Interlaboratory Evaluation of Enterococcus faecium NRRL B-2354 as a Salmonella Surrogate for Validating Thermal Treatment of Multiple Low-Moisture Foods.

ABSTRACT: This multi-institutional study assessed the efficacy of Enterococcus faecium NRRL B-2354 as a nonpathogenic Salmonella surrogate for thermal processing of nonfat dry milk powder, peanut butter, almond meal, wheat flour, ground black pepper, and date paste. Each product was analyzed by two laboratories (five independent laboratories total), with the lead laboratory inoculating (E. faecium or a five-strain Salmonella enterica serovar cocktail of Agona, Reading, Tennessee, Mbandaka, and Montevideo) and equilibrating the product to the target water activity before shipping. Both laboratories subjected samples to three isothermal treatments (between 65 and 100°C). A log-linear and Bigelow model was fit to survivor data via one-step regression. On the basis of D80°C values estimated from the combined model, E. faecium was more thermally resistant (P < 0.05) than Salmonella in nonfat dry milk powder (DEf-80°C, 100.2 ± 5.8 min; DSal-80°C, 28.9 ± 1.0 min), peanut butter (DEf-80°C, 133.5 ± 3.1 min; DSal-80°C, 57.6 ± 1.5 min), almond meal (DEf-80°C, 34.2 ± 0.4 min; DSal-80°C, 26.1 ± 0.2 min), ground black pepper (DEf-80°C, 3.2 ± 0.8 min; DSal-80°C, 1.5 ± 0.1 min), and date paste (DEf-80°C, 1.5 ± 0.0 min; DSal-80°C, 0.5 ± 0.0 min). Although the combined laboratory D80°C for E. faecium was lower (P < 0.05) than for Salmonella in wheat flour (DEf-80°C, 9.4 ± 0.1 min; DSal-80°C, 10.1 ± 0.2 min), the difference was ∼7%. The zT values for Salmonella in all products and for E. faecium in milk powder, almond meal, and date paste were not different (P > 0.05) between laboratories. Therefore, this study demonstrated the impact of standardized methodologies on repeatability of microbial inactivation results. Overall, E. faecium NRRL B-2354 was more thermally resistant than Salmonella, which provides support for utilizing E. faecium as a surrogate for validating thermal processing of multiple low-moisture products. However, product composition should always be considered before making that decision.

Evaluation of Enterococcus faecium NRRL B-2354 as a surrogate for Salmonella in ground black pepper at different water activities.

Thermal inactivation kinetics of Salmonella in low moisture foods are necessary for developing proper thermal processing parameters for pasteurization. The effect of water activity on thermal inactivation kinetics of Salmonella and Enterococcus faecium NRRL B-2354 in ground black pepper has not been studied previously. Identification of a suitable surrogate assists in conducting in-plant process validations. Ground black pepper was inoculated with a 5-serotype Salmonella cocktail or E. faecium NRRL B-2354, equilibrated to water activities of 0.25, 0.45 or 0.65 in a humidity-controlled chamber, and isothermally treated at different temperatures. The survivor data were used for fitting the log-linear models to obtain the D and z-values of Salmonella and E. faecium in ground black pepper. Modified Bigelow models were developed to evaluate the effects of temperature and water activity on the thermal inactivation kinetics of Salmonella and E. faecium. Water activity and temperature showed significant negative effects on the thermal resistance of Salmonella and E. faecium in ground black pepper. For example, significantly higher D values of Salmonella were observed at water activity of 0.45 (D70°C = 20.5 min and D75°C = 7.8 min) compared to water activity of 0.65 (D70°C = 3.9 min and D75°C = 2.0 min). D-values of E. faecium were significantly higher than those of Salmonella at all three water activities, indicating that E. faecium is a suitable surrogate for Salmonella in thermal processing validation.