Purpose: The aims of this work were: 1) to model the survival response of L. monocytogenes strains in a lactic soft cheese stored at 4°C, after exposure to acid and salt stress; and 2) to evaluate the effect of lactate and diacetate salts on the inactivation kinetics.
Methods: Listeria monocytogenes strains T69, 159/10, 243 and ATCC19115 were subjected to acid and salt (NaCl) stress; then inoculated into soft cheese, which was stored at 4°C for 15 days. Survival data was fitted into four primary inactivation models. A secondary, second order polynomial function was used to model the effect of sodium lactate and diacetate addition on the rate parameter of the Weibull model. Suvivor diversity was assessed by (GTG)5-rep-PCR fingerprinting.
Results: Inactivation of stress treated cells was described by nonlinear models (R2 > 0.90). When unstressed, inactivation was best described by a convex double Weibull model (R2 = 0.86). Stressed cells significantly reduced (p < 0.05) inactivation rates. Addition of sodium lactate and diacetate (0.5 – 2.5% (m/v)) to the soft cheese resulted in an increase in the rate parameter in the Weibull model described by a second order, polynomial function (R2 = 0.84). Of 40 isolates, strain 159/10 remained the dominant strain representing survivors of acid and salt treated L. monocytogenes after 15 days.
Significance: Stressed L. monocytogenes have enhanced survival in acid soft cheese. Lactate and diacetate salts are not effective in controlling L. monocytogenes in lactic soft cheese. In contaminated foods, recent food isolates survived better than laboratory strains and could be more useful in predicting L. monocytogenes survival response.