Fate of Listeria Monocytogenes and Salmonella spp. in Sphagnum Peat-Based Casing Soils and Pre-Harvest Food Safety Interventions to Reduce Contamination of Fresh Mushrooms
Open Access
- Author:
- O'Patchen, Rachel Lynn
- Graduate Program:
- Food Science
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- May 20, 2011
- Committee Members:
- Luke F LaBorde, Thesis Advisor/Co-Advisor
- Keywords:
- Listeria monocytogenes
food safety
mushrooms
peat soils
Salmonella - Abstract:
- Efforts are continually made in the mushroom industry to reduce the risk of contamination with human pathogens. Numerous random product samplings reveal that Listeria monocytogenes and Salmonella spp. are found consistently on whole and sliced fresh mushrooms, which has prompted several recalls over the years. The purpose of this study was to investigate multiple hurdles at pre-harvest that might prevent microbial contamination and growth of human pathogens on mushrooms. Two potential hurdles were investigated: natural microbial populations in casing soils and irrigation water supplemented with sanitizers. Light peat casing soil alone exhibited a suppressive effect on L. monocytogenes and Salmonella spp., and dark peat casing soils also had a suppressive effect on the pathogens, however, it took approximately 14 days for pathogens to reach the same low levels in the dark peats as they did in the light peat. Indigenous populations of total aerobic bacteria, Actinomycetes, and yeasts and molds were significantly lower in dark peat, as compared to the light peat, and with less competition in the dark peats, pathogen levels did not decrease as rapidly as in the light peat. Combinations of light and dark peat were used in a model mushroom growing system in ratios of 100:0, 80:20, and 60:40 light:dark and inoculated with L. monocytogenes and Salmonella spp. for a challenge study. Pathogen levels were sampled over the course of the growing cycle and emerging mushrooms were tested for presence of pathogens. Pathogen populations were lowest in the 100:0 light:dark peat and were highest in the 60:40 light:dark peat, however, in all three combinations pathogen levels decreased by at least 3.18 logs between inoculation and harvest. Average frequency of pathogen transfer from soil to mushroom was between 45 and 66 percent for Salmonella spp. and between 53 and 56 percent for L. monocytogenes. To replicate the scenario where mushroom pins are contaminated with Listeria and/or Salmonella spp., pins in a model mushroom growing system were spot-inoculated with the pathogens and irrigation water was supplemented with chlorine dioxide, sodium hypochlorite, hydrogen peroxide, and Tsunami 100- a commercial sanitizer containing peroxyacetic acid. When applied to experimentally inoculated mushrooms, log10 CFU/g reductions of both pathogens were between 2.5 and 3.9 and there were no significant differences between the control (water) and the sanitizers. Results of this research can be used to provide growers with information about peat casing soils and sanitizers so they can make informed decisions about what practices to use on their farms to grow and sell safe mushrooms.