8E Identification of Impact Odorants of Wines 405
GC-MS system. The similarity of odors and odor intensities (at relatively equiv-
alent levels) and odor thresholds (in AEDA or Charm experiments) must be used
as an additional confirmatory criterion.After this process, the identity of mostof the “easy” odorants, i.e. those odorants
present at concentrations large enough to produce clear peaks in the GC-MS system,
will have been satisfactorily confirmed. Nevertheless, some of the most important
wine impact aroma compounds belong to the class of difficult odorants and it will
be nearly impossible to get a clear mass spectrum for such compounds in the orig-
inal extract. For these compounds it will be necessary to obtain a more refined and
concentrated fraction. The exact procedure to follow depends on different questions,
some of which are detailed below:
a) The targeted odor zone is at the final part of the chromatogram. This means that
the odorant is quite heavy and/or polar and that therefore, it is very difficult to
concentrate in extracts obtained by headspace techniques (static or dynamic).
In this case it should be advisable to get a total extract following some of
the procedures recommended in the literature (see for instance Kotseridis and
Baumes 2000; Ferreira et al. 2001a; Cullere et al. 2004a) and to apply fur-
ther a fractionation of the extract. This can be carried out on silica (Kotseridis
and Baumes 2000; Campo et al. 2006a), on reversed phase HPLC (Ferreira
et al. 1999; Aznar et al. 2001), on normal phase HPLC (L ́opez et al. 1999), or
even in a polymeric sorbent (Cullere et al. 2003). If the odorant is very polar a
reversed phase fractionation on C18 or in a polymeric sorbent is the best option.
The polarity of the odorant can be assessed from the difference between their
Retention Index on polar and nonpolar stationary phases (Ferreira et al. 1998a):
the higher the difference, the more polar the compound. In these cases even a
direct isolation in the same solid phase extraction cartridge could be attempted
(Ferreira V et al. 2003a).b) The targeted odor zone corresponds to a very volatile compound. In this case
the best isolation strategy is via headspace. A headspace SPME extraction with
a Carboxen fibre is a good starting choice that should be complemented by the
use of a chromatographic column with a very thick stationary phase or even by a
Porous Layer Open Tubular (PLOT) column.c) The targeted odor zone corresponds with a reasonable probability to an ultra-
trace odorant belonging to the families ofpolyfunctional mercaptans or metho-
xypyrazines. In these cases, several procedures are described for the selective
isolation of these odorants. See Tominaga et al. (1998), Schneider et al. (2003),
Tominaga and Dubourdieu (2006), and Ferreira et al. (2007) for the selective iso-
lation of polyfunctional mercaptans and Allen et al. (1994) and Sala et al. (2002)
for those of methoxypyrazines. Some selective separation schemes for carbonyls
have been also described in the literature (Ledauphin et al. 2006b), but they
are extremely complicated, work intensive and some of the steps require harsh
operating conditions under which some molecules could be formed (artifacts) or
degraded.