Impact of environment and interspecific interactions between spoilage fungi and Aspergillus ochraceus on growth and ochratoxin production in maize grain

Using layers of irradiated but still fertile maize grain, the effects of water activity (0.995, 0.95 aw) and temperature (18, 30°C) on interspecific interactions between Aspergillus ochraceus and five other spoilage fungi were examined. Asp. ochraceus was not competitive against Asp. flavus, Asp. niger, or Alternaria alternata at 18°C when water was freely available (0.995 aw), while at 0.95 aw it was dominant against Asp. candidus, Asp. flavus and Alt. alternata. At 30°C and 0.995 aw, Asp. ochraceus was dominated by other fungi, except Alt. alternata, and was mutually antagonistic to Asp. candidus and Eurotium amstelodami. However, at 30°C and 0.95 aw it was competitive against Asp. candidus and Alt. alternata, but not against the other species examined. The overall Index of Dominance showed that Asp. ochraceus was not competitive under the conditions examined here. At 18°C ochratoxin production by Asp. ochraceus was inhibited significantly by Asp. candidus (0.995 and 0.95 aw) and Asp. niger (0.995 aw). When grown on maize grain at 30°C, ochratoxin production by Asp. ochraceus was significantly inhibited by other spoilage fungi when both were grown on maize grain, especially by Asp. niger and E. amstelodami (0.995 aw) and Asp. flavus at 0.95 aw. These results suggest that, to a large extent, A. ochraceus is not as competitive as some other spoilage fungi in primary resource capture on maize grain at aw of 0.95 or above, although it may modify resource quality and influence secondary colonisation by other species under the conditions tested.