Modelling the behaviour of Listeria monocytogenes in cheese

Cheese is a complex biological system where environmental dynamics take place between pH, moisture, water activity and temperature. Bacteria growing in or on cheese behave differently depending on the cheese types or in the processing step. In this work, the behaviour of Listeria monocytogenes during cheesemaking and ripening was studied given a milk contamination or surface cross-contamination scenario. When milk was contaminated, L. monocytogenes initiated growth with a different probability during cheesemaking compared to milk or laboratory media, indicating that models built from laboratory media would not accurately predict the behaviour of L. monocytogenes in farmhouse cheese. Lower limits of growth at aw values of 0.94-0.96, depending on contamination level, were identified using an Ordinary Logistic Regression model. In the range tested, the pH showed no effect on growth initiation. Furthermore, L. monocytogenes presented considerable differences in behaviour in pasteurised or raw milk during cheesemaking and ripening. In contrast with the pasteurised milk cheeses, raw milk did not support growth of L. monocytogenes during cheesemaking but during ripening, growth occurred only in raw milk cheeses. The growth observed was modelled with a Logistic Cardinal model. When the contamination was present on the surface only, Listeria was inactivated in cheeses made with pasteurised milk (with pH < 5.4) during ripening and when aw decreased considerably. The inactivation was modelled with a Log-linear response surface model with aw as the factor with higher effect. In contrast, cheese made with raw milk or with pH around neutrality supported the growth of L. monocytogenes and was modelled with the Logistic Cardinal model with pH as the most influencing environmental factor on the growth rate. In summary, the need for food models has been demonstrated and pH and aw were the most significant variables conditioning the behaviour of L. monocytogenes in the various cheese scenarios.BIOTRACE

Modeling the growth of Listeria monocytogenes on the surface of smear- or mold-ripened cheese.

Surface-ripened cheeses are matured by means of manual or mechanical technologies posing a risk of cross-contamination, if any cheeses are contaminated with Listeria monocytogenes. In predictive microbiology, primary models are used to describe microbial responses, such as growth rate over time and secondary models explain how those responses change with environmental factors. In this way, primary models were used to assess the growth rate of L. monocytogenes during ripening of the cheeses and the secondary models to test how much the growth rate was affected by either the pH and/or the water activity (aw) of the cheeses. The two models combined can be used to predict outcomes. The purpose of these experiments was to test three primary (the modified Gompertz equation, the Baranyi and Roberts model, and the Logistic model) and three secondary (the Cardinal model, the Ratowski model, and the Presser model) mathematical models in order to define which combination of models would best predict the growth of L. monocytogenes on the surface of artificially contaminated surface-ripened cheeses. Growth on the surface of the cheese was assessed and modeled. The primary models were firstly fitted to the data and the effects of pH and aw on the growth rate (mumax) were incorporated and assessed one by one with the secondary models. The Logistic primary model by itself did not show a better fit of the data among the other primary models tested, but the inclusion of the Cardinal secondary model improved the final fit. The aw was not related to the growth of Listeria. This study suggests that surface-ripened cheese should be separately regulated within EU microbiological food legislation and results expressed as counts per surface area rather than per gram

Comparison of growth limits of Listeria monocytogenes in milk, broth and cheese.

AIM: To determine growth initiation differences of Listeria monocytogenes between a cheesemaking context, milk and tryptic soy broth (TSB). METHODS AND RESULTS: A laboratory-scale cheese was made with a mix of two strains of L. monocytogenes at four initial pH values, five water activity (a(w)) values and two contamination levels at 30 degrees C. Counts of L. monocytogenes were determined at time 0 and after 8h of cheese manufacture. Milk and TSB at the same pH and a(w) conditions were inoculated with the L. monocytogenes mix in multi-well plates. Growth was determined by plating each well onto Agosti & Ottaviani Listeria Agar after 8h of incubation at 30 degrees C. Each condition was repeated six times, and growth initiation probability was modelled with logistic regression models. Growth initiation boundaries were obtained for each matrix type. The results showed that the growth limits were matrix dependent. In the three matrix types, a(w) was the most important factor affecting the probability of growth initiation. Contamination level affected growth TSB and cheesemaking conditions. CONCLUSIONS: The interface wideness and position in cheese, milk and TSB were dissimilar, indicating that the use of models evaluated in TSB or milk could not be used to predict the behaviour of L. monocytogenes under cheesemaking conditions. SIGNIFICANCE AND IMPACT OF THE STUDY: Predictive models generated in liquid media are not necessarily adaptable to solid food, and the generation of real food models is necessary

Deterministic and stochastic behaviour of L. monocytogenes suspended cells or detached from stainless steel surfaces during cheese manufacture

Growth probability and kinetic models for Listeria monocytogenes in response to multiple hurdles occurring during cheese anufacturing are mainly focused on suspended L. monocytogenes cells. This study aimed to compared: (i) the growth/no growth interface of L. monocytogenes cells attached on stainless steel (SS) surfaces, or in suspension, within adjusted media and (ii) the behavior of planktonic and detached Listeria cells during manufacturing and ripening of two popular Greek cheeses: Feta and Graviera. A multi-strains composite of L. monocytogenes isolates from cheese, factory and farm in Greece and Ireland, were grown in TSBYE, MRD, Milk, Feta and Graviera cheese in the presence of SS coupons (2x5cm) for 3d at 20 °C, to obtain the following inocula: planktonic cells (P), and cells detached from the SS coupons (D). Detachment took place by the bead vortexing method. For growth/no growth evaluation P and D cells were inoculated in TSBYE, adjusted to 5 pH (6.8-4.8) by lactic acid and at 4 aw (0.945-0.995) by NaCl. For evaluation of L. monocytogenes kinetics in cheese, P and D cells were inoculated at three simulated stages of Feta and Graviera manufacture: in pasteurized milk, after cutting the curd and after the first ripening. The growth of D cells slightly delayed compared to P cells while it was more affected by aw than pH. On cheese, L. monocytogenes survived throughout the ripening at low levels. The differences in probability of growth of single cells for both inocula (P and D) were assessed by stochastic approaches. Furthermore, PFGE analysis resulted that 91 % of the cells of any tested condition belonged to the cheese factory isolate. The re- sults may address safety implications relevant to the potential of attached cells to proliferate, whereas data may contribute to filling data gaps on risk assessment of L. monocytogenes isolates from the dairy industry