The biocontrol yeast Pichia anomala J121 prevents mould spoilage by Penicillium roqueforti and prolongs the safe storage of moist cereal grain in malfunctioning airtight storage systems. P. anomala is naturally occurring on grain and is classified as safe, i.e. biosafety level 1 microorganism. Strain J121 does not grow above 37°C and is sensitive to commonly used antifungal drugs. The ability of P. anomala to prevent mould growth during 14 months of wheat grain storage was evaluated in outdoor silos with different air permeability. P. anomala rapidly consumed O2 leaking into the silo during the spring, and P. roqueforti did not grow in treatments inoculated with P. anomala. A connection between production of the ester ethyl acetate and biocontrol activity was found. The biocontrol effect was enhanced when complex medium or glucose was added to grain minisilos. The addition did not markedly influence yeast cell numbers, but products of glucose metabolism, ethyl acetate in particular and ethanol, increased. When a diploid and a haploid strain of Pichia anomala were tested at two water activities (aw) for biocontrol ability in grain minisilos, the two yeast strains grew and inhibited mould growth equally well and showed similar patterns of ethyl acetate production at the higher aw. However, at lower aw the growth, biocontrol performance and ethyl acetate production of the haploid strain were reduced. The biocontrol activity of an additional 57 yeast species was evaluated in grain minisilos. Most yeast species grew to levels comparable to that of P. anomala J121, but only six other species strongly inhibited P. roqueforti. The biocontrol activity of Candida fennica and Candida silvicultrix is reported for the first time. The ability of 27 yeast species to grow to high CFU values without inhibiting mold growth, and the increase in biocontrol activity after nutrient addition, indicates that nutrient competition is not the main reason for mould inhibition in the system. Instead, a combination of ethyl acetate production and rapid consumption of oxygen leaking into the system could be the main inhibitory mechanism
