104 M. Ugliano
and malolactic fermentations. Also, the extent of certain enzymatic reactions can
determine the efficiency of specific technological steps considered to be of primary
importance in the modern wine industry, such as juice and wine clarification, color
extraction, and protein stabilization. Therefore, understanding the role played by
enzymes during winemaking can help in the development of rational and effective
strategies for optimizing wine processing to modulate wine composition and sensory
properties.
Grape berries and wine yeasts are the major sources of enzymes involved in the
various biochemical transformations that take place during winemaking. However,
typical winemaking conditions such as high sugars and ethanol concentrations, low
pH and high concentrations of polyphenols, can potentially inhibit the activity of
grape and microbial enzymes, often with synergistic interactions which result in
enhanced inhibitory effects. For this reason, the reactions catalyzed by grape and
microbial enzymes during winemaking are often incomplete, with a significant por-
tion of the substrate remaining untransformed and therefore available for further
reaction. Because many of these reactions are considered beneficial to wine quality
or to the efficiency of specific technological operations, the addition of exogenous
enzymes that exhibit higher efficacyunder winemaking conditions is frequently
carried out in the winery to obtain the desired level of substrate transformation.
Exogenous enzymes are an important component of modern winemaking, and many
industrial preparations are now commercially available, particularly to assist during
juice and wine clarification and to increase liberation of aroma compounds from
odorless precursors.
This chapter provides an updated overview of the current knowledge on the role
played by enzymes in the process of transforming grapes into wine and on the pos-
sibility offered by new technologies tomodify wine composition though a more
effective control of enzymatic reactions.
4.2 Polyphenol Oxydases
Oxidative browning during the production of white wine is a well known phe-
nomenon that can occur at various stages of the winemaking process. In sound
grapes, enzymatic oxidation of phenolic compounds takes place in the presence of
oxygen during pre-fermentative operations,due to the action of grape tyrosinase.
Following grape crushing, tyrosinase is partially released from berry chloroplasts
into the juice (Dubernet 1974), where itcan react with cinnamic acids and their
tartaric esters to formo-quinones (Fig. 4.1). These highly reactive species can then
enter two different reactions. In one possible mechanism, quinones can condense
with other phenolic compounds to form polymerized products that exhibit a more
or less brown color, according to the degree of polymerization (Singleton 1987).
Alternatively, quinones can react with reductive species like glutathione, a tripeptide
commonly found in grapes and containing an –SH group. This reaction forms a
colorless derivative that is stable to tyrosinase action, and therefore does not result in