Hurdles to Prevent Outgrowth of Listeria monocytogenes: Evaluation of Factors in Gouda Cheese

European Union regulation EC 2073/2005 contains microbiological food safety criteria for Listeria monocytogenes in ready-to-eat foods. Product categories are specified based on the potential of this pathogen to grow in a food. Some foods do not support growth of L. monocytogenes; for instance, when the pH ≤ 4.4, or the water activity (aw) ≤ 0.92, or when pH ≤ 5.0 and aw ≤ 0.94. In other cases, evidence that a product does not support growth can be obtained by predictive microbiological modelling, historical data, information from scientific literature, and/or durability or challenge testing.

Knowledge of treatments to eliminate L. monocytogenes from foods and of the most important hurdles to control growth in actual food products is critical for defining process, product, and storage conditions that warrant safety. Here, the case of nature-ripened Gouda cheese is presented. This is a ready-to-eat product made from pasteurized milk with a pH just above 5.0 and aw above 0.94. This cheese has not been associated with foodborne outbreaks of listeriosis and various challenge tests have shown that L. monocytogenes does not grow in it. To explain the absence of growth, we evaluated the individual factors relevant to Gouda cheese for their potential to inhibit growth of L. monocytogenes. Factors included aw, pH, undissociated acetic and lactic acid, diacetyl, free fatty acids, lactoferrin, nitrate, nitrite, and nisin. This revealed that undissociated lactic acid is the main growth inhibitor of L. monocytogenes in Gouda cheese. This factor can be included as a criterion in product specifications of this cheese in relation to the risk of outgrowth of L. monocytogenes. In addition, this knowledge is applicable in product development of cheeses and other products.