Validating a physically heat-treated process for poultry litter in industry settings using the avirulent Salmonella surrogates or indicator microorganisms

Poultry litter is an excellent source of nutrients for the growth of agricultural crops. To reduce the microbiological risks associated with the use of raw poultry litter as a soil amendment or organic fertilizer, heat treatment is recommended to reduce or eliminate potential pathogenic microorganisms. Our recent studies have demonstrated that thermal resistance of Salmonella in chicken litter is increased significantly when cells are adapted to desiccation or when aged chicken litter with low moisture content is heat treated. By increasing the moisture level in chicken litter or applying a two-step heat treatment (wet heat followed by dry heat), Salmonella can be inactivated more rapidly. Our preliminary results indicate a good correlation in thermal inactivation rates between desiccation-adapted Salmonella and indigenous enterococci in chicken litter, suggesting enterococci as a potential indicator for heat process validation. We will collaborate with two large poultry litter processors to validate their heat-treatment processes in industrial settings by using Salmonella surrogate and indicator microorganisms identified in this study. Results from this research will provide some valid guidelines and tools for the fertilizer industry to produce Salmonella-free heat-treated poultry litter, thereby ensuring safe production of fresh produce.

Poultry litter, commonly used as a soil amendment or organic fertilizer, may contain human pathogens such as Salmonella. Heat treatment of raw poultry litter is recommended to inactivate pathogenic microorganisms and reduce the microbiological risks associated with the use of raw poultry litter. Results from our recent CPS-funded project demonstrated that thermal resistance of Salmonella in chicken litter was increased significantly when cells were adapted to desiccation or when dry chicken litter was heat treated. By increasing the moisture level in chicken litter or by applying a two-step heat treatment consisting of a moist-heat treatment at 65°C for 1 h and a sequential dry-heat treatment at 85°C for 1 h, Salmonella can be inactivated rapidly. Our preliminary data also indicated a strong correlation in thermal inactivation between desiccation-adapted Salmonella and indigenous enterococci in chicken litter. To apply our laboratory findings to real-world use, a pilot study in industrial settings is urgently needed. Due to biosafety concerns, surrogates for Salmonella need to be selected and validated prior to being released in a commercial processing environment. Therefore, in this proposed study, five potential surrogates will be desiccated and subjected to thermal inactivation at 75 and 150°C in poultry litter with a 30% moisture level. Salmonella Senftenberg, identified in our previous study as the most heat-resistant serotype during chicken litter processing, will be used as the benchmark microorganism for surrogate selection. The correlation analysis will be performed on thermal inactivation data for the selected Salmonella surrogates, indigenous enterococci and total aerobic bacteria in poultry litter (Obj. 1). For process validation, we have teamed up with two industry leaders, one in turkey litter processing (Suståne Natural Fertilizer, Inc.) and another in chicken litter processing (Perdue AgriRecycle LLC.), to conduct pilot runs in their processing plants. We will first determine the residence time of poultry litter during the drying process inside a 15-meter-long industrial dryer using color-coded magnetic MicroTracer®, and the indigenous enterococci and total aerobic bacterial populations in the litter at different residence times. The desiccation-adapted Salmonella surrogate in poultry litter with 20, 30, and 40% moisture levels at initial populations of ca. 107 CFU/g (placed in a customized stainless steel sampler) will be shipped overnight to our industry collaborators, added into the inlet of the dryer, and collected at the exit end. Each sample, shipped overnight back to the lab, will be analyzed immediately for the presence or population of Salmonella surrogate along with indigenous enterococci and total aerobic bacterial population (Obj. 2). Also, we will identify indigenous microbial species in poultry litter that survive the physical heat treatment, which may lead to future studies on the biological control of pathogens in soil amendments. The results from this study will validate current processes for physically heat-treated poultry litter in industrial settings, and will provide tools (surrogate and/or indicator microorganism for Salmonella) or guidelines (time/temperature, moisture level) for litter processors to modify their existing process parameters to produce microbiologically safe products. The ultimate goal of this study is to help the produce industry safely grow fresh produce.

1. Select avirulent Salmonella surrogates and indicator microorganisms for process validation of physically heat-treated poultry litter. 

2. Validate thermal processes for physically heat-treated poultry litter in industrial settings (turkey litter and chicken litter processing plants) using Salmonella surrogate and indicator microorganisms.

Objective 1: Select avirulent Salmonella surrogates and indicator microorganisms for process validation of physically heat-treated poultry litter. Our results clearly demonstrated that presumptive indigenous enterococci and desiccation adapted E. faecium NRRL B-2354 were more heat resistant at 75, 85, and 150°C as compared to desiccation-adapted S. Senftenberg 775/W in both turkey and chicken litter/compost, suggesting the use of these indicator and surrogate microorganisms can provide sufficient safety margin when validating the thermal processing of poultry litter. However, the thermal resistance of these microorganisms is affected by heating temperature, moisture content of poultry litter, and other factors. As these thermal inactivation data were collected under controlled experimental conditions in the lab, the effectiveness of using these as surrogate and indicator microorganisms should be validated in a commercial thermal processing plant of poultry litter under various processing conditions. 

Objective 2: Validate thermal processes for physically heat-treated poultry litter in industrial settings (turkey litter and chicken litter processing plants) using Salmonella surrogate and indicator microorganisms. We have used E. faecium NRRL B-2354 and presumptive indigenous enterococci in successfully validating the thermal processes of three poultry litter plants under different seasons. Even though the processing conditions in these plants varied greatly, the validation results showed that Salmonella levels could be reduced by at least 5 log, based on the reductions of surrogate and indicator microorganisms. The custom-designed sampler basket was capable of withstanding the harsh environment (high temperature, strong physical forces) inside the industrial dryers, and provided a carrier for the inoculated poultry litter samples to be exposed to the actual thermal process inside the dryer. Therefore, both the indicator and surrogate microorganisms along with the custom-designed sampler can be practical tools for poultry litter processors to routinely monitor or validate their thermal processes without introducing pathogens into the industrial environment. In consideration of the low cost to enumerate indigenous enterococci, poultry litter processors could do routine monitoring of their thermal process by using indicator microorganisms if minor changes occur to their processing parameters. Due to the fluctuation of indigenous enterococci populations in animal waste–based soil amendments, the use of spiking with E. faecium NRRL B-2354 is preferable for a validation study. Additionally, the microbial community analysis of poultry litter products will expand our knowledge on microbial community structure and functions, and identify potential beneficial microorganisms for agricultural applications. Overall, our findings from this study will assist poultry litter processors in optimizing their existing process parameters to produce Salmonella free physically heat-treated poultry litter, which can be used by the produce industry to grow microbiologically safe products.