Risk assessment of Listeria monocytogenes in Gouda cheese

In this PhD thesis, the potential of outgrowth of L. monocytogenes was assessed in Dutch-type Gouda cheese. It was demonstrated that L. monocytogenes, which can cause listeriosis, is not able to grow in or on Dutch-type Gouda. The factors present in Gouda cheese that can lead to full inhibition of growth of L. monocytogenes were identified and safety criteria aiming for complete inhibition of growth of L. monocytogenes in Dutch-type Gouda cheese are suggested.

Dutch-type Gouda cheese is a semi-hard cheese made from bovine milk that is pasteurized when produced at an industrial scale. It is a ready-to-eat food with a pH > 5.0 and water activity aw > 0.94. In absence of scientific evidence that this product does not support growth, Dutch-type Gouda is classified by the European legislation as a ready-to-eat food product able to support growth of L. monocytogenes.

In two challenge studies described in this thesis, it was demonstrated that Dutch-type Gouda cheese does not support growth of L. monocytogenes when inoculated in and on the product. During the cheese making process, entrapment but no growth of L. monocytogenes in the curd was observed. During subsequent ripening of the cheeses, no growth was observed, and upon prolonged ripening periods (>2 months) inactivation was found. In the second challenge study, a limited transfer of L. monocytogenes from brine to the outer layers of cheeses was observed, and during brining and ripening viable numbers of L. monocytogenes did not increase.

The variation in aw inside Gouda cheese was assessed by determining the profiles of water and NaCl and the resulting aw in nature-ripened and foil-ripened Gouda cheese during brining and ripening. An empirical model was derived for Gouda cheese in which aw is expressed as a function of the NaCl-in-moisture content.

Dutch-type cheeses contain organic acids that are known to have potential inhibitory effects on L. monocytogenes. The MICs of organic acids for 6 different L. monocytogenes strains were established at pH values that are relevant to Dutch-type Gouda. The MICs were established for lactic acid (which is the main organic acid in Gouda), acetic acid, propionic acid, and citric acid. Variations in MICs between strains were observed.

Ifn an overall review of the factors present in Gouda cheese that are relevant to growth inhibition of L. monocytogenes, undissociated lactic acid was evaluated as the primary growth-inhibiting factor that can lead to full growth inhibition in Gouda. Additionally, low aw in the cheese rind and after prolonged ripening times can cause full growth inhibition.

This thesis lends support to categorizing Gouda as a ready-to-eat food product that does not support growth of L. monocytogenes. Furthermore, it is justifiable to include undissociated lactic acid (together with pH and aw) in future food safety criteria for ready-to-eat products related to absence of growth of L. monocytogenes.