8E Identification of Impact Odorants of Wines 395
volatile molecules. Many of these volatile molecules have some aroma, but only a
limited number of volatiles can be foundin wines at concentrations high enough
to be perceived. Obviously, these compounds are the ones which will have a
major influence on the aroma characteristics of wine and, therefore, are the ones
which will have to be analyzed and considered further in sensory studies. This
is something that all researchers in wine chemistry should remember: there are
so many volatiles that it is absolutely essential to screen the real odor-active
molecules from all the volatiles present.- The second point is related to the simultaneous presence of odorants at g/L levels
and of others that can be active at levels as low as ng/L. This means that although
it makes sense to use a general screening procedure for detecting by olfactometry
the potentially most relevant aroma molecules, it will not be possible to use a
single isolation or preconcentration scheme to identify and further quantify the
different aroma molecules. Rather, it will be necessary to have an array of chem-
ical isolation and quantification procedures if a comprehensive aroma analysis is
our objective.- The third and fourth influential factors on wine aroma chemistry arise from
the problems caused by the matrix on both the isolation and preconcentration
of molecules and on the sensory assessment of the role played by the impact
odorants. The presence of major volatiles, such as ethanol and fusel alcohols,
complicates the isolation of the other wine odorants, particularly of those present
at low levels. This has a marked influence on the way in which the extracts for
screening and for further qualitative or quantitative studies should be prepared.
On the other hand, ethanol and the major fermentation volatiles have also a deep
influence on the way in which the odor chemicals are released and perceived.In conclusion, the study of the wine aroma chemicals and the understanding of
the role they play in the different wine aroma nuances have to be structured into a
numbers of steps strongly constrained by the previous considerations. Such steps
will be the subject of thischapter. The first step is about the screening of aroma
molecules, which will be carried out by using gas chromatography-olfactometry.
The second will be the isolation and identification of odorants. The third is the
quantitative determination, for which only a very brief outline will be given, and
the fourth is about the sensory tools used to assess the sensory role played by the
different odorants.
8E.2 Screening for Aroma Impact Molecules
Since all aroma molecules are more or less volatile, the technique that “a priori” is
best suited to screen the odor active molecules from the rest of molecules is Gas
Chromatography-Olfactometry (GC-O). This technique makes use of the human
nose as detector for the compounds eluting out of the chromatographic column, typ-
ically a fused silica capillary column (Acreeet al. 1984). There are several different
approaches for GC-O differing in the way in which the olfactometric signals are