Factors that inhibit growth of Listeria monocytogenes in nature-ripened Gouda cheese: A major role for undissociated lactic acid

- Factors relevant to Gouda cheese with potential to inhibiting growth of L. monocytogenes were assessed.
- Undissociated lactic acid was evaluated to be the most important growth inhibitor of L. monocytogenes in Gouda.
- Undissociated lactic acid should be included as a factor in predictive models and in setting criteria for Listeria in cheese.

In this study, factors relevant to nature-ripened Gouda cheese were evaluated for their potential to inhibit growth of Listeria monocytogenes. Factors included water activity, pH, undissociated acetic and lactic acid, diacetyl, free fatty acids, lactoferrin, nitrate, nitrite and nisin. In addition, the effect of temperature was evaluated. For each factor, the actual concentrations and values relevant to Gouda cheese were obtained and the inhibitory effect of these individual factors on growth of L. monocytogenes was assessed. This evaluation revealed that undissociated lactic acid is the most important factor for growth inhibition of L. monocytogenes in Gouda cheese and that, additionally, low water activity as present in the cheese rind and after prolonged ripening times can also cause full growth inhibition. Gouda cheeses have a typical total lactic acid content of 1.47% w/w. In a 2-week old Gouda cheese, with a pH value of 5.25 and a moisture content of 42% w/w, the concentration of undissociated lactic acid in the water phase is 10.9 mM. Growth of L. monocytogenes is not supported when the undissociated lactic acid concentration is >6.35 mM. Concentrations of undissociated lactic acid in the water phase of Gouda cheese will be higher than this value when the total lactic acid content is >0.86% w/w at a pH < 5.25 (relevant to young Gouda cheese), or >1.26% w/w at a pH < 5.50 for mature Gouda cheese (moisture content of 35% w/w). This study underlines the importance of undissociated lactic acid as growth inhibitor for L. monocytogenes in Gouda cheese.