Influence of water activity and temperature on the production of zearalenone in corn by three Fusarium species

The production of zearalenone in corn by seven isolates of Fusarium belonging to F. graminearum, F. culmorum and F. oxysporum was investigated for different culture conditions (water activity, temperature and incubation time). Two isolates belonging to F. graminearum were the highest producers of zearalenone (5150 and 7100 mg/kg dry weight of culture medium). The culture conditions that gave the highest yields were a(w) = 0.97, 40 days of incubation and temperature conditions including 2 weeks at 28 degrees C followed by incubation at 12 degrees C. These conditions were also optimal for the two isolates of F. oxysporum but for the remaining isolates the maximum levels of zearalenone were obtained at room temperature and 30 days of incubation. The maximum rate of mycotoxin production was observed between day 15 and 30 for all strains tested. At a(w) = 0.95 the maximum yield of zearalenone was 9.3 mg/kg dry weight of culture medium, produced by an isolate of F. graminearum. At 37 degrees C zearalenone was not detectable under any of the conditions assayed.