Volatile constituents of uncooked rhubarb (Rheum rhabarbarum L.) stalks

Volatiles of rhubarb (Rheum rhabarbarum L.) stalks were isolated by means of vacuum headspace method and analyzed by capillary gas chromatography-mass spectrometry. Fifty-nine components were reported for the first time in rhubarb. A striking feature of the extracts obtained is the preponderance (similar to65%) of compounds with C-6 skeletons. In addition to unsaturated C-6 aldehydes and alcohols, substantial amounts of the less common (E)-2- and (E)-3-hexenoic acid were detected. Gas chromatography-olfactometry and determination of odor activity values revealed the sensory importance of the C-6 compounds to the aroma of rhubarb. Comparative experiments involving the inhibition of enzyme activities revealed that the initial spectrum of C-6 components is changed due to subsequent isomerization and reductions. Thus, contributions of (Z)-3-hexenal and the unsaturated acids decrease, and (E)-2-hexenal/(E)-2-hexenol play major sensory roles.