Studies on irradiation of pre-packaged prepared vegetables and improvement of microbiological safety of some sous-vide meals by gamma radiation

In the first part of the work, various aerobically packaged pre-cut vegetables, heavily inoculated with Listeria monocytogenes were gamma-irradiated with 1 kGy thereby not changing the sensory quality of the produce. This low-dose irradiation was able to improve the microbiological safety and stability of the refrigerated products stored at various temperatures. The radiation treatment significantly diminished the population of L. monocytogenes and increased the growth temperature requirement of its surviving cells. Losses of vitamin C as a direct effect of low-dose irradiation proved to be no higher than those which occurred in the non-irradiated samples during their shelf-life. The studies underlined the importance of good temperature control in the chill chain. Package conditions of minimally processed, low-dose irradiated chilled vegetables must be aerobic enough to prevent growth of non-proteolytic Clostridium botulinum. In the second part of the work, experimental batches of prepared meals such as chopped beef in tomato sauce, chopped beef in paprika gravy, and smoked-cured pork in stewed beans sauce were inoculated with spores of psychrotrophic Bacillus cereus, more heat- and radiation-resistant than spores of non-proteolytic C. botulinum. After vacuum packaging, the meals were treated with combinations of pasteurising heat treatments and gamma irradiation in the dose range of 2.5 to 5 kGy. Prior to and after treatments, and periodically during storage at 10 deg. C , total aerobic and total anaerobic viable cell counts, and, selectively, the viable cell counts of B. cereus and sulphite-reducing clostridia were determined. The effects of the treatment order as well as addition of nisin to enhance the preservative efficiency of the physical treatments were also studied. Sensory testing with uninoculated samples proved that the combination-preserved meals were of acceptable quality at the beginning of their storage. The microbiological investigations showed that the quality friendly sous-vide cooking in combination with mediumdose gamma irradiation and/or nisin addition may increase considerably the microbiological safety and keeping quality of the meals studied. (author)