Enhancement of the antimicrobial activity of eugenol and carvacrol against Escherichia coli O157:H7 by lecithin in microbiological media and food

Essential oils (EOs) or their isolated components, such as eugenol and carvacrol, have strong antimicrobial activities against both Gram-positive and Gram-negative bacteria and are generally recognized as safe by the FDA. However their hydrophobic properties limit their dispersion and stabilization in aqueous food systems. This requires higher concentrations, which in turn negatively affect the quality of foods. The objective here was to determine the effect of the natural emulsifier lecithin on the antimicrobial activity of eugenol and carvacrol and possible food applications. Escherichia coli K12 and E. coli O157:H7 strains ‘Cider’ and ATCC 43889 were used. Homogenized eugenol and carvacrol, with and without lecithin, were screened for antimicrobial activity. The stability of the samples measured by particle size and zeta potential was not affected by different concentrations of lecithin. For all strains, the antimicrobial activity of carvacrol and eugenol was enhanced significantly (P<0.05) by low concentration of lecithin. The D-value (time at a specific concentration of antimicrobial necessary to cause a 90% reduction in viable cells) for E. coli K12 exposed to 0.047% v/v eugenol or 0.015 % v/v carvacrol was reduced from 13.3 to 6.3 min and 17.4 to 9.7 min, respectively, with the addition of 0.0025% lecithin (w/v). Similarly 0.0025% w/v lecithin in the presence of 0.058% v/v eugenol or 0.0188% v/v carvacrol, caused the D-value to decrease from 4.0 to 1.2 min and 10.2 to 6.9 min, respectively, for E. coli strain ‘Cider’ and from 6.2 min to 3.6 min and 9.9 to 5.4 min, respectively, for E. coli ATCC 43889. Higher lecithin concentrations (> 0.005% w/v) increased D-values compared to lower concentrations. Similar results were found in vegetable juice. The results showed that a small amount of lecithin can enhance the antimicrobial activities of essential oils. Addition of lecithin had no effect on oil-water emulsion droplet particle size and the stability of the samples was not affected by different concentration of lecithin. We believe that lecithin enhances the antimicrobial activity of eugenol and carvacrol droplets by improving the ionic interactions between the positively charged lecithin-containing essential oil components and negatively charged bacterial cells.