HEAT-RESISTANCE OF LISTERIA-MONOCYTOGENES IN DAIRY-PRODUCTS AS AFFECTED BY THE GROWTH-MEDIUM

Listeria monocytogenes strains 1151 and Scott A were grown in broth at 30 degrees C and transferred to half cream, double cream and butter s tored at 5 degrees C to determine the influence of dairy product compo sition on heat resistance at 52, 56, 60, 64 and 68 degrees C. Strain 1 151 showed a higher heat resistance than strain Scott A. The heat resi stance of both strains was higher in the dairy products than in broth, particularly at lower temperatures. A significant difference was obse rved between log 10 of the D-values in the different dairy products. T he D-values obtained for both strains resuspended in all the dairy pro ducts would result in efficient elimination of the pathogen at 72.7 de grees C for 15 s. The highest D-value was 11.30 at 68 degrees C and by using a z-value of 6.71 degrees C it can be determined that at 72.7 d egrees C the D-value would be 1.5 s. The 15 s process would therefore achieve 10 log reductions. The effect of growth conditions on the heat resistance at 60 degrees C of L. monocytogenes Scott A was also inves tigated. When the cells were grown in the dairy products themselves, a nd particularly butter, the heat resistance of Scott A was enhanced; f or example, the D-values were 7.15 times higher than in broth. Further studies are required to investigate if this protection against heatin g exists at higher temperatures, in which case the efficiency of paste urization treatments or other heat treatments would be considerably lo wered.