured in electron volts (eV). The usual apparatus runs at 5 to 10 million electron volts (MeV).

Source: CDC
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