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Determining the Environmental Factors Contributing to the Extended Survival or Regrowth of Foodborne Pathogens in Composting Systems


This study examines the effectiveness of composting for inactivating pathogens in manure, given that raw or inadequately composted animal waste applied to growing fields is a potential pre-harvest source of produce contamination. The primary mechanism for pathogen inactivation during outdoor composting is microorganism-related heat generation. In practice, the effectiveness of pathogen inactivation varies with environmental factors, including temperature, rainfall, nutrient sources, compost ingredients, and pathogens' induced heat resistance

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Key environmental factors related to the regrowth and survival of three foodborne pathogens in compost are evaluated in this study. Data from this project can be applied to the design of composting practices in California, the identification of environmental factors conducive to pathogen regrowth, and to the prediction of pathogen inactivation during on-farm composting.

<li>The specific objectives are to:
Identify the optimal conditions for E. coli O157:H7, Salmonella spp., and L. monocytogenes regrowth in composts under field application; The most desirable environmental conditions for inactivation, such as heavy rain/overhead sprinkling/flooding, nutrient sources (types of compost), and warm temperatures, will be evaluated for pathogen regrowth in compost. A cocktail of three strains for each pathogen will be heat-shocked and then inoculated into the compost with low initial populations. The inoculated compost will be maintained inside a greenhouse for four weeks and sampled periodically for pathogen analysis. Different varieties of compost produced in California will be used for studying pathogen regrowth.

<li>Determine the fate of heat-adapted avirulent pathogens during composting on the farm; Composting practices in California will be followed and simulated, using on-farm composting of vegetables with dairy manure and ingredients used in California (which can also be found in South Carolina.) The green wastes from either summer or fall organic crops will be collected from Clemson University's Calhoun Field Laboratory farm. Duplicate compost heaps will be set up on site. The survival of compost-adapted and heat-adapted avirulent strains of E. coli O157:H7, Salmonella spp., and L. innocua will be determined at three locations (surface, center, and bottom) of composting heaps. Field studies will be conducted during summer months (25-35 C) and winter months (5-15 C) to determine the influence of outdoor temperatures and precipitation on the fate of pathogens.

<li>Study the impact of initial heat resistance of pathogens on their survival during composting. Three strains for each pathogen will be heat-shocked at 45 C prior to the inoculation of compost mixture separately. Cultures without heat-shock treatment will be used as control. The inoculated compost held in a Tyvek pouch will be kept in an incubator at a range of temperatures (45, 50, 55 and 60 C) to simulate the conditions inside compost heaps during thermophilic composting. At certain intervals, samples will be taken out and analyzed for surviving bacterial counts. Mathematic models based on these data will be developed to predict the thermal inactivation of pathogens during composting.</ul>

Zehnder, Geoff; Luo, Feng; Jaing, Xuiping
Clemson University
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