**3. Gamma irradiation**

Gamma rays use irradiation given off by Cobolt-60, a radioisotope of cobalt (Steele and Engel, 1992). It is reported that all radiation facilities in the world use Cobalt-60 rather than Cesium-137 (WHO, 1987). Cobalt-60 is derived from cobalt-59 which is placed in a nuclear reactor and bombarded with neutrons until an extra neutron is absorbed forming the unstable radioisotope cobalt-60. Over 80% of the cobalt-60 available on the world market is being produced in Canada (Diehl, 1995). For use as a radiation source the activated cobalt pellets are encapsulated in a stainless steel linear in form of pin or pencil to minimize absorption of the cobalt and to minimize heat build up. The stainless rods are placed on racks which are stored in approximately 25 feet of water and raised into concrete irradiation chamber to dose the food (Jones, 1992). As the food go through the chamber, the stainless steel linears are raised above the water so that it is exposed to gamma rays. The irradiation dose applied to food is measured in terms of kiloGyray (kGy) and is usually measured in a unit called the Gray, abbreviated Gy. The newer unit,1 Gy = 100 rads; 1 kGy = 1,000 Grays). The practical working range of food irradiation is generally from 50 Gy to as high as 10,000 Gy, depending upon the food in question and the effect desired (Satin, 1993). There are three general application and dose categories that are referred to when foods are treated with ionizing radiation (Urbain, 1986): (1) Low dose (radurization) up to approximately 1 kGy for sprout inhibition, delay of ripening, and insect disinfection, (2) Medium dose (radicidation)- 1 to 10 kGy for reduction of non-spore forming pathogens, delay of ripening, and reduction of spoilage microorganisms, and (3) High dose (radappertization)-10 to 50 kGy for reduction of microorganisms to the point of sterility. In the United States, the amount of irradiation dose applied to food is controlled by plant quality personnel and United States Department of Agriculture (USDA) and inspectors (Giddings and Marcotte, 1991).
