Food Protection Trends - August 2010 - 475

TABLE 1.

Listeria prevalence in chillers, pre- and post-chiller interventions Pre-intervention Blast (Jan. 2005 – Dec. 2005) Holding (Jan. 2005 – April 2006) n Detections (%) 24 (1.63) 25 (1.36) 49 (1.48) Post-intervention Blast (Jan. 2006 – Dec. 2006) Holding (May 2006 – Dec. 2006) n 1470 1102 2572 Detections (%) 7 (0.48) 1 (0.09) 8 (0.31)

Blast Chiller Holding Chillers All Chillers

1470 1838 3308

prevalence. Since numbers of detections were low, all samples types were combined for analysis. The percentage of samples with Listeria spp. detections for both chillers was 1.48% pre-intervention and 0.31% post-intervention. The decrease in numbers of Listeria detections before and after the introduction of chiller intervention was significant (P < 0.001) for both blast and holding chillers. The sampling was completed for Listeria species only. It would have been possible to look specifically for L. monocytogenes, but the additional resources required to perform this testing was not in line with its key outcomes, namely to evaluate the hypothesis that heating chillers reduces the presence of all Listeria species.

DISCUSSION
Heat treatment of the food processing environment to manage Listeria. The main focus when managing L. monocytogenes contamination of cooked meals is on preventing contamination by the post-cook factory environment. Heat can be used to manage L. monocytogenes in the post-cook factory environment, as this pathogen is not unusually heat resistant among vegetative Gram positive bacteria (12). The maximum growth temperature of L. monocytogenes is 45oC (12) and heat inactivation takes place above that limit, with the rate of inactivation being a function of both time and temperature. Heat has been used, for example, to surface pasteurize and reduce L. monocytogenes on vacuum-sealed precooked ready-to-eat meat products (11). Heat can also be applied as steam directly onto surfaces and equipment that need to be sanitized. Of course, the potential of “caking-on” of product needs

to be considered individually, based on the particular food matrix. The application of steam onto equipment can be optimized for complex machinery by covering the equipment to be treated with a tarpaulin so as to maximize steam contact time and penetration. Cookrooms also manage environmental Listeria by “pasteurizing” mobile equipment capable of surviving such a heat treatment (17). It has been observed that heating air within a room can be effective for removing moisture at the end of cleaning sanitation (17). A noteworthy observation from staff at the facility we worked in was how the wet chillers were transformed to dry via the heat treatment. Chmielewski and coworkers (6) used predictive modeling to suggest that with proper control of time and temperature, hot water sanitation of stainless steel surfaces could serve as an efficient method for elimination of L. monocytogenes in biofilms.

Listeria biofilms in the food processing environment
The persistence of L. monocytogenes in food processing facilities has been ascribed to the ability of this pathogen to live in biofilms. A biofilm may be defined as “a microbially derived sessile community characterized by cells that are irreversibly attached to a substratum or interphase, or to each other, are embedded in a matrix of extracellular polymeric substance that they have produced, and exhibit an altered phenotype” (13). The Listeria present in chillers, targeted by this intervention, are likely to be in this biofilm state and would be expected to be more resistant to both heat and sanitizers

than their suspended counterparts (13); if they are in the biofilm strata where nutrients are depleted, cell growth is slow and may induce stress response and clustering. Dense clustering of cells and production of extracellular polymers effectively changes the heating menstruum, providing additional heat tolerance (5). In this survey, the presence of all Listeria spp. was monitored. The presence of any Listeria species in food may indicate poor hygiene (12). Previous biofilm formation by one species (e.g., a non-pathogenic species) may provide a niche for another species (2). It may be possible for a nonpathogenic bacterial species (Listeria or another genus) to take residence and develop a biofilm, and a pathogenic species such as L. monocytogenes may then establish residence in the pre-existing biofilm (5). Indeed, many Listeria species can exist within the same environmental site (8). Biofilms are more difficult to remove when formed in the presence of food residues (3, 4) as soil can have a protective effect on the heat inactivation of planktonic or sessile microorganisms. High fat substrates increase heat resistance of L. monocytogenes (3, 4). Food residues may also promote bacterial growth, subsequently influencing heat inactivation (7). There may be a degree of synergy between chiller heat treatment and desiccation-related stress. It has been suggested that simultaneous stressors may achieve an antimicrobial effect greater than the individual sum of each individually.

Application of heat treatment of chillers
In the intervention described here, a multi-discipline approach was taken at

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Food Protection Trends - August 2010

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