Antifungal activity of some essential oil components

Fifteen essential oil components were evaluated for antifungal activity towards five spoilage-causing fungi. In liquid shake cultures, unsaturated aldehydes (citral, cinnamic aldehyde and cittronellal) followed by geraniol, an unsaturated alcohol were most inhibitory to Aspergillus niger, Fusarium oxysporum and Penicillium digitatum; their minimal inhibitory concentrations (MIC) was 100 μg ml⁻¹. Menthol, a terpene alcohol was most inhibitory to Rhizopus stolonifer and Mucor sp. with a MIC of 200 μg ml⁻¹. Hydrocarbons like camphene, limonene and α-terpinene were least inhibitory. When incorporated in agar medium different patterns of activity were found. Thus citral, cinnamic aldehyde, citronellal, geraniol and menthol not only failed to completely inhibit A. niger, F. oxysporum and P. digitatum but were more active against R. stolonifer and Mucor sp. than in liquid medium. The differences were due to the vapour of the volatile test compounds which accumulated over the agar medium. The growth inhibition due to the vapours alone was measured by using structurally modified petri-plates. The vapours were more active against R. stolonifer and Mucor sp. than against A. niger, F. Oxysporum and P. digitatum. Fungal growth inhibition by volatile compounds in agar medium reflects the combined activity of the vapour and the compound incorporated in the medium and the inhibition is different from that obtained in liquid medium. Regarding the structure-activity relationship of citral, the-CHO group in conjugation with a carbon to carbon double bond (C=C) was found to be responsible for the antifungal activity of the molecule.