SURVIVAL AND GROWTH-CHARACTERISTICS OF LISTERIA-MONOCYTOGENES AND SALMONELLA-TYPHIMURIUM ON STAINLESS-STEEL AND BUNA-N RUBBER

Microorganisms harbored on food-contact surfaces are part of a complex ecosystem. The interactions of temperature, relative humidity (RH), soil and surface on the survival of Listeria monocytogenes and Salmonella typhimurium were studied. Survival and growth were monitored at 25 degrees C and 6 degrees C and 32.5% RH and 75.5% RH. Survival in phosphate-buffered saline and dilute pasteurized whole milk on both stainless steel and buna-n was highest at 6 degrees C and 75.5% RH. Both organisms were recoverable on the two surfaces after 10 days storage at 6 degrees C and 75.5% RH. Survival of L. monocytogenes and S. typhimurium at 25 degrees C and 75.5% RH was increased in dilute pasteurized whole milk on stainless steel, but not on buna-n. Organisms grew in pasteurized whole milk on stainless steel at 25 degrees C and 75.5% RH, but failed to grow on buna-n. At 25 degrees C and 75.5% RH, S. typhimurium was not recoverable on buna-n after 10 days in whole milk; however, L. monocytogenes remained close to initial levels. The survival and growth of both organisms in raw milk soil was similar to that in pasteurized whole milk soil. Buna-n was not bacteriostatic towards all organisms, as the total viable count in raw milk increased by more than a factor of IO after 1 day storage at 25 degrees C and 75.5% RK. Unlike other soils tested, survival of S. typhimurium at in conditions and L. monocytogenes at 25 degrees C and both RHs in whey was higher on buna-n than on stainless steel. At 6 degrees C and both RHs, L. monocytogenes levels remained constant on both surfaces in whey. The bacteriostatic effect of buna-n was not affected significantly by exposure to 20 cycles of a simulated clean-in-place (CIP) process.