Validating Salmonella inactivation during thermal processing of the physically heat-treated chicken litter as soil amendment and organic fertilizer.

There is a growing demand for the physically heat-treated chicken litter being used as soil amendment and organic fertilizer for plant growth. However, chicken litter is often contaminated with human pathogens such as Salmonella spp. Heat treatments have been recommended to reduce or eliminate these pathogens, but no scientific research has been reported to validate if these treatments are adequate to produce the finished products free from pathogenic microorganisms. This proposed study will determine the thermal resistance of a mixture of four Salmonella serotypes at temperatures recommended for heat treatment of chicken litter. Several key environmental factors such as moisture level, nutrient variation, and the freshness of chicken litter will be evaluated. Furthermore, a practical method for combining moist heat treatment with drying process will be investigated for rapidly inactivating Salmonella in broiler chicken litter. The results from this study will provide some practical guidelines on time-temperature combination to treat chicken litter of different properties to produce the finished products as Salmonella-free. The ultimate goal of this project is to help the produce industry to grow safe fresh produce using organic fertilizers free from human pathogens.

Chicken litter, commonly used as soil amendment and organic fertilizer, may contain harmful human pathogens such as Salmonella spp. Although the physically heat-treated chicken litter has been recommended and used by produce growers, there is a lack of scientific data to prove if the heating processes in terms of time-temperature combination are adequate to kill human pathogens. This proposed study is to validate Salmonella inactivation during the heating processes as recommended for the physically heat-treated chicken litter by taking consideration of several factors such as type, dryness and freshness of chicken litter, and develop a two-step heat treatment for rapid pathogen inactivation. In this proposed study, we will use a mixture of four Salmonella serotypes (Typhimurium, Enteritidis, Heidelberg, and Senftenberg) which are inoculated at level of ca. 7 log CFU/g broiler chicken litter with different levels of moisture (20, 30, 40, and 50%). To determine the thermal inactivation rates of Salmonella, the inoculated chicken litter will be exposed to 75, 80, 85, and 150oC for a period of time until the final moisture level is below 10~12% as recommended by several guidelines (Objective I). Since the chicken litter varies significantly in chemical composition, microbial flora and physical properties, we will compare the thermal resistance of Salmonella in both broiler chicken and egg-laying hen litter, and determine the change of Salmonella heat resistance as affected by different storage ages of the broiler chicken litter collected from the same farm (Objective II). Furthermore, we’ll evaluate a two-step heat treatment of chicken litter by applying moist heat at 65oC first to inactivate Salmonella rapidly, followed by drying the chicken litter to the desired moisture level (<12%) at 80oC (Objective III). The results from this study will validate the effectiveness of the recommended thermal processes for producing the physically heat-treated chicken litter free from Salmonella contamination, and provide the industry with an effective heat treatment method for treating raw chicken litter. The ultimate goal of this proposed study is to help the produce industry to grow safe products for human consumption using pathogen-free soil amendment and fertilizer. This proposal will address the identified research area (1.1) of “Compost, Soil Amendment Fertilizer Use and Cultivation Practices”.

1. Validating the thermal inactivation of Salmonella spp. at different temperatures in broiler chicken litter 

2. Evaluating the effect of type and freshness of chicken litter on thermal resistance of Salmonella spp. 

3. Developing a two-step heat treatment for chicken litter to expedite Salmonella inactivation

Objective 1: Validating the thermal inactivation of Salmonella spp. at different temperatures in broiler chicken litter Our results demonstrated that the thermal resistance of Salmonella in aged chicken litter was increased significantly when the cells were adapted to desiccation or dry chicken litter were heat-treated. In addition, pronounced tailing was also observed in the survival curves of desiccation-adapted Salmonella at 70, 75, 80 and 85°C. Our observation implies that desiccation adapted cells from the tailing in survival curves should be considered sufficiently by chicken litter processors when applying thermal treatment to chicken litter. Otherwise, inadequate processing would lead to the survival of a few heat-resistant Salmonella cells that could contaminate produce in the field. In addition, the use of S. Senftenberg, verified as the most heat resistant serotype in this study, as indicator microorganism can assure microbial risk assessment of the ‘worst-case scenario’ when evaluating the thermal processing of chicken litter in future heat challenge studies. Overall, our findings have important implications for the chicken litter processors to validate and modify their heating process depending on the conditions of incoming raw chicken litter in order to eliminate Salmonella that may be subjected to dry stress during storage. 

Objective 2: Evaluating the effect of type and freshness of chicken litter on thermal resistance of Salmonella spp. Our results revealed that desiccation-adapted Salmonella in different types and storage ages of chicken litter displayed different survival profiles during heat treatment, and changes in moisture level, ammonia, electrical conductivity, heavy metals and indigenous microbial community of these samples could contribute to this difference. Overall, the desiccation-adapted Salmonella in fresh chicken litter was more susceptible to heat inactivation as compared in aged chicken litter of the same type or different type. Therefore, our recommendation to chicken litter processing industry is to process the chicken litter as soon as possible since the presence of ammonia and moisture in fresh chicken litter can enhance the inactivation rate of Salmonella during thermal processing. 

Objective 3: Developing a two-step heat treatment for chicken litter to expedite Salmonella inactivation due to the increased resistance of Salmonella in dried chicken litter during thermal processing, a few heat resistant cells may survive current physical heat processing and result in the contamination of the finished products. In order to provide temperature-time recommendations for processing physically heat-treated chicken litter, the most heat-resistant form of Salmonella, desiccation-adapted cells, was used to simulate the ‘worst-case scenario’. We have demonstrated that a two-step heat treatment by applying moist heat to the contaminated chicken litter first followed by dry heat, can not only ensure the fast inactivation of Salmonella but also produce more stable and nutrient dense finished products. Based on our results, a twostep heating technique consisting of a moist-heat treatment for 1 h at 65°C and a sequential dryheat treatment for 1 h at 85°C can be sufficient for achieving >5.5-log reductions of Salmonella in chicken litter with moisture content of ≥40%. Results generated from this study, after actual processing plant validation, will help the chicken litter processors to modify their existing process parameters to produce microbiologically safe organic fertilizers and soil amendment, thereby reducing the possible source of produce contamination on farm.