Purpose: The objectives of this study were: (i) to evaluate silicon dioxide as a carrier to mimic a low moisture environment, and (ii) to assess thermal resistances of Salmonella Enteritidis PT 30 and its potential surrogate Enterococcus faecium NRRL B-2354 over a range of aw at 80°C (aw,80°C).
Methods: The equilibrium isotherms (moisture content versus water activity) of silicon dioxide at 80°C were obtained using a newly designed cell. Bacterial inoculums were inoculated to silicon dioxide and dried in the ambient environment. Samples (~ one gram) were sealed in custom designed test cells, heated isothermally (80°C) at the selected aw,80°C, cooled, and plated on differential media. The aw,80°C (0.1 to 1.0) of inoculated samples was controlled in these cells by balancing water vapor pressure via solutions from 0 to 18mol/kg. The survivors were enumerated and used for D80°C calculation.
Results: Flat equilibrium isotherm curves of silicon dioxide were observed (aw remained at 0.30±0.02). Enterococcus faecium showed higher D80°C values than those of Salmonella Enteritidis at all tested aw,80°C. The D80°C for both bacteria increased exponentially when aw,80°C dropped from 0.7 to 0.1 (R2=0.94 to 0.98, D80°C of E. faecium/Salmonella at aw,80°C 0.7 and 0.1 were 4.4±0.2/2.1±0.2 and 281.8±10.0/171.0±18.5 min, respectively).
Significance: Silicon dioxide can serve as a carrier for evaluating bacterial thermal resistance in low moisture environment. Enterococcus faecium appears to be a valid Salmonella surrogate at 80°C regardless of the aw,80°C. The relationship between D80°C of both microorganisms in silicon dioxide and aw, 80°C can promote understanding of thermal resistances of bacteria in low-moisture foods.