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

Given the continuing concerns worldwide over the safety of foods, Hungarian scientists wanted to study how effectively irradiation could improve the safety of semi-prepared meals. The researchers determined that the potential risk posed by non-sporeforming pathogenic bacteria could be considerably reduced by using gamma irradiation. However, storage temperature remains a crucial factor, and techniques should be developed to counteract the lipid-oxidative effect of the radiation processing.

Investigators prepared experimental batches of a stuffed pasta product--tortellini--and slightly pre-fried breaded reconstituted turkey steaks with cheese and ham filling--Cordon Bleu--according to commercial recipes. They inoculated the tortellini with [10.sup.4] CFU per g of S. aureus and the Cordon Bleu with [10.sup.6] CFU per g of L. monocytogenes before packing the products in plastic bags under a gas atmosphere of 20% CO2 and 80% [N.sub.2].

Then the scientists irradiated the inoculated package of tortellini at 3 kGy and the Cordon Bleu at 2 kGy with a [sup.60]Co 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 C and 9 C.

Unirradiated samples were kept together with the respective irradiated ones. Storage was continued for four weeks. The researchers performed microbiological tests before and after the products were irradiated, 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 the case of 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 it 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 of storage. The Listeria count in Cordon Bleu was reduced by irradiation from the initial count of 6.1 log CFU per g to 3.5 log CFU per g. At 5 C storage, this residual count remained stagnant up to three to four weeks, but started to increase at 9 C after one week of storage.

In the unirradiated samples, the Listeria count increased 100-fold during four weeks at 5 C, and during two weeks at 9 C. Sulphite-reducing clostridia were undetectable (at less than 0.48 log CFU per g) in all samples, even at 9 C. The factor limiting the shelflife of the unirradiated poultry was the growth of lactic acid bacteria at 9 C. Enhanced lipid oxidation was an unwanted side-effect of radiation treatment.

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