Listeria monocytogenes growth under well-controlled CO2, pH, and temperature conditions through a novel gas-controlling system

Abstract

While effectively extending the shelf life of perishable foods at low temperatures by delaying the growth of spoilage microorganisms, modified atmosphere packaging (MAP) may favor the growth of certain pathogens under refrigerated and anaerobic environments. In particular, Listeria monocytogenes poses a major safety concern and has caused a significant increase in foodborne outbreaks over the last five years in EU. Although this pathogen has received great attention in the field of predictive microbiology, the exact relations between its growth and interconnected stress factors (CO₂, pH, and temperature) still need to be better understood, calling for extensive data collection. Thus, this study aims to understand the impact(s) of these factors by analyzing the behavior of L. monocytogenes in a liquid medium under well-controlled anaerobic conditions with varying CO₂ levels (0–20–40-60 %), pH values (7.7, 6.2), and temperatures (4, 7 °C). Three primary growth models – Logistic, modified Gompertz, and Baranyi – were evaluated to fit the growth data from controlled experiments, performed under anaerobic atmospheres employing an innovative gas-washing bottle incubation system (GBIS). The Baranyi model presented the best growth model to fit the growth data. This study showed that higher CO₂ (60 %) individually reduced the maximum growth rate (μ_max, 1/day) from 0.32 to 0.21 at 4 °C and from 0.46 to 0.39 at 7 °C. Similarly, lower pH decreased the μ_max from 0.40 to 0.32 at 4 °C and from 0.68 to 0.46 at 7 °C. Moreover, decreasing temperature from 7 °C to 4 °C led to a reduction in μ_max from 0.68 to 0.40 at pH 7.7 and from 0.46 to 0.32 at pH 6.2, while μ_max decreased from 0.39 to 0.21 in the presence of 60 % CO₂. In terms of interactions, lower temperature enhanced the growth-reducing effect of higher CO₂. The same stressing factor, conversely, reduced the low pH-induced effect on the growth in CO₂-free conditions. Similarly, lower pH decreased the growth-reducing effect of lower temperature. These insights highlighted the importance of studying well-controlled experimental conditions when aiming at designing MAP packaging to achieve safe, sustainable packaging solutions.