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Improvements in Reduction of Feed Contamination: An Alternative Monitor of Bacterial Killing During Feed Extrusion¹

P. O. Okelo^*,,2, S. W. Joseph, D. D. Wagner^*, F. W. Wheaton, L. W. Douglass and L. E. Carr

^* Center for Veterinary Medicine, US Food and Drug Administration, Rockville, MD 20855; Department of Environmental Science and Technology, Department of Cell Biology and Molecular Genetics, and Department of Animal and Avian Sciences, University of Maryland, College Park 20740

Correspondence: ² Corresponding author: Phares.Okelo{at}fda.hhs.gov

Although numerous studies have documented the use of surrogate organisms for the evaluation of pathogens in human food, reports on the use of such organisms for similar studies in animal feed are limited. In the current study, dry feed inocula of Salmonella Typhimurium cells or Bacillus stearothermophilus spores were prepared and used to evaluate the efficiency of sterilization during feed extrusion. The inocula were placed in sealed stomacher bags and kept under 4° C refrigerated storage, where they remained stable for the 8-wk study period. Test feed batches were inoculated with the dry feed inocula of Salmonella Typhimurium or B. stearothermophilus spores, and the batches were then extruded by using a single-screw extruder set to various processing stringencies according to a designed experimental protocol. Only thermophilic B. stearothermophilus could be recovered from the test feed samples over the entire range of extrusion processing stringencies used (245 to 345 g of moisture/kg of feed; 3 to 11 s of feed retention time in the extruder barrel; and 77 to 110° C extruder barrel exit temperature). It was concluded that B. stearothermophilus is a suitable surrogate organism for evaluating the sterilization efficiency of feed extrusion and for identifying the optimal processing conditions to use during feed extrusion to eliminate, or at least minimize, the incidence of mesophilic and thermotolerant pathogens in feed.

Key Words: Bacillus stearothermophilus • Salmonella Typhimurium • spore • surrogate • feed • extrusion • sterilization • animal feed

¹ This article is dedicated to the memory of Lewis E. Carr.