Purpose: The aims of this study were i) to study the effect of the inoculum level of lactic acid bacteria (LAB) on the behaviour of L. monocytogenes during cheese making and ii) to develop of a predictive model to describe this effect, as well as the growth of LAB as a function of temperature.
Methods: Raw milk was inoculated with three different LAB concentrations (4, 6, and 8 log CFU g-1) and, also, contaminated with L. monocytogenes registered strain ATCC 19115. The contaminated milk was used to produce soft cheeses. The obtained data were used to validate a predictive model, which in turn was based on data, partly produced by IZSLER and partly available from the ComBase database (www.combase.cc). The model of Baranyi and Roberts was used to calculate the primary growth parameters, while the model of Ratkowsky was used to describe the growth rate as a function of the temperature. To take the inhibitory effect of the LAB concentration into account, we assumed that no cell division took place after LAB reached a concentration level (Jameson effect).
Results: The concentration of Listeria did not exceed the level of 8 log CFU g-1, which is normally its maximum population density, but stopped at around 5 log CFU g-1 when the LAB reached their threshold concentration. The predicted data were in good agreement with independent observations in cheese, showing a discrepancy between 2.18% and 4.64%.
Significance: This model may be a very useful tool to support the monitoring surveys carried out by officers of the Regional Veterinary Authority.