Improvement of the Microbiological Safety of Two Chilled Semi-Prepared Meals by Gamma Irradiation
József Farkas1,2*, Éva Andrássy1,2 and Katalin Polyák-Fehér1
1Department of Refrigeration and Livestock Products’ Technology, Faculty of Food Science, Corvinus University of Budapest, Ménesi út 45, H-1118 Budapest, Hungary
2Central Food Research Institute, Herman Ottó út 15, H-1022 Budapest, Hungary
Article history:
Received August 30, 2004
Accepted June 28, 2005
Key words:
gamma irradiation, tortellini, Staphylococcus aureus, Cordon Bleu, Listeria monocytogenes
Summary:
Experimental batches of a stuffed pasta product, tortellini, and slightly pre-fried breaded reconstituted turkey steaks with cheese and ham filling, Cordon Bleu, were prepared according to commercial recipes, then inoculated with 104 CFU/g of Staphylococcus aureus (in case of tortellini) and with 106 CFU/g of Listeria monocytogenes (in case of Cordon Bleu) prior to packing in plastic bags under a gas atmosphere of 20 % CO2 and 80 % N2. The inoculated packages were irradiated at 3 kGy (tortellini) and 2 kGy (Cordon Bleu) with a 60Co radiation source. The applied radiation doses were sensorially acceptable for these products. The experimental batches of tortellini were stored at 15 °C, while the Cordon Bleu samples were stored at 5 and 9 °C. Unirradiated samples were kept together with the respective irradiated ones. Storage was continued for 4 weeks and microbiological tests were performed before and after the irradiation, and subsequently after every seven days. Besides selective estimation of the counts of the test organisms, total aerobic counts were evaluated in all samples and in case of Cordon Bleu, colony counts of lactic acid bacteria, Enterobacteriaceae, sulphite reducing clostridia, yeasts and moulds were also selectively estimated. The 3-kGy dose reduced the S. aureus count in tortellini below the detection limit (logCFU=0.26), and it remained undetectably low in the irradiated samples during all 28 days of storage, while the S. aureus count in the unirradiated samples increased up to 108 CFU/g during 8 days. The Listeria count in Cordon Bleu was reduced by irradiation from the initial count of 6.1 to 3.5 logCFU/g. At 5 °C storage, this residual count remained stagnant up to 3–4 weeks, but started to increase at 9 °C after one week of storage. In the unirradiated samples, the Listeria count increased hundred-fold during 4 weeks at 5 °C, and during 2 weeks at 9 °C. Sulphite reducing clostridia were, and remained, undetectable (<0.48 logCFU/g) in all samples even at 9 °C. The limiting factor of the shelf-life of the unirradiated poultry products was the growth of lactic acid bacteria at 9 °C, whereas enhanced lipid oxidation was an unwanted side-effect of radiation treatment. From these studies it can be concluded that the potential risk posed by the investigated non-sporeforming pathogenic bacteria could be considerably reduced by gamma irradiation, however, storage temperature remains a crucial factor of safety and methods should be developed to counteract the lipid-oxidative effect of the radiation processing.
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