Optimization of extrusion conditions for elimination of mesophilic bacteria during thermal processing of animal feed mash

Salmonella and other pathogenic organisms that infect poultry and other livestock can originate from feed and environmental sources. Thus, measures are taken to control Salmonella infection in animals to improve food safety and reduce production losses. The current study was designed to investigate and optimize extrusion conditions for reducing bacterial counts in a surrogate feed matrix. A single-screw extruder was used to process feed artificially inoculated with Bacillus stearothermophilus 12980 (ATCC, Reston, Virginia). Preliminary experiments demonstrated that Salmonella typhimurium (S. typhimurium NALr) was eliminated from feed under conditions of moderate extrusion stringency (285 g moisture/kg mash feed, 83 °C extruder barrel exit temperature, 7 s retention time in the extruder barrel) and, therefore, a more thermotolerant organism was required to conduct the study. Spores of B. stearothermophilus 12980 inoculated into a surrogate feed matrix consisting of 600 g maize meal/kg, 300 g soya bean meal/kg and 100 g animal protein blend/kg, respectively, was used to investigate the effect of three extrusion variables on microbial killing. The three variables were extruder barrel exit temperature (T), mash feed moisture content (Mc), and mean retention time of feed in the extruder barrel (Rt). A rotatable central composite statistical design was used with three independent variables and five levels each. The quadratic response surface model fit to spore count data was used to predict extrusion conditions that maximized bacterial killing. The response surface indicated a stationary point within the design region that was a saddle. An estimated ridge of maximum killing indicated that a maximum reduction of 1.03 log cycles would occur under the following extruder settings: T = 110 °C, Mc = 245 g/kg and Rt = 11 s. Because the moderate stringency condition (T = 83 °C, Mc = 285 g/kg and Rt = 7 s) completely eliminated detectable S. typhimurium in the test feed matrix, it would appear that all S. typhimurium cells and all mesophilic organisms of similar thermal tolerance would be eliminated at most extruder conditions within the central composite design region.