et al. (2017)* found that the major factor (other factors were also involved but to a lesser extent) preventing the growth of L. monocytogenes in Gouda cheese was undissociated lactic acid. Providing the concentration was >6.35 mM, no growth occurred.
The accepted value for the pKa for lactic acid is 3.86. However, this value was not calculated using conditions relevant to cheese. et al. (2017) have calculated a more accurate value for the pka of lactic acid in Gouda cheese, 3.71, which better reflects the ionic and lipid environment. This new value gives gives lower values for undissociated lactic acid compared with the previously accepted value.
This research has been reflected in the article "Functions of starters in dairy fermentations and the relative importance and effectiveness of their antimicrobial mechanisms". that has been added to the website. This article also provides access to a free calculator for determining the concentration of undissociated lactic acid in cheese.
* Wemmenhove, E., van Valenberg, H.J.F., van Hooijdonk, A.C.M.,Wells-Bennik, M.H.J., Zwietering, M.H. (2017). Factors that inhibit growth of Listeria monocytogenes in nature-ripened Gouda cheese: A major role for undissociated lactic acid. Food Control, 84, 413-418.