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Inhibitory Effect of Essential Oils on Aspergillus ochraceus Growth and Ochratoxin A Production

    Ochratoxin A (OTA) is a potentially nephrotoxic and carcinogenic mycotoxin and a common contaminant of grains during storage. Aspergillus ochraceus is the most common producer of OTA. To avoid the risk of OTA consumption, 10 different essential oils were tested by fumigation and contact assays for inhibition of A. ochraceus growth and OTA production. Natural cinnamaldehyde proved to be the most effective against A. ochraceus, followed by cinnamon oil, synthetic cinnamaldehyde, citral, litsea citrate oil, eugenol and peppermint oil. The fumigation assay gave higher inhibition results than the contact assay except for tests of eugenol and peppermint oil. Complete fungal growth inhibition was obtained at 150-250 μL/L with fumigation and 250-500 μL/L with contact assays for cinnamon oil, natural and synthetic cinnamaldehyde, L. citrate oil and citral. The effects of natural cinnamaldehyde, citral and eugenol on ergosterol biosynthesis and OTA production were evaluated. The approximately complete inhibition of ergosterol biosynthesis was verified at ≥ 100 μg/mL natural cinnamaldehyde and 200 μg/mL citral, while total inhibition of ergosterol biosynthesis was not achieved at 200 μg/mL eugenol. However, citral and eugenol completely inhibited OTA production at ≥ 75 μg/mL and ≥ 150 μg/mL respectively, while natural cinnamaldehyde could not fully inhibit OTA production at ≤ 200 μg/mL. These results suggested that the inhibition of OTA production by natural cinnamaldehyde was mainly due to a decrease in fungal biomass, while citral and eugenol significantly inhibited the OTA biosynthesis pathway in addition to having antifungal activity. This is the first study to show that A. ochraceus could convert cinnamaldehyde to cinnamic alcohol which was further degraded and no new metabolite was detected by Reverse Phase-HPLC. These findings support the possibility of the use of cinnamaldehyde, citral and eugenol by vapor exposure as effective non-toxic biopreservatives against OTA contamination in stored grains. 
    The first author are Hua Huijuan and Xing Fuguo.