P3-07 Enterococcus faecium as a Surrogate for Salmonella in Thermal Treatment of Nonfat Milk Powder

Wednesday, July 12, 2017
Exhibit Hall (Tampa Convention Center)
Shuxiang Liu , Washington State University , Pullman , WA
Nurul Ahmad , Michigan State University , East Lansing , MI
Jie Xu , Washington State University , Pullman , WA
Ian Hildebrandt , Michigan State University , East Lansing , MI
Elliot Ryser , Michigan State University , East Lansing , MI
Meijun Zhu , Washington State University , Pullman , WA
Bradley Marks , Michigan State University , East Lansing , MI
Juming Tang , Washington State University , Pullman , WA
Introduction: Nonpathogenic surrogates play an important role in evaluating the efficacy of pathogen-reduction processes for low-moisture food products. Enterococcus faecium NRRL B2354 is now an accepted Salmonella surrogate for validating almond pasteurization processes; however, very limited information is available to support its use in milk powder.

Purpose: The objectives were to: (1) compare thermal resistance of E. faecium and Salmonella in nonfat milk powder (a0.25) at 85, 90, and 95°C and (2) evaluate the repeatability of the protocol by comparing results across two laboratories.

Methods: Nonfat milk powder was inoculated with either E. faecium or a five-strain Salmonella cocktail (Agona, Enteritidis PT30, Tennessee, Montevideo, Mbandaka; all linked to low-moisture foods) and equilibrated to aw 0.25 at 23°C for three to four days at Washington State University (WSU). After vigorously shaking to eliminate caking/clumping, ten 1-g subsamples were plated to assess homogeneity of the inoculum. Half of the samples (108.0±0.2 CFU/g) were shipped to Michigan State University (MSU) for isothermal treatment in parallel with WSU. Inoculated samples (~0.7g) isothermally heated (85, 90, or 95°C) in sealed aluminum test cells were periodically removed, cooled, and plated for survivors to calculate thermal inactivation parameters (D- and z-values).

Results: Both organisms exhibited log-linear kinetics at all temperatures tested. At all temperatures, D-values for E. faecium were significantly higher (P<0.05) than those for Salmonella. D-values for E. faecium/Salmonella were D85°C = 48.8±3.9/13.9±0.3, D90°C = 2.52±0.9/6.7±0.1, and D95°C = 13.2±0.6/3.5±0.1 min, respectively. Enterococcus faecium exhibited a higher z-value (17.6°C) than Salmonella(16.6°C).

Significance: Enterococcus faecium appears to be a valid surrogate for assessing thermal inactivation of Salmonella in nonfat milk powder, based on a higher surrogate thermal resistance.