Chemical analysis and origin of geosmin, an major flavor component of red beets (Beta vulgaris L.)

Geosmin is a major flavor compound found in red beets (Beta vulgaris L.) and imparts an ‘earthy’ sensory characteristic to this vegetable. To date, it is not known whether geosmin is a by-product of beet metabolism or is absorbed by the beet root once the flavor compound is synthesized by soil-borne microorganisms. Research with geosmin is complicated by the fact that most analytical methods require large sample sizes (≥1 kg) and are time-consuming to perform. Thus, the objectives of this study were to (a) develop a simplified method for the analysis of geosmin, (b) determine origins of the flavor compound, and (c) identify other volatile compounds in mature red beet roots. Geosmin was analyzed by headspace solid phase microextraction (HSPME) using a polydimethyl-siloxane/divinylbenzene fiber with (−)-menthone serving as the internal standard. The extraction commenced at 60°C for 2 hr after which the volatile compounds were desorbed and analyzed by gas chromatography. The detection limit of geosmin using HSPME was approximately 1 μg/kg while relative recoveries ranged from 99.2 ± 7.4% for roots to 72.0 ± 4.2% for seedlings. Geosmin was detected in red beet seedlings grown under aseptic conditions in a synthetic growth medium. When geosmin was added to the medium, the seedlings did not absorb the flavor compound. These studies indicate that red beets are capable of endogenous synthesis of geosmin. Besides geosmin, red beet roots contained a number of volatile compounds including 2-methoxy-3-(1-methylethyl)pyrazine, methanethiol, several medium-chain aldehydes (C₇, C₈, C₉, and C₁₀), 2-methylpropanal, benzeneacetaldehyde and others.