Survival and Growth Characteristics of Listeria monocytogenes and Salmonella typhimurium on Stainless Steel and Buna-N Rubber

Abstract

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°C and 6°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°C and 75.5% RH. Both organisms were recoverable on the two surfaces after 10 days storage at 6°C and 75.5% RH. Survival of L. monocytogenes and S. typhimurium at 25°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°C and 75.5% RH, but failed to grow on buna-n. At 25°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 10 after 1 day storage at 25°C and 75.5% RH. Unlike other soils tested, survival of S. typhimurium at in conditions and L. monocytogenes at 25°C and both RHs in whey was higher on buna-n than on stainless steel. At 6°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.