4 Enzymes in Winemaking 105
Fig. 4.1Mechanism of formation of oxidized polymers responsible for must browning
significant modification to must color. The competitive nature of these two reaction
mechanisms imply that, when a higher concentration of glutathione is present, less
quinones will be transformed into brown, polymerized products. It has been shown
that some grape varieties are typically more resistant to oxidative browning, due to a
higher content of reductive species thatcan react with quinones,such as glutathione
and ascorbic acid (Rigaud et al. 1990).
Oxidation of phenolic compounds becomes of greater concern when grapes are
infested byBotrytis, due to the concomitant presence of a second powerful oxi-
dase, namely laccase produced byBotrytis(Dubernet 1974). The activity of this
enzyme, which can also be responsible for the loss of color in red wines, is about
30 times higher than that of tyrosinase. It is stable at juice pH, and is not inhibited
by SO 2. Also, it can react with a variety of phenolic substrates, as well as with
other classes of chemical compounds, including the quinone-glutathione complex
(Salgues et al. 1986). Therefore, more severe browning is expected to occur in juices
from botrytized grapes compared to juices from sound grapes (Fig. 4.1).
Various strategies can be adopted in the winery to reduce oxidation phenomena.
Because oxygen is a cofactor of enzymatic browning reactions, careful protection of
the must from oxygen and addition of ascorbic acid are often recommended. Sulfita-
tion and cooling of grapes and juice during crushing/pressing are both very effective
in controlling tyrosinase-driven juice oxidation, as tyrosinase is strongly inhibited
by SO 2 and its activity is significantly reduced atlow temperatures. Conversely, in
the case ofBotrytis-infected grapes, heating of the must at 50◦C is the only treatment
that can eliminate the risk of oxidation due to the activity of laccase.
Although it is generally accepted that protection of the must against oxidation can
result in wines characterised by higher concentrations of volatile compounds that
are responsible for fruity aromas (Moio et al. 2004), extreme antioxidant protection
of the must can result later in wines that are more susceptible to oxidation. Based
on this observation, a technique known as hyperoxygenation has been proposed