Effect of water activity and temperature on growth and ochratoxin production by three strains of Aspergillus ochraceus on a barley extract medium and on barley grains

Aspergillus ochraceus Wilhelm is a widespread storage fungus that has been isolated from grains such as barley. The objective of this study was to determine the effects of water activity (aw), temperature, time and their interactions on (a) growth on a barley extract agar medium, (b) growth on barley grains and (c) ochratoxin production on barley grains by three strains of A. ochraceus. For the three A. ochraceus isolates examined (NRRL 3174, 3.113 and 3.38), optimal aw levels for growth on agar media were in the range 0.98–0.96, with temperature optima of 30°C for two of the isolates and 25–30°C for the other isolate. Growth was observed at 10 and 37°C, but only at higher aw levels assayed. Two dimensional profiles were constructed for the range of aw and temperature conditions that allow growth of the three isolates. Maximum growth on barley grains was reached at 30°C, at both aw levels assayed (0.96 and 0.98), with fungal growth rates in the 4–5 mm day−1 range. Maximum amounts of ochratoxins were produced at the highest aw treatment (0.98 aw) and after a three-week incubation time at 25–30°C. The range of ochratoxin concentrations varied considerably, from 1.7 to 12,949 ppm, depending on the aw and temperature interactions.