A bulletin for the Australian Food Industry November 2002
Contents: Listeria in ready-to-eat foods | Effect of common sanitisers on Listeria monocytogenes | National Risk Validation Project | Validation of food safety control measures
Effect of common sanitisers on Listeria monocytogenes
The incidents reported above reflect the continuing challenge that Listeria monocytogenes poses to the food industry and particularly the ready-to-eat sector. The unusual growth and survival properties of the organism and its ability to adhere to food contact surfaces contribute to the complexity of eliminating it from the food processing environment. Microorganisms including L. monocytogenes adhered to surfaces are more resistant to disinfectants than those in suspension (Food Technology 46 1992 12 84; Journal of Food Protection 55 1992 246).
A recent study conducted at the Campden & Chorleywood Food Research Association in the UK encapsulates some of the barriers that must be overcome by an effective sanitation program in those environments where L. monocytogenes is found.
This study (Journal of Applied Microbiology Symposium Supplement 92 2002 1115) had a number of aims, one of which was to determine the disinfectant resistance of persistent strains of L. monocytogenes and Escherichia coli found in the UK food industry. An important finding from the study was that conditions are likely to be present in food factories that may give rise to the development of persistent L. monocytogenes (and E. coli) strains. The nature of this persistence, however, is not due to disinfectant resistance but may be due to physical adaptation (surface attachment, biofilm formation) to a whole range of environmental conditions. The authors conclude that current cleaning and disinfection programs, correctly applied to equipment and environments that are hygienically designed, effectively control the presence of potential pathogens in food factories.
Further work in this area should be focused on other aspects of persistence adaptation, particularly the removal of adhered strains from surfaces to suspension environments in which they are inherently less disinfectant resistant.
In the study, selected L. monocytogenes and E. coli strains isolated from five chilled food factories were assessed for any resistance to commercial disinfectants compared with a laboratory L. monocytogenes strain. The disinfectants chosen for testing were a commercial quarternary ammonium disinfectant (QAC) and sodium hypochlorite. The QAC was chosen because a detailed survey of the food industry showed that these disinfectants were the most widely used although many factories use more than one product often for different applications. QACs may be used on food processing equipment and surfaces where they are non-corrosive while sodium hypochlorite may be used for floors and drains.
The results obtained suggest that for the Listeria strains examined, there was no evidence that the three strains isolated from food factories were any more resistant to either the QAC or hypochlorite than the laboratory disinfectant test strain.
This study consolidates earlier work with special reference to L. monocytogenes and the need for a cleaning programme to obtain the cells in suspension as far as possible before the selected disinfectant is applied.
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