Purpose: This study compared the thermal resistance of E. faecium and a Salmonella enterica cocktail in peanut butter and evaluated the reproducibility of the results obtained by two laboratories.
Methods: A five-strain Salmonella cocktail and E. faecium diluted 1:1 with peanut oil and emulsified with Tween 80 were used to inoculate commercial peanut butter. Samples were equilibrated to 25% relative humidity before dissemination to two laboratories. Aluminum test cells were filled with inoculated peanut butter in a humidity-controlled chamber and isothermally treated at three temperatures in the range 85 to 100°C, depending on the organism. After treatment, samples were serially diluted and enumerated on appropriate differential agar. Using organism survival ratios, D- and z-values were calculated using Matlab nonlinear fitting tools.
Results: The D90ºC for E. faecium (21.35±0.20 min) was greater (P≤0.05) than for S. enterica (12.62±0.15 min). Differences in D-values between laboratories were significant for two temperatures, but the nominal difference remained ≤12%. The z-value (14.94±0.34 and 15.49±0.45ºC) for S. enterica was similar (P ≥0.05) between laboratories. While the z-values for E. faecium were significantly different between laboratories (12.97±0.14 and 12.23±0.14ºC); the actual difference was small (0.74°C).
Significance: Enterococcus faecium was more thermally resistant than S. enterica at each temperature evaluated. Overall, results were reproducible between laboratories; however, the instances of unattributable differences reiterate the importance of cross-laboratory validations.