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The effect of wheat kernel medium supplementation with octanoic acid on the formation of PR toxin and some volatiles by Penicillium roqueforti was investigated.Octanoic acid was added to the medium once, prior to inoculation (4·55 mg g−1), or periodically (total 27·3 mg g−1) during the 10 day course of incubation. No octanoic acid was added to the reference sample. Levels of 2-heptanone, 2-heptanol and aristolochene, a volatile intermediate in PR toxin biosynthesis, were monitored using a solid phase microextraction (SPME) technique. The contents of PR toxin and ergosterol were determined after incubation. Aristolochene was observed in the reference sample, and 10·4 mg kg−1 of PR toxin was detected after 10 days. In cultures periodically supplemented with octanoic acid, no aristolochene or PR toxin were observed. However, in samples supplemented with octanoic acid only prior to incubation, the aristolochene level was 25% that in the reference sample, and PR toxin content was 3·4 mg kg−1.These data suggest that a high level of octanoic acid in the medium prevents PR toxin formation by P. roqueforti.