Modeling the growth of Listeria monocytogenes in soft blue-white cheese

Abstract

The aim of this study was to develop a predictive model simulating growth over time of the pathogenic bacterium Listeria monocytogenes in a soft blue-white cheese. The physicochemical properties in a matrix such as cheese are essential controlling factors influencing the growth of L. monocytogenes. We developed a predictive tertiary model of the bacterial growth of L. monocytogenes as a function of temperature, pH, NaCl, and lactic acid. We measured the variations over time of the physicochemical properties in the cheese. Our predictive model was developed based on broth data produced in previous studies. New growth data sets were produced to independently calibrate and validate the developed model. A characteristic of this tertiary model is that it handles dynamic growth conditions described in time series of temperature, pH, NaCl, and lactic acid. Supplying the model with realistic production and retail conditions showed that the number of L. monocytogenes cells increases 3 to 3.5 log within the shelf life of the cheese.

Fig 1

Depth profiles of pH in blue-white soft cheese measured with microprofiling and microsensor equipment 3 days, 21 days, 42 days, 63 days, and finally 84 days after the production was initiated.

Fig 2

Calibration of model parameter μ_opt for ATCC 19111, ATCC 19115, strain 6179, and strain EGD (means and 95% confidence intervals). Strain 6179 grew faster than the other tested strains. Simu., simulated.

Fig 3

Validation of model performance: comparison of simulated (Simu.) and observed CFU of L. monocytogenes strain ATCC 19115 in blue-white soft cheese when inoculated in cheeses of different ages (means and 95% confidence intervals). μ_opt = 0.43 h⁻¹.

Fig 4

Validation of model performance: comparison of simulated (Simu.) and observed CFU of L. monocytogenes strain ATCC 19115 in blue-white soft cheese when inoculated at day 73 and stored at 4°C, 9°C, and 13°C (means and 95% confidence intervals). Applied parameter: μ_opt = 0.43 h⁻¹.

Fig 5

Application of model: comparison of simulated (Simu.) and observed CFU of L. monocytogenes strain ATCC 19115 in blue-white soft cheese when inoculated at day 13 (means and 95% confidence intervals). Applied parameter: μ_opt = 0.43 h⁻¹. The applied time series of pH and temperature similar to the actual conditions at the dairy are shown in the two graphs at the bottom.