Thermal inactivation kinetics of surface contaminating Listeria monocytogenes on vacuum-packaged agar surface and ready-to-eat sliced ham and sausage.

The aim of this work was to study thermal inactivation kinetics of Listeria monocytogenes on vacuum-packaged food surfaces. The kinetics were first determined on model agar systems (BHI agar plates), mimicking cooked meat products, which have the same characteristics (pH, sodium chloride (NaCl) or sodium lactate (NaL) content and thickness) as the cooked meat products. Then, in order to validate how well the thermal inactivation on the model agar system simulated inactivation on real products, inactivation kinetics of L. monocytogenes on slices of cooked ham and cooked sausage were examined. BHI agar plates (pH6.2 or 7.2) were prepared with and without the addition of 3% NaCl or NaL. They were initially inoculated with approximately 10CFU/plate culture, aseptically packaged in linear low-density polyethylene pouches, and vacuum-sealed. Thermal treatments were performed by submerging packages in a water bath maintained at 60°C. For most of the conditions studied, the inactivation curves were linear; shoulders were only observed for curves at conditions of pH6.2 with 3.0% NaL. The t values (time needed to obtain an inactivation of four-log reduction) were calculated based on the best fitting models included in GInaFit. The observed t values for L. monocytogenes on agar surfaces ranged from 6.8 (pH6.2) to 13.7min (pH7.2 with 3.0% NaCl). At pH6.2 addition of NaCl or NaL significantly increased the heat resistance of L. monocytogenes while at pH7.2 this effect was not significant. NaL seemed to affect the heat resistance to the same extent as NaCl. Inactivation curves of L. monocytogenes on slices of cooked ham at pH6.2 with or without addition of NaCl or NaL appeared to be log-linear in shape. However, the curves obtained from cooked sausages were markedly concave and the Weibull model was used for fitting. Concerning heat resistance of L. monocytogenes on meat products, t values increased approximately two-fold compared to those corresponding on model agar surfaces. The addition of 3.0% NaCl and NaL in cooked ham increased t values of L. monocytogenes from 11.8 to 24.9min and 24.3min, respectively. Similar effects were observed on cooked sausage. Survival on the cooked sausage, containing about 33% of fat, was not significantly different from that on cooked ham. Meanwhile, the addition of NaCl or NaL decreased the average proportions of injured cells substantially. The results of this study can be used by food processors to validate thermal processes with regard to the expected inactivation of L. monocytogenes post-contaminating meat product surfaces.