Survival characteristics and the applicability of predictive mathematical modelling to Listeria monocytogenes growth in sous vide products

Survival and growth of Listeria monocytogenes isolates during sous vide processing and storage, and the applicability of predictive modelling in determining the potential for growth of L. monocytogenes in broth models and in sous vide products was investigated. L. monocytogenes grew in anaerobic tryptose phosphate broth and in chicken and beef samples by 2 log cycles in 8 days at 3°C and 4–5 log cycles in 6 days at 8°C. However, heating to an internal temperature of 70°C resulted in a 4–5 log reduction and 70°C/2 min resulted in a reduction greater than 7 log cycles. Lowering the product pH to 5.0 was effective in inhibiting L. monocytogenes growth, whereas a sodium chloride concentration of 2% had a negligible effect on growth rates. The square root model (Ratkowsky et al., 1983) predicted L. monocytogenes growth rates at 0–25°C with a coefficient of determination (R2 value) of 98.36–99.63% and a bias factor of 1.08 to 1.21 in beef, chicken and broth substrates of unmodified pH. In addition, the Response Surface Polynomial Model (Version 3.1, Buchanan et al., 1989) predicted generation times at 5–25°C with a 0–17.4% difference between observed and expected generation times in tryptose phosphate broth at pH 7.3. There were however, large differences (25.5 vs. 5.3 h) between observed generation times at pH 5.6 (8°C) and those predicted by the Pathogen Modelling Program in tryptose phosphate broth. A divergence from predicted values was also noted at lower temperatures (0–3.5°C) in the square root model.