Water-soluble antimicrobial active components

The in vitro most antimicrobially active essential oils regularly (or normally) contain substances as main components, which are themselves known to exhibit pronounced antimicrobial properties. These are cinnamic aldehyde (cinnamon bark and cassia oil) and the phenolic compounds eugenol (clove and cinnamon leaf oil) and thymol (thyme oil) (Pauli, 2001). All these essential oils reveal a broadband spectrum of activity in various in vitro test systems (agar diffusion, dilution, and VP) due to their considerable water solubility and volatility. The evaluated antimicrobial inhibitory data of the essential oils obtained in agar dilution tests, serial DIL, and VP tests are summarized in Table 14.81. 

As p-hydroxybenzoic acid esters present a relatively high pKa value (approx. 8.5), their antimicrobial action is less dependent on the pH value of the type of food to be preserved. In this respect they are superior to the organic preservative acids. Unfavourably, as in the case of phenol, p-hydroxybenzoic acid esters may be linked to some extent to proteins, emulsifiers or other food components on account of their phenolic hydroxylic-group and thus be inactivated. The antimicrobial action of the p-hydroxybenzoic acid esters is proportional to the alkyl chain length. Thus the antimicrobial action of the methyl ester is some 3 to 4 times, that of the ethyl ester some 5 to 8 times, that of the propyl ester about 25 times as powerful as phenol (Thompson, 1994). However, superimposed on this effect may be the water solubility of the esters, which is inversely proportional to the alkyl chain length. Since the activity spectra of the individual p-hydroxybenzoic acid esters are different, the application of mixtures can be profitable. 

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