Antifungal activity and synergistic effects of thyme, cinnamon, rosemary and marjoram essential oils in combination, against apple rot fungi

The complex composition of the essential oils and the variety of chemical structures of their constituents are responsible of a wide range of biological activities many of which are of increasing interest in the field of foodstuff preservation (Romanazzi et al. 2012). Regarding that higher amount of EOs are required for food preservation, their application can negatively influence taste or odor. To avoid this unwanted side effect, several EOs can be combined. On the other hand the synergistic possible effect produced by the combination of plant essential oils was referred as an efficient strategy to combat microbial development(Wagner. 2011). The main purpose of this study was to evaluate the antifugal synergistic effect of combination of some essential oils including thyme, cinnamon, rosemary and marjoram aginst apple rot fungi during posthrvest storage. In this experiment, chemical composition of the essential oils was analyzed by gas chromatography. For evaluation of antifungal activity based on MIC, against Alternaria alternanta, Penicillium expansum and Botrytis cinerea, an agar dilution method was used. EOs interactions were assessed using a checkerboard microdilution method. The evaluated concentrations were in the range of 5 dilutions below the MIC to twice the MIC. For the double combinations, a two-dimensional checkerboard with twofold dilutions of each EOs was used (six double combinations). The maximum antifungal activity was demonstrated by thyme oil which showed MIC values 1250 μg/mL for A. alternanta and P.expansum and 625 μg/mL for B. cinerea .Cinnamon oil with MIC values1250 μg/mL against A. alternanta and B. cinerea and 2500 μg/mL against P.expansum displayed more inhibitory effects than rosemary and marjoram essential oils. In antifungal synergy testing, some double combinations (thyme/rosemary, thyme/cinnamon and cinnamon/marjoram) were found to be synergistic (FICi≤0. 5). The in vivo assay also demonstrated considerable inhibitory effects of EO combination treatments. Results from artificial wounding of trated apple indicated that double combination of thyme/cinnamon (156, 312 μg/mL) has more inhibitory effect than single EO treatments. Although the major components of thyme and cinnamon (especially thymol, carvacrol and cinnamaldehyde) are very important for their antifungal activity, other less-active compounds in rosemary and marjoram (such as α–Pinene, 1.8-cineole, Terpinene-4-ol and γ-Terpinene) play a significant role, as they can strengthen the effects of major components, though synergistic effects have also been observed