There's a rather comprehensive article about food irradiation in Wikipedia
http://en.wikipedia.org/wiki/Food_irradiationIn a nutshell, it plays havoc in the living matter by breaking organic molecules and damaging the DNA. The bacteria naturally present on the food are killed. Other kind of bacteria can replace them, but it may be prevented by enclosing the food in a tightly sealed package.
It’s a mode of sterilization equivalent to others such as pasteurization. Perhaps the damage done by those processes is not as great as a thorough cooking, but it’s similar. Killing bacteria means killing the cells as well, therefore the food is dead.
Selected excerpts:
Food irradiation acts by damaging the target organism's DNA beyond its ability to repair. Microorganisms can no longer proliferate and continue their malignant or pathogenic activities. Spoilage-causing microorganisms cannot continue their activities. Insects do not survive, or become incapable of reproduction. Plants cannot continue their natural ripening processes.[1]
The energy density per atomic transition of ionizing radiation is very high; it can break apart molecules and induce ionization, which is not achieved by mere heating. This is the reason for both new effects and new concerns. The treatment of solid food by ionizing radiation can provide an effect similar to heat pasteurization of liquids, such as milk. The use of the term "cold pasteurization" to describe irradiated foods is controversial, since pasteurization and irradiation are fundamentally different processes.
Food irradiation using cobalt-60 is the preferred method by most processors, because the deeper penetration enables administering treatment to entire industrial pallets or totes, reducing the need for material handling.[30] A pallet or tote is typically exposed for several minutes to hours depending on dose. Radioactive material must be monitored and carefully stored to shield workers and the environment from its gamma rays. During operation this is achieved by substantial concrete shields. With most designs the radioisotope can be lowered into a water-filled source storage pool to allow maintenance personnel to enter the radiation shield. In this mode the water in the pool absorbs the radiation. Other uncommonly used designs feature dry storage by providing movable shields that reduce radiation levels in areas of the irradiation chamber.
One variant of gamma irradiators keeps the cobalt-60 under water at all times and lowers the product to be irradiated under water in hermetic bells. No further shielding is required for such designs.
Concerns and objections include the possibility that food irradiation might do any of the following:
* Mask spoiled food
* Discourage strict adherence to good manufacturing practices
* Preferentially kill "good" bacteria and encourage growth of "bad" bacteria
* Devitalize and denature irradiated food
* Impair flavor
* Not destroy bacterial toxins already present
* Cause chemical changes that are harmful to the consumer
* Be unnecessary in today's food system[73]