Response Surface Methodology for Salmonella Inactivation during Extrusion Processing of Oat Flour

Abstract

An increase in the number of foodborne outbreaks and recalls due to Salmonella in low-moisture foods has resulted in the need for the development and validation of process controls to ensure their microbiological safety. Furthermore, the Food Safety Modernization Act Preventive Controls for Human Food final rule requires food processors to validate their process controls to ensure food safety. The objective of this study was to develop a response surface model to predict Salmonella inactivation in oat flour, as affected by moisture, fat content, screw speed, and temperature. Oat flour was adjusted to different moisture (14 to 26% wet basis) and fat (5 to 15% [w/w]) contents and was then inoculated with a five-strain cocktail of Salmonella. Inoculated material was extruded through a single-screw extruder running at different screw speeds (75 to 225 rpm) and temperatures (65 to 85°C), without a die. Once steady-state conditions were attained, extruded samples were collected, cooled, and stored under refrigeration, and Salmonella survivors were enumerated. A split-plot central composite second-order response surface design was used, with the central point replicated six times. Temperature showed a significant ( P < 0.0005) positive effect on microbial reduction. Moisture content showed significant linear ( P = 0.0014) and quadratic ( P = 0.0005) effects, whereas higher fat content showed a significant ( P < 0.0001) protective effect on Salmonella destruction. The screw speed did not play a major role in inactivating Salmonella, but it had a significant ( P = 0.0004) interactive effect with temperature. Results indicated that a >5.5-log reduction was achieved in oat flour extruded at a temperature above 85°C at all moisture and fat contents evaluated at a screw speed of 150 rpm. The developed response surface model can be used to identify the extrusion process conditions to achieve a desired reduction of Salmonella based on the moisture and fat contents of the product.

Evaluation of Enterococcus faecium NRRL B‐2354 as a Surrogate for Salmonella During Extrusion of Low‐Moisture Food

“…The study done here had a slightly higher temperature range (75 to 95 °C), which resulted in a slightly higher moisture loss (1.0% to 3.0%). These results are also in agreement with the study done by Verma et al (), which reported a loss of 1.5% to 2.5% moisture during extrusion at similar conditions to those reported here. In industrial applications moisture losses obtained by flash‐off as hot products expand at the die during the extrusion may be as high as 4.0% to 7.0%.…”

“…In industrial applications moisture losses obtained by flash‐off as hot products expand at the die during the extrusion may be as high as 4.0% to 7.0%. At different fat levels (5%, 10%, and 15%), the water activity of the samples ranged from 0.70 to 0.95 for various moisture content (14% to 26% wb) at room temperature (Verma et al ).…”

“…To further evaluate the use of E. faecium NRRL B‐2354 as a surrogate for Salmonella , the effect of moisture content, fat content, temperature, and screw speed on their inactivation was compared. Because of their individual response to the process of extrusion, data for E. faecium NRRL B‐2354 inactivation were collected in the temperature range of 75 to 95 °C, whereas Salmonella inactivation data were collected in the range of 65 to 85 °C (Verma et al, ). To compare the behavior of both organism during extrusion, both models were extrapolated to the same temperature range of 65 to 95 °C (Figure and ).…”

“…To simulate such conditions, extrusion was then performed with the barrel filled at 75% capacity. Information from a preliminary experiment done by Verma et al () was used to identify the feed rate to achieve 75% capacity for different screw speeds in the lab‐scale extruder.…”

“…Before subsequent trials were performed, a push‐through cleaning procedure of the extruder barrel was done using uninoculated oat flour to clean the barrel. Verma et al () reported that feeding uninoculated oat flour for 20 min was sufficient for this push‐through cleaning at all different screw speeds.…”