The qualities of beer are deteriorated by the presence of either di- or tribydroxyoctadecenoic acids, which reduce the beer 'head' and produce an astringent flavor. In this study we found that native extracts of malt mash transformed linoleic acid into di- and trihydroxyoctadecenoic acids, but this transforming activity and lipoxygenase activity were inactivated by heating the mash at 70degreesC for 30 min. Recombinant barley lipoxygenase I was not able to transform linoleic acid into di- and trihydroxyoctadecenoic acids. The transforming activity of mash extract heated at 70degreesC for 30 min could be restored by the addition of recombinant barley lipoxygenase 1; in contrast, the activity of boiled mash extract was not substantially restored by the recombinant enzyme. These results indicate that di- and trihydroxyoctadecenoic acids are generated from linoleic acid by both lipoxygenase and a heat-stable enzymatic factor present in the mash.