(-)-Geosmin (GEO) (Figure 1) is an aromatic volatile metabolite the presence of which in
grapes is associated with an earthy smell of crucial importance in grape products (e.g. grape
juice). Two fungi are involved in the accumulation of geosmin in grapes: Botrytis cinerea and
Penicillium expansum (La Guerche et al., 2005). Other compounds of fungal origin such as
fenchone, fenchol, anisoles and methyl isoborneol (MIB) can also transmit undesirable
aromas (Boutou and Chatonnet, 2007)
According to literature (Boutou and Chatonnet, 2007; Baghery et al., 2007; Salto et al., 2008
and Prat et al., 2008), Head Space Solid-Phase Microextraction (HS-SPME) coupled with Gas
Chromatography/Mass Spectrometry (GC/MS) methodology was used to detect and quantify
GEO. The fiber used was divinilbenzene/carboxen/polidimetilsiloxane (DVB/CAR/PDMS).
Grape juice spiked with standards of the analytes was analyzed to validate the method.
Optimization of the method was performed by determining the efficiency of GEO extraction.
The influence of the ionization of the analytes was tested by extracting at different pH (from
ca. 3.5 to 7.0) and/or addition of NaCl (from none to saturation). Also, the optimal ethanol
concentration in the sample was assessed (from none to 6% v/v). Time of extraction (from 20
to 65 min) and temperature of extraction (from 30 to 70 ºC) were assayed as was the position
of the Head Space fiber. The results underwent ANOVA tests to evaluate the significance of
the differences in geosmin extraction. Graphics of extraction of GEO as a function of the
factors tested were plotted to choose those conditions that maximized the detection of GEO
and permitted the detection of the other analytes. Finally, internal standards were tested (4-
Nonanol; Methyl-4-pentan-2-ol and 2,3,4-Trichloroanisole) to obtain calibration curves.
The calibration curves of GEO were linear with high correlation coefficients. Geosmin, MIB
and most of the other metabolites were detected. Quantification of GEO was possible in the
range of concentrations in naturally contaminated grape juice