Use of ionizing radiation in refrigerated foods

The effect of irradiation on the shelf-life of ready to eat meals prepared by a local caterer was studied. The meals, comprising beef, mashed potatoes, beans and peas, trout, rice and carrots, and pork, potatoes, rice and green beans (nitrogen or air packed), were irradiated at 0.5, 1 and 1.5 kGy, or frozen (-55 deg. C) before irradiation at 1.5, 2 and 3 kGy, and stored at 3 deg. C. Batches were prepared in winter, spring or summer. The mesophilic aerobic counts (MACs), psychrotrophic aerobic counts (PACs), total and faecal coliforms, and Salmonella spp., Staphylococcus aureus, Clostridium spp. and Listeria monocytogenes were determined. A MAC of 1x106 CFU/g was considered to be the cut-off value. The organoleptic, 2-thiobarbituric acid (TBA) and free fatty acid values, as well as the available lysine and soluble starch, were determined. At the same time, golden conger fish mince inoculated with different loads of a non-toxigenic mutant strain of Vibrio cholerae was frozen before irradiation (1-7.5 kGy) to determine the survival of vibrios. The initial MACs of the non-irradiated meals ranged between 102 (winter) and 106 CFU/g (summer). The shelf-life of the 0.5, 1 and 1.5 kGy meals (with initial MACs of <104 CFU/g) was at least 7 days longer than that of the controls, and 21 days at a dose of 1.5 kGy (with initial MACs of 102 CFU/g). The 1.5 kGy dose was not effective in extending the shelf-life of meals with high initial MACs (106 CFU/g), but it did retard the growth of L. monocytogenes and eliminate faecal coliforms and S. aureus. The shelf-life was 14-28 days at 2 and 3.5 kGy. No differences (P>0.005) were found for the free fatty acids or the available lysine on day 0 and after 14 days of storage between the non-irradiated and irradiated beef, trout and pork at a dose of up to 3.5 kGy. The TBA values of the irradiated samples packed in nitrogen were always lower (P<0.005) than the corresponding control packed in air during storage. No starch hydrolysis was found in beans and potatoes at a dose of up to 3.5 kGy and in rice at up to 15 kGy, the dose needed to destroy Bacillus cereus spores. No vibrios survived at 3-7.5 kGy (at an initial load of 11x106 CFU/g), which indicates that a dose of 3 kGy could be sufficient to kill the V. cholerae present in infective loads, e.g. 107 CFU/g. It is concluded that irradiation at a dose of up to 3.5 kGy, combined with other processes, does not affect the chemical quality and can extend the shelf-life of ready to eat meals. (author)