Inhibitory effect of essential oils on Aspergillus ochraceus growth and ochratoxin A production

Abstract

Ochratoxin A (OTA) is a mycotoxin which is a common contaminant in grains during storage. Aspergillus ochraceus is the most common producer of OTA. Essential oils play a crucial role as a biocontrol in the reduction of fungal contamination. Essential oils namely natural cinnamaldehyde, cinnamon oil, synthetic cinnamaldehyde, Litsea citrate oil, citral, eugenol, peppermint, eucalyptus, anise and camphor oils, were tested for their efficacy against A. ochraceus growth and OTA production by fumigation and contact assays. Natural cinnamaldehyde proved to be the most effective against A. ochraceus when compared to other oils. 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. Essential oils had an impact on the ergosterol biosynthesis and OTA production. Complete inhibition of ergosterol biosynthesis was observed at ≥ 100 µg/mL of natural cinnamaldehyde and at 200 µg/mL of citral, but total inhibition was not observed at 200 µg/mL of eugenol. But, citral and eugenol could inhibit the OTA production at ≥ 75 µg/mL and ≥ 150 µg/mL respectively, while natural cinnamaldehyde couldn't fully inhibit OTA production at ≤ 200 µg/mL. The inhibition of OTA by natural cinnamaldehyde is mainly due to the reduction in fungal biomass. However, citral and eugenol could significant inhibit the OTA biosynthetic pathway. Also, we observed that cinnamaldehyde was converted to cinnamic alcohol by A. ochraceus, suggesting that the antimicrobial activity of cinnamaldehyde was mainly attributed to its carbonyl aldehyde group. The study concludes that natural cinnamaldehyde, citral and eugenol could be potential biocontrol agents against OTA contamination in storage grains.

Figure 1. The effect of ten essential oils on the growth of A. ochraceus with fumigation.

Figure 2. The effect of ten essential oils on the growth of A. ochraceus in contact assay.

Figure 3. SEM for the nonfumigated and fumigated mycelia.

A. Fumigated with 100 µL/L of natural cinnamaldehyde; B. Fumigated with 100 µL/L of citral; C. Fumigated with 250 µL/L of eugenol.

Figure 4. Effect of natural cinnamaldehyde (A), citral (B) and eugenol (C) on ergosterol biosynthesis and OTA production from A. ochraceus determined by HPLC.

The experiment was conducted during 5 days of incubation under agitation at 28°C (n = 4).

Figure 5. RP-HPLC detection of cinnamaldehyde of 50 mg/L (A) and cinnamic alcohol of 50 mg/L (B).