Enterococcus faecium as a Salmonella surrogate in the thermal processing of wheat flour: Influence of water activity...

This study investigated the influence of temperature-dependent water activity (a_w) on thermal resistances of Enterococcus faecium NRRL B-2354 (E. faecium) and Salmonella Enteritidis PT 30 (S. Enteritidis) in wheat flour. The a_w for wheat flour samples at 20, 40, and 60 °C was determined by a vapor sorption analyzer and at 75, 80 and 85 °C using custom-built thermal cells with high temperature humidity sensors. Full-factorial isothermal inactivation studies of both strains in sealed aluminum-test-cells included three temperatures (75, 80, and 85 °C) and three a_w,25°C levels (0.30, 0.45 and 0.60 within ±0.02 range, prior to the thermal treatments). Isotherm results of wheat flour demonstrate a significant increase (P < 0.05) of a_w as temperature rises (e.g. a_w,25°C = 0.45 ± 0.02 became a_w,80°C = 0.71 ± 0.02 in a closed system). Inactivation kinetics of both microorganisms fitted a log-linear model, the yielded D-values varied from 2.7 ± 0.2 min (D_85°C of S. Enteritidis at a_w,25°C 0.60 ± 0.02) to 65.8 ± 2.5 min (D_75°C of E. faecium at a_w,25°C 0.30 ± 0.02). The z_T of E. faecium and S. Enteritidis decreased from 16.4 and 16.9 °C, respectively, to 10.2 °C with increased moisture content (dry basis) from 10 to 14%. Under all tested conditions, E. faecium exhibited equal or higher (1.0-3.1 times) D- and z_T-values than those of Salmonella. Overall, E. faecium should be a conservative surrogate for Salmonella in thermal processing of wheat flour for control of Salmonella over a moisture content of 10-14% and treatment temperatures between 75 and 85 °C.