The Alpha and Omega of Gamma
For the time being, gamma irradiation using cobalt-60 is the most commonly used food irradiation technology because it can penetrate big sacks of spices and herbs, which is most of the food currently irradiated in the United States.
I visited one of these gamma ray facilities this spring. Nestled between the docks of the Salem River and a quiet working class neighborhood in Salem, N.J., the facility owned by Ion Beam Applications SA (IBA) is an aluminum-sided building attached to a warehouse with a few administrative offices and a laboratory. A 1.8-meter-thick wall of solid concrete on one side of the building separates the warehouse from where the source rack of cobalt-60 is housed.
Sitting alongside boxes of medical instruments and empty milk cartons are giant sacks of black pepper and paprika, waiting for their trip on a conveyor belt around the source rack. When not in use, the rack sits immersed in a 9-meter-deep pool of deionized water. It contains 16 modules, each comprised of a frame in which several stainless steel rods called pencils, loaded with pellets of cobalt-60, sit side by side. Each rod is doubly encapsulated, measuring 46 cm long and 0.95 cm in diameter. The radionuclide’s 5.27-year half-life necessitates replenishmentnew pelletsabout once a year.
Suddenly, the conveyor belt springs to life. Over the din of clanging doors, whining forklifts, and blaring alarms, a worker loads a pallet of bags of spice onto the pneumatically controlled belt, which snakes along a maze-like path around the raised cobalt-60 source. Unlike e-beam or X-ray, which can be turned on and off, gamma sources give off radiation in the form of photons, all the time in every direction.
Every product processed in the facility has a minimum dose and a maximum dose. For food, the minimum dose is the point at which the target pathogen is destroyed. The maximum dose is the point beyond which the color, odor, or flavor of the food is changed. To control radiation dosages, each pallet dwells in a certain position along the maze for a specific amount of time to expose each side of the box or bag. How long depends on the density of the product and the desired dose.
"A sack of spices needs a minimum dose of 5 kGy," explains Chip Colonna, president of the GUARDiON Food Safety Division of IBA SA (IBA North America, Memphis, Tenn.). "But you can irradiate spices with up to 30 kGy and not notice any flavor or aroma change."
But what’s good for spice isn’t good for the gooseor chickenwhich has a higher water content than spices. It’s therefore more vulnerable to the creation of excess hydroxyl radicals that produce off-flavors, off-odors, and off-colors.