Modelling the fate and serogroup variability of persistent Listeria monocytogenes strains on grated cheese at different storage temperatures.

Processed cheese from cow's milk is one of the most consumed dairy products worldwide. Since this product is defined as ready-to-eat, foodborne pathogens such as Listeria monocytogenes can represent a health concern for susceptible populations. In this study, the individual and combined kinetic behaviour of four L. monocytogenes serogroups (namely, 1/2a, 1/2b, 1/2c and 4b) persistently isolated from a Spanish cheesemaking factory was modelled on grated cheese at different isothermal conditions (4 and 12 °C) during a 120-days period. The serogroup variability was characterized over the storage time by the isolation and identification of the different serogroups in the cocktail containing the four strains. This processed cheese did not support the growth of L. monocytogenes during storage. Survival patterns described by the log-linear type model indicated a high variability of L. monocytogenes serotypes at the tested temperatures: L. monocytogenes serogroup 4b showed a more rapid decrease rate at 4 °C than at 12 °C, while the opposite trend was found for the rest of serogroups and the L. monocytogenes cocktail containing all the strains. Survival rate of L. monocytogenes serogroup 1/2c at 4 °C was 0.007 log CFU/d being the most resistant serotype while at 12 °C, serogroup 1/2a showed the lowest survival rate (0.011 log CFU/d), thus showing a prolonged survival at this temperature. This study highlights the potential implications of L. monocytogenes contamination in processed cheese and shows that serogroup variability should be considered when evaluating survival patterns in contaminated products. Finally, the predictive models developed here could be useful to assist food operators to set specific storage conditions and formulations to avoid L. monocytogenes growth and survival in grated cheeses.