SCANNING ELECTRON-MICROSCOPE OBSERVATIONS OF GROWTH AND OCHRATOXIN-A PRODUCTION OF ASPERGILLUS-ALUTACEUS VARIETY ALUTACEUS (FORMERLY A OCHRACEUS) ON GAMMA-IRRADIATED BARLEY

Scanning electron microscope examination, colony counting and biochemical studies were conducted to describe the effect of gamma-irradiation on growth and ochratoxin A production by Aspergillus alutaceus. Irradiation at a dose of 1.0 or 2.0 kGy reduced the level of mold growth greatly relative to unirradiated controls. Growth in the irradiated samples after 7 to 12 day incubation was mainly in cracks in the hull, and less mycelium was seen on the grain surface. In unirradiated controls, mycelial growth was heavy and, although conidial heads were most abundant in cracks in the hull, they were seen over the whole surface. When the barley was inoculated before irradiation, the number of colony forming units (cfu) at 5 days after 1.0 or 2.0 kGy irradiation was lower than in the unirradiated controls; however, the number increased over the control by 30 days. A dose of 4.0 kGy eliminated viable fungi. Ochratoxin A production decreased from the control level of 17.6-mu-g/g with increased dose and was below the detection limit above 4.0 kGy. When barley was inoculated after irradiation the spore count and the ochratoxin A level were higher than the unirradiated control after 27 days. We conclude that the difference in growth and ochratoxin A production on irradiated and unirradiated barley is due to the effect of irradiation on the natural competitive microflora on the grain surface and the reduction of inoculum size of the A. alutaceus by radiation.