Improve safety of chilled semi-prepared meals using gamma irradiation.

Scientists based in Hungary looked to optimize the safety of semi-prepared chilled meals. They prepared experimental batches of a stuffed pasta product, tortellini, and slightly pre-fried breaded reconstituted turkey steaks with cheese and ham filling, Cordon Bleu.

The tortellini was inoculated with [10.sup.4] CFU per g of S. aureus. The Cordon Bleu was inoculated with [10.sup.6] CFU per g of L. monocytogenes. The products were packed in plastic bags under a gas atmosphere of 20% C[O.sub.2] and 80 % [N.sub.2]. Using a cobalt-60 radiation source, the inoculated tortellini was irradiated at 3 kGy. The Cordon Bleu was irradiated at 2 kGy.

It appears that the potential risk posed by non-spore-forming pathogenic bacteria can be considerably reduced by gamma irradiation. However, storage temperature remains a crucial factor, and methods should be developed to counteract the lipid-oxidative effect of radiation processing.

The investigators found that the irradiation doses were sensorially acceptable for these products. They stored the experimental batches of tortellini at 15 C, and the Cordon Bleu samples at 5 C and 9 C. Unirradiated samples were kept together with the irradiated ones. Storage was continued for four weeks. Microbiological tests were performed before and after the products were irradiated, and subsequently after every seven days.

The researchers evaluated total aerobic counts in all samples. For the Cordon Bleu, colony counts of lactic acid bacteria, Enterobacteriaceae, sulphite-reducing clostridia, yeasts and molds were also selectively estimated. The 3-kGy dose reduced the S. aureus count in tortellini below the detection limit, and the count remained undetectably low in the irradiated samples during all 28 days of storage. The S. aureus count in the unirradiated samples increased up to [10.sup.8] CFU per g during eight days.

The Listeria count in the Cordon Bleu declined from 6.1 to 3.5 logs CFU per g. At 5 C in storage, this residual count remained stagnant for up to four weeks. But it started to increase at 9 C after one week of storage. In the unirradiated samples, the Listeria count increased 100 hundred-fold during four weeks of storage at 5 C, and during two weeks at 9 C.

Sulphite-reducing clostridia remained undetectable at less than 0.48 log CFU per g in all samples at 9 C. What limited the shelf life of the unirradiated poultry products was the growth of lactic acid bacteria at 9 C. Enhanced lipid oxidation was an unwanted side-effect of radiation treatment.

Further information. Jo zsef Farkas, Central Food Research Institute, Herman Otto ut 15, H-1022, Budapest, Hungary; phone: [phone]; fax: [phone]; email: [e-mail].