Purpose: The objective of the study was to develop a response surface model to predict Salmonella inactivation in oat flour, as affected by product composition and process parameters.
Methods: Whole grain oat flour was adjusted to different moisture content (MC, 14 to 26% w.b.) and fat content (FC, 5 to 15%). Oat flour was inoculated with five-strain cocktail of Salmonella to attain ca. seven log CFU/g. Inoculated material was then extruded in a laboratory-scale single screw extruder running at different screw speeds (SS, 75 to 225 rpm) and different temperatures (T, 65 to 85°C) without a die. Samples were collected once steady-state conditions were attained, cooled, stored under refrigeration and Salmonella populations were determined. Samples were plated on both nonselective media (tryptic soy agar with yeast extract, TSAYE) and selective media (TSAYE overlaid with xylose lysine deoxycholate), incubated at 37°C for 24 h. A central composite 2nd order incomplete block design was used, with the central point replicated six times.
Results: The T showed the highest effect in microbial reduction. MC showed a significant quadratic effect whereas increase in FC resulted in significant lower reduction. The SS did not play a major role, but it had an interactive effect. The response surface for inactivation of Salmonella spp. in selective media is: log(Nt/No)=-7.9212+0.342*T-0.268*FC-1.007*MC+0.0762*SS-0.0011*SS*T+0.0277MC2 (R2=0.84).
Significance: Even under worst conditions, a significant reduction of Salmonella was achieved, which indicates that extrusion is an effective preventive control. Also, the developed model can be used to identify process conditions for different product matrices to achieve a desired reduction of Salmonella.