Salmonella inactivation during extrusion processing

Extruded products, such as snacks, cereals, textured vegetable protein and pet foods, remain a highly recalled category of low moisture foods.  In the production of extruded products, an initial dry product formulation is fed into the hopper, where it is combined with added moisture, mixed, cooked, and formed.  The residence time in combination with temperature achieved (through added heat, shear and pressure) of many extruded products, should be adequate to ensure the inactivation of pathogens, but data did not exist to confirm this hypothesis.

Previously we discussed the importance of conducting validation studies to ensure the safety of low moisture foods.   This month, we will dive into one such challenge study that was conducted at the Institute for Food Safety and Health in collaboration with Wenger Manufacturing.  This experiment is unique in that it was conducted on pilot-scale equipment using pathogenic microorganism in a state of the art BSL-3 BioContainment Pilot Plant.

Typical conditions used in the production of extruded snack foods for human consumption include extrusion in a double-screw extruder reaching temperatures between 130-180oC.  The aim of this research was to study Salmonella inactivation during extrusion processing of a model cereal product using ‘worst-case’ scenarios.  These criteria included; low temperature (<100oC), low water activity/moisture content, low/no pressure (only backpressure to ensure the extruder barrel was full), low shear screw profile and a single-screw extruder.

Oat flour, a simple model product with high-indigenous fat levels, was inoculated with Salmonella Agona at levels ~7 log CFU/g.  Contaminated product was then processed using a Wenger X85 single-screw extruder at a feed rate of 75 kg/hr and a screw speed of 500 rpm, over a range of 0.72-0.96 water activities and 65-100oC temperature using a factorial design.  Product samples were collected at the end of the extruder barrel, immediately cooled, and enumerated to detect surviving populations.

Data collected was used to build a response surface model (population, temperature and water activity) for Salmonella inactivation.  Results indicated that using these ‘worst-case’ situations, extrusion can provide a 5-log CFU/g reduction of Salmonella.  While processing technologies may be adequate to provide a safe product, post-processing contamination of low moisture extruded products remains an issue and needs to be maintained to ensure the safety of these products. Full results can be found in this Journal of Food Science article.

salmonella inactivation

The project team included scientists, microbiologists, engineers, statisticians, biosafety, and students from a collaborative effort between the U.S. Food and Drug Administration, Illinois Institute of Technology/Institute for Food Safety and Health, and Wenger Manufacturing.