Summary
Compost as soil amendment and organic fertilizer is a major source of nutrients for plant growth. Although the high temperatures generated by microbial activities during active composting can inactivate pathogens, the survival or regrowth of foodborne pathogens during composting process or in the finished compost can be problematic for vegetable production. This proposed study uses a systems approach to address pathogen control during composting process and subsequent storage and handling of finished products, develop and validate some practical strategies, which can be readily adopted by composting operators or growers. In this proposed study, we'll validate the thermal inactivation data of E. coli O157:H7 and Salmonella in compost using naturally occurring isolates; optimize and validate the finished compost as physical covering and straw as the base of passive static compost heaps and windrow compost piles; apply the pathogen growth model to determine the potential of finished composts to support the pathogen growth, and investigate the growth, survival, and control of food borne pathogens in the finished compost. The results from this study will provide practical methods or practices on compost production and handling to eliminate or reduce pathogen contamination of compost, thereby helping produce industry to grow safe products for human consumption.
Technical Abstract
As a major source of nutrients for crop production, the safety of composts as soil amendment and organic fertilizer is critical for ensuring produce safety. Although the high temperature generated by microbial activities during active composting can inactivate human pathogens, the survival or regrowth of these pathogens during composting process or in the finished compost can be problematic for the vegetable production. This proposed study uses a systems approach to address this biological hazard control during composting process and subsequent storage and handling of finished products, develop and validate some practical strategies, which can be used by growers. In this proposed study, first we will use naturally occurring Escherichia coli O157:H7 and Salmonella strains isolated from compost to validate those thermal inactivation data acquired from outbreak strains, which will be conducted inside an environmental chamber to mimic early phase of composting process (Objective I). Since the compost surface is where the pathogens survive the longest due to lower temperature being exposed to, our strategies are to apply the finished compost as covering material and the straw as the base of compost heaps to minimize the heat loss, and optimize the depth of covering over the freshly formed passive static compost heaps commonly found on small or medium farms (Objective II). Furthermore, we’ll validate this practical pathogen control method in a windrow composting system, and also evaluate the feasibility of weed seed germination as a composting temperature indicator which can be easily adopted by farmers. For Objective III, we’ll apply the pathogen growth model to predict the potential of finished composts (n=30) to support the growth of human pathogens. By comparing with currently required microbiological and maturity tests, we’ll identify the correlation of certain species or population level of background microorganisms with the safety of compost. Finally, we’ll determine the growth and survival of pathogens in the finished compost with different particle sizes, and apply the competitive exclusion (CE) microorganisms isolated and characterized from the previous project to some finished composts with potential for supporting pathogen growth, which will be conducted under greenhouse condition to simulate the storage condition on farm (Objective IV). The results from this study will provide the practical methods or practices on compost production and handling to eliminate or reduce pathogen contamination of compost, thereby helping produce industry to grow safe products for human consumption. This proposal will address the identified research area (1.1) of “Compost, Soil Amendment Fertilizer Use and Cultivation Practices”.
Research Objectives
1). Validating the thermal inactivation data collected from outbreak strains in compost using naturally occurring Escherichia coli O157:H7 and Salmonella.
2). Optimizing and validating the finished compost as physical covering and straw as base of freshly formed static compost heaps or windrow compost piles.
3). Applying the pathogen growth model to determine the potential of finished composts to support the growth of human pathogens.
4). Investigating the growth, survival, and control of foodborne pathogens in the finished compost.
Findings & Recommendations
Obj. 1: The naturally occurring strains of E. coli O157:H7 and Salmonella survived the thermophilic composting phase better than the corresponding outbreak strains in dairy and poultry composts, respectively. Therefore, the time/temperature combination data reported for these 2 pathogens using outbreak strains should be considered as the minimum and need to be verified for specific composting process in order to reduce the risk of pathogen survival during composting. Due to the prolonged survival of a few resistant NS cells in compost, it is recommended to validate the complete killing of the pathogens in the finished compost by using sensitive detection methods coupled with enrichment step.
Obj. 2. The finished compost as cover can increase the temperature at the interface of freshly constructed compost surface and the finished compost cover, while hay at the base of the composting heaps showed little impact on the composting temperature. Due to the presence of ammonia, Salmonella was inactivated rapidly even when the depth of finished compost only 10 cm thickness. The effectiveness of finished compost as a covering material on pathogen reduction was also verified in static and windrow composting systems at a commercial scale. Additionally, there is a strong association between inactivation of weed seed germination and E. coli O157:H7 growth, further studies to validate weed seed germination as an indicator of pathogen reduction in finished compost would be helpful. Based on the results of this study, it is recommended to cover the composting heaps or piles with about 10~30 inches of the finished compost, esp. during winter months when the ambient temperature is low, and use less finished compost for covering poultry compost heaps.
Obj. 3: By analyzing some representative agricultural wastes-based composts, we have found certain types of compost may have the potential for supporting pathogen growth due to the types and levels of indigenous microorganisms, although all these composts met the microbiological criteria and maturity of finished compost. Further studies are needed to identify those key microbial species responsible for pathogen control in the compost. Therefore, just relying on microbiological test results on fecal coliforms, E. coli, Salmonella, and E. coli O157:H7, and maturity or stability tests may not be sufficient to predict if pathogen growth can occur in the finished compost.
Obj. 4: Our results revealed that compost with larger particle size supports pathogen survival more than the compost with small particle size, and the initial rapid moisture loss in compost contributes to fast inactivation of pathogens in the finished compost. By applying competitive exclusion microorganisms to the finished compost with at least 30% moisture, up to 99% population of those E. coli O157:H7 cells due to cross-contamination can be effectively inactivated within 2 days during colder seasons (winter and fall). As for those heat-adapted E. coli O157:H7 cells surviving the thermophilic composting process, longer treatment with CE cultures is needed, suggesting the cross-resistance of those heat-adapted population. Additionally, both compost moisture and season of application may affect the efficacy of this biological control method as well. Based on the results of this study, we’d recommend covering the fresh compost surface with the finished compost or other physical barrier to reduce the aerosolization of compost particles. Produce field in very close proximity to the composting site should be checked periodically for possible pathogen transmission from the fresh compost heaps. To avoid the pathogen growth in the finished compost due to cross contamination, a cocktail mixture of CE can be applied a few days prior to the use of the finished compost, preferably in the colder seasons.