Direct exposure to these powerful photons is very damaging to biological tissues, due to their ability to ionize (ie. strip electrons from) atoms they encounter, causing undesired chemical reactions to occur, often damaging genetic material. This is why it's so useful to us in terms of sterilization. In some cases, however, this ionizing radiation will damage the food itself (changing the flavour, texture, destroying vitamins), so gamma sterilization isn't necessarily appropriate for all foodstuffs.
However, in order to cause a material to become radioactive itself, you need to interact with the nuclei of the atoms directly, to change them from stable (non-radioactive) to unstable (radioactive) isotopes. This effect can in fact be achieved through exposure to gamma radiation, if said radiation is energetic enough.
This is why standards organizations (such as the FDA) place strict limits on the intensity of the gamma radiation used in food sterilization, to ensure that it stays well below the level where radioactivity can be induced in the food being sterilized. The FDA limits the energy of sterilizing gamma radiation to (I believe) 4 million electron volts, which is well below the threshold required to induce radioactivity.
Answered by: Colby Hayward, Computer Support Technician, Ontario, Canada
For something to become radioactive it has to have an unstable nucleus. For example, stable hydrogen has one proton in the nucleus. When you add one neutron you have deuterium which is stable. Add another neutron and you have tritium which is unstable and radioactive. When the isotope decays, they frequently release gamma rays, however exposing something to gamma rays, doesn't make it radioactive, since gamma rays cannot add or remove a neutron or proton from the nucleus.
However, there is a interaction called the photoneutron production where a gamma interacts with an atom and releases a neutron. This is only possible however if the energy of the gamma exceeds the binding energy of the atom. The two lowest atoms for binding energy are hydrogen and beryllium. Both about 2 Mev. A free neutron then could be absorbed by another atom and possibly make it radioactive.
The two isotopes most commonly used for food irradiation are co-60 and cs-137. The cesium isotope has a peak at 662 Kev and cobalt has 2 peaks. One at 1.3 Mev and one at 1.2 Mev. All the gammas being less than the 2 Mev needed.
From a practical standpoint, photoneutron production only generates meaningful results with gammas above 5-8 Mev.
So the short answer, is the gammas being used don't have enough energy to produce neutrons which could be absorbed to possibly make food radioactive.
As a side note, you may be interested to know many foods have naturally occuring radioactive isotopes present in them already. For example bananas and spinach have potassium 40. Nuts, such as peanuts and brazil nuts have high levels of radium. And the tritium talked about before is plentiful in the oceans.
Answered by: Scott Grasmick, Health Physicist
As the USDA website explains:
"Irradiation is a process that exposes products to ionizing radiation. Ionizing radiation has sufficient energy to remove electrons from atoms [hence the name "ionizing"], creating positive and negative charges that harm or kill the rapidly growing cells of insects, molds and microbial pathogens... FDA has established maximum energy levels for these machines [the electron accelerators that produce the radiation] to prevent the treated foods from becoming radioactive. The energy levels of the gamma rays are too low to induce radioactivity."
To change a stable isotope into a radioactive one takes thousands of times more energy than it takes to merely strip off an electron from a molecule. In fairness, any radiation that can ionize an atom can affect chemical changes in a substance. Many people worry that the irradiated foods are chemically altered in such a way as to be dangerous. But this concern is ill-founded. While some small changes occur, they have never been shown to be harmful in any way. In fact, many of these so-called radiolytic products are nothing more than substances already present in raw meats anyway.
People fear radioactivity -- and with good reason. But it is worth noting that the largest source of dangerous radioactivity in our lives is the sun -- not irradiated food, cell phones, nuclear power plants or anything else we tend to worry about! What's more, every year many thousands of Americans become sick (some of them dangerously so) from food-borne illnesses caused by bacteria or parasites.
For more information I would suggest the information from the US Dept. of Agriculture:
and the fact sheet from Rutgers University on the subject:
Answered by: Brent Nelson, Ph.D., Research Fellow, University of Michigan