Predicting Listeria growth in cheese

The bacterium Listeria monocytogenes is found almost everywhere in nature. This rather persistent pathogen can grow at very low temperatures, in high salt concentrations, and at low pH. By attaching to surfaces and producing biofilms, it can also create problems in food processing environments. 

Pasteurisation and other heat treatments kill this bacterium. However, ‘ready-to-eat’ (RTE) products such as cheese are generally not heated before consumption. So post-processing contamination of your cheese by L. monocytogenes could result in public health issues, expensive recalls, food wastage – and critical damage to your brand’s image.

Both the European Commission and the FDA strongly regulate microbiological criteria for L. monocytogenes in RTE foods[1]. To safely market and export your cheese, you need to comply with food safety criteria and determine the risk of L. monocytogenes growth. For some products, this is easy: for instance, if the pH is below 4.4 or the water activity is below 0.92, Listeria will not grow. But many cheeses fall outside of these limits.

A validated model for assessing the risk

To gain fast insight into whether a cheese is at risk for supporting L. monocytogenes growth, we created a predictive model. We began by researching Gouda, which has never been associated with listeriosis, despite a pH above 5.0 and a water activity above 0.94. Our research determined that the undissociated lactic acid concentrations (above 6.35 millimolar) in the Gouda, were fully inhibiting L. monocytogenes growth.

Our study also demonstrated that, even when undissociated lactic acid concentrations fall below the critical level, L. monocytogenes growth in cheese can still be sufficiently inhibited by additional factors including pH, water activity and temperature. Using this scientific insight, we developed and successfully validated a model to calculate the potential of Listeria growth in many other cheeses. These include soft cheeses (blue cheese, Camembert, cottage cheese, ricotta, queso fresco, mozzarella), semi-hard cheeses (cheddar, feta-style) and cheeses prepared with milk from animals other than cows (goats, sheep and buffalos).

Solutions for the entire production chain

Predictive models like this one are powerful tools to assess potential pathogen risks in the food chain. But determining whether or not Listeria can grow in your cheese is only one step in pathogen control.

For example, what happens if our model indicates that there is a risk of pathogen growth in your cheese? To determine the outcome of such bacterium in your product, we can use our unique mini-cheese, high-throughput platform to quickly and effectively evaluate the true survival rates of the pathogens throughout your product’s shelf life.

Furthermore, it is clear that chain control is critical to ensure sufficient pathogen inactivation during processing, and prevent post-processing contamination and outgrowth. By assessing foodborne hazards, including Listeria, in your entire production chain, we can evaluate the impact of processing and storage conditions on pathogen inactivation, survival and growth. Then, we can provide you with independent expert advice for solutions and process improvements.

[1] EC regulation 2073/2005 specifies that, if your product supports growth of L. monocytogenes, there may be no Listeria present at all when it leaves the production facility. Even if your product does not support Listeria growth, Listeria levels must remain below 100 cfu/g throughout its shelf-life.

1. Wemmenhove et al. 2018. Factors that inhibit growth of Listeria monocytogenes in nature-ripened Gouda cheese: A major role for undissociated lactic acid. Food Control. 84: 413-418.
2. Wemmenhove et al. 2013. Fate of Listeria monocytogenes in Gouda microcheese: No growth, and substantial inactivation after extended ripening times. Int Dairy J. 32(2): 192-198
3. Wemmenhove et al. 2014. The fate of Listeria monocytogenes in brine and on Gouda cheese following artificial contamination during brining. Int Dairy J. 39(2): 253-258
4. Wemmenhove et al. 2016. Minimal inhibitory concentrations of undissociated lactic, acetic, citric and propionic acid for Listeria monocytogenes under conditions relevant to cheese. Food Microbiol. 58: 63-67.