Chemistry Reference
In-Depth Information
new world winemaking countries; this trend is accompanied by a change in global consumer
tasteinwine.
Consumers want aromatic fruity wines and in the case of red wines; soft tanins, but
above all they want consistency in the profile of the product they buy. This trend has lead
to an industrialization of winemaking processes and a more consistent use of recipes and
processing aids such as enzymes. The use of pectinases was introduced in winemaking in the
1970s to improve white must clarification. From the 1980s onwards enzymes have been used
for colour extraction, filtration and aroma release. Nowadays, enzymes are used not only to
improve the efficiency of processes such as pressing and clarification but also to improve
wine quality through aroma, mouthfeel and structure.
Although the grape berry as well as the microorganisms performing the transformation
of grape juice into wine do perform enzymatic reactions, this chapter only reviews the
application of added exogenous industrial enzyme preparations with a special focus on
enzymes working on grape-derived substrates.
9.2.2 Grape structure and composition
Although grape berry composition depends on the variety, soil and climatic conditions, there
is little variation in the plant cell structure (see chapter on enzymes in fruit and vegetable
processing and juice extraction).
The grape berry skin accounts for 6-9% of the total berry weight. Within skin cells are
found essential compounds such as anthocyanins (responsible for the red colour of wines),
tannins (essential for the structure of red wines) and aroma or their precursors. The thick
pecto-cellulosic wall of the skin cells provides rigidity to the berry but when making wine it
prevents the diffusion into the must of intracellular components.
The pulp represents 75-85% of the ripe berry weight. It comprises large cells with
fine pecto-cellulosic walls offering limited mechanical resistance. In the cell, vacuole is
a concentrated solution of organic acids, fermentable sugars, some aroma and precursors.
Pectin is located between the cells, in the primary wall and lamella.
9.2.3 Pectin
Pectin is probably one of the most complex macromolecules found in nature. Its content in
grapes varies with the variety and maturity.
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The pecto-cellulosic wall is a complex structure.
It is composed of cellulose microfibrilles, linked together by a matrix of xyloglucan, mannan,
xylan (generally known as hemicellulose) and pectin, all consolidated by a secondary protein
network. Some neutral sugars (galactose and arabinose) make up part of the structure of the
lateral pectin chains and form macromolecules with proteins.
Pectin three main components
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9.2.3.1
Homogalacturonan (HG) is a non-ramified chain of
-1,4-linked galacturonosyl acid
residues, partially esterified at C-6 with methanol groups. They are also described as the
'smooth region' of pectin.
α
Rhamnogalacturonan I (RG-I). The main chain, comprises an alternating rhamnose and
galacturonic acid, bears lateral chains of arabinans and arabinogalactans to form areas
known as 'hairy regions'.
