Interaction between toxigenic Aspergillus flavus Link and mites (Tyrophagus putrescentiae Schrank) on maize grains: effects on fungal growth and aflatoxin production

The ability of mites of the species Tyrophagus putrescentiae to spread the toxigenic fungus Aspergillus flavus from contaminated maize to sterile grains was studied under controlled conditions. Maize samples were distributed among boxes, each divided into two compartments by a barrier of polystyrene and having restricted access from one compartment to the other. One box contained only uncontaminated maize (negative controls). The other boxes contained two compartments. In compartment A the grains were inoculated with mites and or fungi. Compartment B contained sterile grain. The boxes were maintained at 85% relative humidity and at two temperatures (25 and 30 degrees C) for 30, 60 or 90 days prior to analysis. Moisture content values varied between 15.4 and 16.9%, being greater in grain samples containing both mites and fungi. Water activity levels ranged from 0.86 to 0.89. The numbers of adult mites, eggs and fungal colony forming units as well as the concentrations of aflatoxins increased with time of incubation. The presence of A. flavus contributed to the growth of mites and these, in turn, efficiently dispersed viable fungal spores from the inoculated compartment to the uncontaminated one. After 30 incubation days, the fungal population in all samples exceeded the international norms (10(4) CFU/g), and after 90 days of incubation, aflatoxin was detected at over 23.7 microgram/kg, exceeding the limit established by Brazilian legislation (20 microgram/kg) in food for human consumption. Samples of grains inoculated with A. flavus alone or a mixture of A. flavus and mites developed concentrations of aflatoxins above 27.4 microgram/kg. The current data confirms T. putrescentiae is a means of dispersal for toxigenic fungi in stored grain kept under warm and moist conditions.