Enantioselective Analysis of Methyl-Branched Alcohols and Acids in Rhubarb (Rheum rhabarbarum L.) Stalks

The enantiomeric compositions of 2-methylbutanol (1), 4-methylhexanol (2), 2-methylbutanoic acid (3), and 4-methylhexanoic acid (4) present in rhubarb (Rheum rhabarbarum L.) stalks were determined. Enantiodifferentiation was achieved via multidimensional gas chromatography using heptakis(2,3,6-tri-O-ethyl)-β-cyclodextrin as a chiral stationary phase. For all compounds the enantiomeric ratios were in favor of the (R)-enantiomers. The alcohols (1 and 2) exhibited generally high excesses of the (R)-enantiomers, the ratios varying slightly from batch to batch. For the acid (3) a rather narrow range averaging 65% (R):35% (S) was observed. The procedure applied to isolate the volatiles (vacuum headspace technique, simultaneous distillation-extraction, liquid-liquid extraction) had no significant impact on the enantiomeric ratios. The study describes for the first time a plant used as food material in which 2-methyl-branched volatiles are not nearly exclusively present as (S)-enantiomers. This information enlarges the current regulatory knowledge regarding the classification of these important flavor compounds as "natural" on the basis of their enantiomeric ratios.