Adequate surrogate identification is critical for validating in-plant thermal process controls for Salmonella inactivation in different food matrices. This study compared the thermal inactivation parameters (D- and z-values) and evaluated the heat resistance of Enterococcus faecium (8459) as a surrogate for a 5-serovar Salmonella cocktail in cornmeal. The cornmeal was spray inoculated with the respective bacteria to achieve ~9 log CFU/g population and set to the desired moisture contents (16, 22, and 28% w.b.). The inoculated cornmeal was then heat-treated at pre-determined temperatures (60, 64, and 68 °C) in sealed aluminum thermal-death-time disks in hot water baths for pre-determined time intervals. Injury-recovery media [brain heart infusion (BHI) agar overlaid with xylose lysine deoxycholate (XLD) agar for Salmonella or BHI agar overlaid m-enterococcus agar for E. faecium] were used for microbial enumeration to account for thermally injured bacterial cells. The D-values of Salmonella in cornmeal at 16, 22, and 28% moisture content were 37.5, 8.4, and 2.4 min at 60 °C, 19.9, 3.5, and 1.1 min at 64 °C, and 10.1, 1.4, and 0.5 min at 68 °C, respectively. The D-values of E. faecium in cornmeal at 16, 22, and 28% moisture content were 140.4, 18.9, and 3.3 min at 60 °C, 78.4, 7.1, and 1.6 min at 64 °C, and 37.3, 2.8, and 0.8 min at 68 °C, respectively. The z-values of E. faecium and Salmonella in cornmeal at 16, 22, and 28% moisture content were 13.9, 9.7, and 12.5 °C, and 14.0, 10.4, and 11.7 °C, respectively. These results indicated similar or higher thermal resistance (D-values) and equivalent thermal sensitivity (z-values) of E. faecium compared to Salmonella at different moisture contents and respective temperatures (P ≤ 0.05). Therefore, E. faecium could be used as a surrogate for Salmonella during thermal process validation of cornmeal processing.
Keywords: D-value; Extrusion; Surrogate; Z-value.
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