Lactic starter cultures have a well-deserved reputation for inhibiting, and or in some instances killing, spoilage and pathogenic bacteria not just in dairy foods but in meat and vegetable products also.

While the importance of undissociated lactic acid as an inhibitor of the growth of pathogens by starters has been known for many decades it is only recently that the effect has been quantified for Listeria monocytogenes in Gouda cheese.

Wemmenhove 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. Wemmenhove 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 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". which 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.