Antioxidants
Antioxidants behave in a manner
similar to electrochemically-active
species, because they respond to the
potential for oxidation or reduction
in their environment. For example,
examining a series of chemicals that
are thought of as antioxidants
(Figure 4), one can see that tannins
are more effective antioxidants than
SO2.
In white and rosé wines, no significant
amounts of tannin are present,
and the next most effective antioxidant
in such wines is glutathione. Clearly,
maximization of GSH will lead to better
wine preservation. The goal of
winemakers should thus be to maximize
the GSH concentration in these
wines.
What makes GSH an effective
antioxidant? It is the presence of a
thiol group in the molecule. Thiols
provide many of the strongest fruity
characters found in certain wines
(such as Sauvignon Blanc), and these
aromas can be susceptible to oxidation
precisely because they are good
antioxidants, like GSH. The problem
is that when they react, as antioxidants,
their contribution to wine
aroma is lost or altered, hence wine
quality deteriorates.
The thiol group (–SH) is a reduced
form of sulfur (S). Sulfur is able to
react with oxygen to form oxides (as
in SO2), sulfites
(SO32–2–)
and, ultimately, sulfate
(SO42–).
Since an informal
definition of oxidation is “combination
with oxygen,” the
progression of sulfur from –SH to
SO42–
constitutes the behavior of an
antioxidant (Figure 5).
With glutathione, the situation can
be slightly more complicated in that,
for the more chemically-minded,
GSH acts as a nucleophile and regenerates
hydroquinone and quinone
species, which are oxidized forms of
phenols (see Figure 6). The end result
is typical antioxidant behavior, since
the oxidized form of the molecule is
reduced to the original phenolic form,
with glutathione now bound to the
phenolic core to form the “grape reaction
product.”
Preserving wine aroma
Some simple practical adaptations
can be made to preserve the glutathione
content of wines. Maintenance
of the SO
2 level
