Relating spore population of acid-producing thermophilic organisms and artificially inoculated Bacillus stearothermophilus to processing variables during extrusion of a standardized feed formulation

Salmonella and other pathogenic organisms that infect poultry and other livestock can originate from feed or the environment. Technologies to control Salmonella infection in poultry are important measures taken to reduce turkey and poultry production losses. A study was designed to determine extrusion conditions that optimized microbial inactivation in feed using the Extru-tech E325 single-screw extruder. Bacillus stearothermophilus was selected as the indicator organism after no viable cells of Salmonella typhimurium were detected when 28.5% moisture content mash feed (wet basis) was extruded at 83°C extruder barrel exit temperature with a 7 second feed retention time in the extruder barrel. Spores of B. stearothermophilus inoculated into a standardized feed formulation consisting of 60% corn meal, 30% soybean meal, and 10% animal protein blend, by mass, respectively, was used to investigate the effect of 3 extrusion variables on microbial inactivation. The 3 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 3 factors and five levels. The quadratic response surface model fit to acid producing, thermophilic organisms and artificially inoculated Bacillus stearothermophilus spore inactivation data was used to predict extrusion conditions that maximized inactivation. The response surface indicated a stationary point within the design region that was a saddle at T = 81°C, Mc = 27.6% wet basis (wb) and Rt = 8 s resulting in 0.170 log cycles of acid producing, thermophilic organisms and B. stearothermophilus spore inactivation. An estimated ridge of maximum inactivation showed a maximum of 1.03 log cycles at T = 110°C, Mc = 24.5% wb and Rt = 11 s. Because the least severe extrusion conditions (T = 83°C, Mc = 28.5% wb and Rt = 7 s) completely inactivated S. typhimurium in the standardized feed, it was speculated that all S. typhimurium cells would be inactivated at any set of extruder conditions within the central composite design region.