Date of Award
Master of Science (MS)
Nutrition and Food Sciences
Catherine W. Donnelly
Washed-rind, or smear-ripened, cheeses are considered high-risk for contamination with Listeria monocytogenes due to favorable growth conditions on the cheese rind and multiple points for contamination during post-production cheese care and aging. Foodborne illness outbreaks have implicated wash solutions and washing tools as potential vectors for transmission of L. monocytogenes. In order to evaluate the risk posed by the wash solutions and cheese brushes independently, a three-objective study was developed to investigate the growth potential of L. monocytogenes in cheese washing solutions (Objective I) and the transfer potential of L. monocytogenes between wash solution and cheese brush bristles of three different types when stored at 15˚C (Objective II). A final objective assessed the efficacy of commercial dairy sanitizers at eliminating L. monocytogenes from brush bristles (Objective III). Brushes were immersed in sanitizer solutions for 1-hour and results were compared to a heat treatment of 20-minutes submerged in boiling water in order to determine the most efficacious method of decontamination.
Wash solutions of four different compositions were found to support survival of L. monocytogenes for a minimum of 24h at two storage temperatures of 5˚C and 15˚C, and a solution composed of a commercial ripening blend of cultures in 3% NaCl solution supported 3.04 log and 5.79 log growth over 7 days when stored at 5˚C and 15˚C respectively. All three brush types were found to have an equally high risk for harborage and transmission of L. monocytogenes cells and no significant difference was found between percent recovery (p > .05). Similarly, all three brush types were found to transfer L. monocytogenes from brush bristles to an uncontaminated wash solution after overnight drying, illustrating the importance of decontamination prior to use and after use in cheese aging practices. Commercial sanitizers were found to be less efficacious at eliminating L. monocytogenes contamination in brush bristles when compared to a heat treatment of 20 minutes submerged in boiling H2O (no viable cells recovered).
Best practice guidelines for cheesemakers should include preparing wash solutions immediately before use, disposing of wash solutions immediately after use, and employing a stringent cross-contamination program when washing batches of cheese. The transfer of L. monocytogenes to and from brush bristles occurred at the introduction of brushes to contaminated wash solutions, irrespective of bristle type. These findings illustrate the importance of decontaminating brushes using a validated method before and after use to decrease the risk of contaminating cheese rinds with pathogenic bacteria. Boiling cheese brushes for 20 minutes in H2O successfully eliminated viable L. monocytogenes from brush bristles, while commercial dairy sanitizers were shown to be insufficient methods of decontamination for all three brush types. The findings of this study can assist cheesemakers in understanding the risks posed by this method of cheese production, and the methods investigated in this study provide cheesemakers with validated methods for decontamination that can easily and cost-effectively be implemented in a cheese production and aging facility.
Number of Pages
Neale, Rosalind Grace, "Assessing growth and methods of decontamination of Listeria monocytogenes in brushes and brines used for production of washed-rind cheese" (2020). Graduate College Dissertations and Theses. 1212.
Available for download on Thursday, February 16, 2023