No abstract

An in vitro initial screen of twenty-four essential oils for inhibition of spore germination and mycelial growth of mycotoxigenic strains of Penicillium verrucosum and Aspergillus ochraceus was made at 25°C and 0.99 water activity (aw). Based on a measurement of clearing zones, six essential oils were found to be effective in inhibiting growth: bay leaf (west Indian), cinnamon leaf, clove bud, clove leaf (cleaned), lemongrass and thyme. Based on those giving clearing zones greater than 5 mm, three were chosen for a more detailed study (clove leaf, cinnamon and bay leaf). The detailed screen was at 0-500 ppm on a 2% milled wheat agar modified to 0.99-0.90 aw for 56 days at 15°C and 25°C for isolates of both species. Generally, growth of the A. ochraceus and P. verrucosum was slower at 15°C than 25°C. Generally, regardless of temperature or aw there was significant (>90%) inhibition of growth with 500 ppm of these essential oils. However, at suboptimal concentrations, especially 50 ppm, some essential oils stimulated growth of both A. ochraceus and P. verrucosum by up to 10-15%. Analysis of the impact on ochratoxin A (OTA) production supported these findings. At 50 ppm there was stimulation of OTA production regardless of aw or temperature. However, at 500 ppm significant control of OTA production by A. ochraceus was observed with the best essential oil treatments. OTA production by P. verrucosum was completely inhibited at the maximum concentration and under the environmental conditions used.