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Modeling the Growth and Death Kinetics of Salmonella in Poultry Litter as a Function of pH and Water Activity

J. B. Payne^*, J. A. Osborne, P. K. Jenkins^* and B. W. Sheldon^*,1

^* Department of Poultry Science and Department of Statistics, North Carolina State University, Raleigh 27695

¹ Corresponding author: brian_sheldon{at}ncsu.edu

Contaminated poultry litter, serving as a reservoir for Salmonella, can be linked to both food safety concerns when contaminated birds enter processing plants and environmental concerns when used as a fertilizer. Predictive modeling allows for the estimation of microbial growth or inactivation as a function of controlling environmental growth factors. A study was conducted to observe the combined effects of pH and water activity (A_w) at a constant temperature on Salmonella populations in used turkey litter to predict microbial response over time. Litter, first pH-adjusted and then inoculated with a 3-strain Salmonella serovar cocktail to an initial concentration of ~10⁷ cfu/g, was placed into individual sealed plastic containers with saturated salt solutions for controlling A_w. A balanced design including 3 A_w values (0.84, 0.91, 0.96), 3 pH values (4, 7, 9), and a constant temperature of 30°C was used, with litter samples periodically removed and analyzed for Salmonella populations, pH, and A_w. At each combination of environmental factors, the Churchill or exponential inactivation mathematical models were used to describe the growth and death of Salmonella over time. Salmonella populations exhibited growth (~2 log) with little decline up to 42 d in litter environments of pH 7 and 9 and a A_w of 0.96. As litter A_w and pH levels were reduced, populations declined, with the most drastic reductions (~5 log in 9 h) occurring in low-pH (4) and low-A_w (0.84) environments. Generalized models for bacterial growth and death under grouped pH environments were successfully developed to predict Salmonella behavior in litter over time. These findings suggest that the best management practices and litter treatments that lower litter A_w to 0.84 and pH to 4 are effective in reducing Salmonella populations. The use of a single equation to predict the growth and decline of Salmonella populations as a function of pH and A_w has potential application for use in the development of effective pathogen control strategies at the farm level.

Key Words: Salmonella • mathematical modeling • litter • pH • water activity