Use of residence time versus screw speed in the response surface model for microbial inactivation during single-screw extrusion of low-moisture food

The implementation of the Food Safety Modernization Act requires the food industry to validate the processing interventions as a kill-step for foodborne pathogens. During extrusion process, the mean residence time (MRT) of the material plays a critical role in microbial inactivation. Therefore, the objective of this study was to develop a response surface model for the MRT as a function of moisture, fat content, screw speed, and temperature. This model was further incorporated into the previously developed models for microbial inactivation by replacing screw speed with MRT and evaluated for goodness-of-fit. The oat flour formulated to different moisture (14–26%) and fat (5–15%) contents was extruded in a single-screw extruder at different temperatures (65 and 95 °C) and screw speeds (75–225 rpm). The MRT was measured using 0.5 g of Congo red/kg of formulated sample. With this wide range of product and process variables tested, the range of MRT was minimal (111–144 s). The temperature had no significant effect on MRT in the range tested. Screw speed, moisture, and fat content showed a significant linear and quadratic effect on the MRT. The use of MRT instead of screw speed in the previously developed inactivation models slightly improved the R2 value from 0.83 to 0.85 for Salmonella and 0.84 to 0.89 for E. faecium. As MRT is challenging to measure with high repeatability when compared to screw speed, the slight increase in the goodness-of-fit measure does not warrant the use of MRT in inactivation models.