388 Abstracts

RADIORESISTANCE OF LISTERIA MONOCYTOGENES IN MEAT AND CHEESE

JACEK SZCZAWIfiSKI, MALGORZATA SZCZAWIhSKA and BOkENNA STAI+2ZAK

Department of Food Hygiene, Faculty of Veterinary Medicine, Warsaw Agricultural University, Nowoursynowska 166, 02-787 Warsaw, Poland

Abatrac-Samples of ground beef, decontaminated with ionizing radiation, were placed in polyethylene (PE) or polyamide-polyethylene (PA-PE) pouches and inoculated with L. monocytogenes. Pouches were sealed in the presence of air or under vacuum and irradiated with increasing doses of Cobalt 60 gamma rays at 4°C or at - 18°C. D,, values for L. monocyrogenes irradiated in beef placed in PE pouches were 0.265 kGy (4°C) and 0.376 kGy (- 18°C). D,, values for listeria radiated in meat placed in PA-PE pouches were 0.322 kGy (4°C) and 0.393 kGy (- 18°C). It was found that packaging materials and temperature of irradiation exerted statistically significant effects on radiation resistance of L. monocyfogenes. In the second experiment samples of sliced cheeses packed in PA-PE foil were inoculated with L. monocylogenes and irradiated at ambient temperature. D,, values for listeria irradiated in the most popular ripening cheeses in Poland ranged from 0.258 to 0.273 kGy. D,, values for inherent microflora of cheeses were significantly higher and ranged from 0.398 to 0.420 kGy.

EFFECTS OF COMBINED TREATMENTS ON SPORE FORMING BACTERIA-POTENTIALITIES OF THE

MALTHUS INSTRUMENT

I. VIDkSt and J. BECZNER Central Food Research Institute, Department of Microbiology, Budapest

Ahatract-In a MALTHUS measurement the received results are evaluated with the help of a calibration curve. Generally the suspension of different initial counts are prepared from dilution series, and the initial cell counts obtained by plating method are plotted against the measured detection times. However there is no guarantee that the microbes of an unknown sample would behave as the ones used for making the calibration curve. And if the growth rate, lag-time or both differ, highly different detection times are obtained from the same initial count. In a case like this the difference between the calculated results and reality can be as big as several orders of magnitude. If we compare after a treatment the real survival with the results given by the MALTHUS instrument, the two curves differ from each other. The cause of this difference is that a calibration curve prepared with uninjured bacteria is used for the calculations of the initial count from the measured detection times. As lower numbers of uninjured microbes are necessary to give the same detection times as injured bacteria, the survival given by the MALTHUS instrument is lower than reality. The difference indicates the injury of the spores due to treatment. The MALTHUS instrument can be very useful in the study of combined treatments. With the help of special calibration graphs the effects of the different agents can be studied separately. The measurements were carried out in the RIKILT-DLO, Wageningen, The Netherlands, under the guidance of Mr. Henk Stegeman.

tPh.D. student at the University of Horticulture and Food Industry, Budapest.