The occurrence of Listeria monocytogenes in soft cheeses and raw milk and its resistance to heat

Abstract

Listeria monocytogenes was detected by a colony count procedure in 7 out of 69 samples of imported soft cheese; numbers varied between 10 3 and 10 6/g. The organism could be detected by enrichment in another 3 samples. All samples containing L. monocytogenes had been prepared from raw milk. L. monocytogenes was detected in 6 out of 137 samples of raw milk obtained in The Netherlands. Heat resistance experiments carried out in open tubes suggested that L. monocytogenes could survive heating at 80°C for 1 min. These results could not be confirmed in heat resistance experiments with small impermeable plastic bags, enabling total submersion of the sample in the water bath. These experiments showed that L. monocytogenes inoculated at a level of 1.8 × 10 4/ml did not survive heating at 67°C for 20 s or more. Since these latter results were confirmed in experiments with a plate pasteurizer and the validity of results of open tube tests is questionable, it is concluded that normal pasteurization of milk would prevent the contamination of (soft) cheeses with Listeria spp., provided that recontamination during production and ripening is prevented.