Effect of gamma irradiation on the shelf-life and nutritional quality of ready made meals

The effect of ionizing radiation on the microbiological and nutritional quality of chilled ready meals was assessed. The ready meals used for the purposes of this experimental work are normally produced for consumption in a number of hospitals in the Belfast area or for use as 'meals-on- wheels' for elderly people. The meals analysed consisted of roast pork and gravy, boiled potatoes and mixed vegetables or pureed portions of chicken and gravy, boiled potatoes and garden peas. Following arrival at the laboratory, the meals were either left unirradiated or irradiated with doses ranging from 1 to 3 kGy after which they were stored for up to 14 days at 3 deg. C or 10 deg. C being sampled at suitable intervals. Total viable counts (TVCs) were performed on the samples along with analyses for thiamine (vitamin B1) and ascorbic acid (vitamin C). The usefulness of the 2-alkylcyclobutanone and electron spin resonance (ESR) spectroscopy methods for the detection of irradiation treatment of the pureed meals was also assessed. Results showed that an irradiation dose of 2 kGy can be used to extend the shelf-life of ready meals for up to 14 days and that the irradiated meals must be stored under good refrigeration conditions (<3 deg. C) in order to obtain maximum benefit from the irradiation treatment. In terms of nutritional quality, it was found that losses of thiamine and ascorbic acid due to irradiation treatment were comparable to losses induced by cooking. Storage over the 14 day period at 3 deg. C was found to reduce the thiamine and ascorbic acid content of the roast pork and boiled potatoes, respectively, in the whole meals. The thiamine content of the pureed chicken and gravy was not affected by storage. The overall thiamine and ascorbic acid content of the whole and pureed meals, including the non-irradiated controls, was found to be low especially for the vegetables. The 2-alkylcyclobutanone method was successfully used to detect irradiation treatment in the pureed chicken and gravy with both 2-dodecylcyclobutanone (2-DCB) and 2-tetradecylcyclobutanone (2-TCB) being detected over the dose range of 1 to 3 kGy. Neither compound was detected in the nonirradiated control samples. Both 2-DCB and 2-TCB were found to increase linearly in concentration with increasing irradiation dose and there was no significant diminution in concentration when samples were stored for 14 days at 3 deg. C. ESR spectroscopy was successfully used to detect irradiation treatment via the cardboard packaging of the pureed meals. ESR spectra typical for irradiated samples containing cellulose were derived from the packaging of samples irradiated over a dose range of 1 to 5 kGy and were absent in non-irradiated samples. The intensity of the ESR signals increased with increasing irradiation dose. (author)