Modeling carbon dioxide effect in a controlled atmosphere and its interactions with temperature and pH on the growth of L. monocytogenes and P. fluorescens.

The effect of carbon dioxide, temperature, and pH on growth of Listeria monocytogenes and Pseudomonas fluorescens was studied, following a protocol to monitor microbial growth under a constant gas composition. In this way, the CO dissolution didn't modify the partial pressures in the gas phase. Growth curves were acquired at different temperatures (8, 12, 22 and 37 °C), pH (5.5 and 7) and CO concentration in the gas phase (0, 20, 40, 60, 80, 100% of the atmospheric pressure, and over 1 bar). These three factors greatly influenced the growth rate of L. monocytogenes and P. fluorescens, and significant interactions have been observed between the carbon dioxide and the temperature effects. Results showed no significant effect of the CO concentration at 37 °C, which may be attributed to low CO2 solubility at high temperature. An inhibitory effect of CO appeared at lower temperatures (8 and 12 °C). Regardless of the temperature, the gaseous CO is sparingly soluble at acid pH. However, the CO inhibition was not significantly different between pH 5.5 and pH 7. Considering the pKa of the carbonic acid, these results showed the dissolved carbon under HCO form didn't affect the bacterial inhibition. Finally, a global model was proposed to estimate the growth rate vs. CO concentration in the aqueous phase. This dissolved concentration is calculated according to the physical equations related to the CO equilibriums, involving temperature and pH interactions. This developed model is a new tool available to manage the food safety of MAP.