The infection of stored apples by the fungus Penicillium e., pansum Causes the contamination of fruits and fruit-derived products with the mycotoxin patulin, which is a major issue in food safety Fungal attack can be prevented by beneficial microorganisms, so-called biocontrol agents Previous time-course thin layer chromatography analyses showed that the aerobic incubation of patulin with the biocontrol yeast Rhodosporidium kratochvilovae strain LS11 leads to the disappearance of the mycotoxin spot and the parallel emergence of two new spots, one of which disappears over time In this work, we analyzed the biodegradation of patulin effected by LS11 through HPLC The more stable of the two compounds was purified and characterized by nuclear magnetic resonance as desoxypatulinic acid, whose formation was also quantitated in patulin degradation experiments After R. kratochvilovae LS11 had been incubated m the presence of (13)C-labeled patulin, label was traced to desoxypatulinic acid, thus proving that this compound derives from the metabolization of patulin by the yeast Desoxypatulinic acid was much less toxic than patulin to human lymphocytes and, in contrast to patulin, did not react in vitro with the thiol-bearing tripeptide glutathione The lower toxicity of desoxypatulinic acid is proposed to be a consequence of the hydrolysis of the lactone ring and the loss of functional, groups that react with thiol groups The formation of desoxypatulinic acid from patulin represents a novel biodegradation pathway that is also a detoxification process