[Isomeric trihydroxy-octadecenoic acids in beer: Evidence for their presence and quantitative determination (author's transl)].

Evidence is presented that beer contains the following isomerric trihydroxy acids: 9,12,13-Trihydroxy-10 trans-octadecenoic acid (G1), 9,10,13-Trihydroxyl-11 trans-octadecenoic acid (G2), and 9,10, 11-Trihydroxy-12 trans-octadecenoic acid (G3). The compounds were isolated from a CHCl3: MeOH extract of beer by column chromatography and their structures determined by comparison of their mass spectra, Rf-values (TC) and retention times (GC) with the data obtained with the reference compounds G1, G2, and G3 prepared by enzymic oxidation of linoleic acid with barley flour. For the quantitative determination a CHCl3: MeOH extract of 100 ml acidified beer was methylated, silylated and separated by GC. Depending on the specific beer source, the beer contained 4.7--9.0 mg/1 G1, 1--2.4 mg/1 G2 and 0.4--0.7 mg/1 G3. The mechanism of the formation of these compounds from linoleic acid is discussed as well as their possible contribution to the stale flavor of beer which develops upon prolonged storage.