394 V. Ferreira and J. Cacho
the odor of the product is closely related to the odor of such chemical (Belitz
and Grosch 1988). In wine this only happens in some particular cases, such as
Muscat. For convenience, however, we will retain the denomination of impact
molecule to refer to a molecule which is a neat contributor to an aroma nuance.
This fact has practical consequences for the researcher, since in many cases it is
not possible to establish a clear and univocal link between a sensory descriptor
and a single aroma molecule. Such a link will have to be established a posteri-
ori in sensory studies working with the potentially relevant aroma molecules in
model solutions or in wine.- A second question related to the previous one is the relatively large numbers of
potential impact compounds in wine. The simultaneous presence of many differ-
ent odor chemicals causes that the final perception will be the result of a complex
brain processing in which some odors are integrated into a single perception;
some act in a competitive or even destructive way (Atanasova et al., 2004),
while others interact to form a new and different perception. A particular case
is the presence in wine of whole sets of aroma chemicals members of a chemical
homologous series and displaying similar odors. In this case the practical con-
sequences are that the role of some chemicals should be considered as a part of
acombination and, of course, that the final role can only be assessed via different
sensory experiments.- The third question derives from the large diversity in physico-chemical proper-
ties of the different wine impact compounds. Some of them are quite hydropho-
bic and are released very easily from the wine matrix, while some others are quite
hydrophilic and will be released with difficulty (Ferreira et al. 2006). The former
are major constituents of the headspaces on a glass of wine, while the latter will
reach the pituitary only when the level of liquid in the glass is very small or
when the wine is swallowed. These differences in behavior, still not completely
understood, may explain why different odors are perceived when the glass of
wine is full or when it is nearly empty (Petka et al. 2006) and why sometimes
there are marked differences between ortho and retronasal perceptions.- Finally, some of the most powerful wine aroma compounds take part in reversible
interactions which evolve during wine aging and that can be reversed, at least
in part, when the wine takes contact with air. These aspects have not yet been
studied in depth, but it is well known that carbonyls form reversible associations
with sulfur dioxide and that mercaptans take part in complex redox equilibria.
These molecules involved in interactions or in redox equilibria are the most likely
cause of the aromatic changes noted during the aging of wine or after the bottle
is opened.This aroma chemical complexity together with some relevant aspects of the wine
as analytical and sensory matrix strongly constrains the way in which the study of
wine impact molecules must be carried out. Some of the most important points are
the following:
- The first concerns the general composition of wine. Wine contains thousands
of different molecules, and a number approaching one thousand is formed by