Synergistic inhibitory effect of citral with selected phenolics against Zygosaccharomyces bailii

Antifungal susceptibilities of Zygosaccharomyces bailii to individual and binary mixtures of citral with selected phenolics were evaluated to identify synergistic combinations. Individual effects of citral, vanillin, thymol, carvacrol, and eugenol concentrations and combined effects of citral with the other phenolic compounds on the growth of Z. bailli were evaluated in potato dextrose agar, adjusted with sucrose to a water activity of 0.99 or 0.95, and hydrochloric acid to pH 4.5 or 3.5. MICs for individual and binary antimicrobial mixtures were identified and then transformed to fractional inhibitory concentrations. Inhibitory concentrations of citral and vanillin were higher than 650 ppm, whereas for thymol, eugenol, and carvacrol, concentrations were lower than 250 ppm for several of the studied water activity-pH conditions. Combining citral with the other phenolic compounds, fractional inhibitory concentration (FIC) and FICIndex varied from 0.216 to 0.582. FICIndex demonstrated synergistic effects on Z bailii inhibition when citral was used in combination with vanillin, thymol, carvacrol, or eugenol. Therefore, the relative amount of antimicrobials could be greatly reduced.