Abstract
Undissociated lactic acid has been shown to play a major role in complete growth inhibition of Listeria monocytogenes in Gouda cheese. In addition, low water activity conditions may contribute to growth inhibition. In the current study, it was assessed whether the major factors that inhibit growth of L. monocytogenes in Gouda cheese are the factors that determine growth in other types of ready-to-eat cheese as well.Various types of cheeses were selected, some of which had been associated with listeriosis, while others had not. Based on the composition of the different cheese types, the concentrations of undissociated lactic acid were calculated for each type. The ability to support growth of L. monocytogenes was predicted using the Gamma model, based on literature data on total lactic acid content, moisture content, fat content, pH, Aw, and temperature, and optimal growth rates in milk at 30–37 °C. In addition, the actual specific growth rates of L. monocytogenes in the various cheeses were calculated based on available experimental growth data. In 9 out of the 10 RTE cheeses reviewed, the undissociated lactic acid concentrations and aw determined growth/no growth of L. monocytogenes. No growth was correctly predicted for feta, Cheddar and Gouda, and growth was correctly predicted for ricotta, queso fresco, Camembert, high-moisture mozzarella, cottage and blue cheese. Growth of L. monocytogenes was not observed in practice upon inoculation of Emmental, whereas growth in this cheese type was predicted when including the above mentioned factors in the models. Other factors, presumably acetic and propionic acid, are thought to be important to inhibit growth of the pathogen in Emmental. The results from our study show that for cheeses in which lactic acid is a main acid, our model based on undissociated lactic acid, temperature, pH and aw gives a good prediction of potential outgrowth of L. monocytogenes. Implications for L. monocytogenes legislation are discussed per type of RTE cheese reviewed.
Highlights
Most cheeses are intended to be ready-to-eat (RTE) products and do not undergo a heating step prior to consumption
These data show that high concentrations of undissociated lactic acid are found in Cheddar, feta and Gouda, even higher than the Minimal Inhibitory Concentration (MIC) of 6.35 mM required to fully inhibit growth of L. monocytogenes (Aryani et al, 2015)
Outgrowth of L. monocytogenes was correctly predicted for ricotta, queso fresco, Camembert, high-moisture mozzarella, cottage and blue)
Summary
Most cheeses are intended to be ready-to-eat (RTE) products and do not undergo a heating step prior to consumption. L. monocytogenes may be present at low concentrations in raw milk (Table 1) and RTE cheeses manufactured from raw milk may pose a risk (Jackson et al, 2018). It is unlikely that L. monocytogenes survives pasteurization of the cheese milk (den Besten and Zwietering, 2012). This pathogen can survive in highly acidic, salty and lowtemperature environments (ICMSF, 1996), it can form biofilms (Bor ucki et al, 2003; Cossart et al, 2003; Carpentier and Cerf, 2011) and it may be persistent in the RTE cheese processing environment. Contam ination of finished products must be prevented, especially as the product does not undergo a heat treatment prior to consumption
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