Agriculture Reference
In-Depth Information
consisting of a benzene ring (C
6
H
6
) with a hydroxyl group (-OH) substituted for
one hydrogen atom. A compound with more than one phenolic group is termed a
polyphenolic compound.
As indicated in Table
7.6
, and described in the Wine Production Section, the
concentration of phenolic compounds is typically higher in red wines compared
to due to the extended extraction from skins in red wine production. The phenolic
compounds in wine are often classified as flavonoids or non-flavonoids. Given that
non-flavonoids are mainly located in the pulp of the grape, white wines contain
mainly this component of phenolic compounds and only minor amounts of flavo-
noids (Table
7.6
). Alternatively, red wines will contain similar non-flavonoid levels
as white wines, but g/L levels of flavonoids extracted from the skin and seeds of the
grape. The non-flavonoid concentration of the wine may be supplemented by ex-
traction of phenolic compounds from oak, and both flavonoids and non-flavonoids
may be supplemented by endogenous additions during wine production. Within the
group of flavonoid compounds are anthocyanins and tannins (i.e., flavonoid poly-
mers) that impart the colour of red wine and contribute to the astringency of wine,
respectively. The monomeric and lower molecular weight flavonoid polymers are
suggested to be important contributors to bitterness sensation in wines, and tannins
of 'medium' length contributors towards astringency, however much debate still oc-
curs on the type, range of sizes and confirmation of tannin that are most important
for astringency (Scollary et al.
2012
). Current research is focused on better under-
standing astringency mechanisms and linking wine tannin structure to viticultural
parameters.
To modulate the perception of bitterness and astringency, soluble or insoluble
proteins are added to wines and are termed 'protein fining' agents. The proteins are
able to interact with phenolic material and ultimately lead to the aggregation and
precipitation of a fraction of the phenolic material which is then removed by settling
and/or filtration. This alteration of the phenolic composition of the wine leads to
dramatic changes in the astringent/bitterness profile of the wine. Due to the variable
composition of both the wine and the protein fining agents, the action of the fining
agent is difficult to predict and small scale trials are usually conducted to ascertain
the most suitable fining agent and the addition rate. Over addition of protein can
cause a detrimental loss of aroma compounds and result in an unbalanced wine. The
source of the protein material added to the wine is mainly animal-based (gelatine,
isinglass, egg white, milk/casein). Insoluble synthetic polymers (polyvinylpolypyr-
rolidone, PVPP) have been developed that allow also modification of the phenolic
profile of a wine (Sims et al.
1995
).
Red wines typically have only trace levels of residual proteins present due to
their high concentration of phenolic compounds, which induces the precipitation
of grape-derived proteins after crushing and during fermentation. However, white
wines with lower phenolic concentrations can have significant concentrations of
protein remain in the wine. This protein may stem from fining additions or derive
from the native protein of the grape. During storage of the wine in the bottle, a
fraction of the protein can aggregate and form a haze in the white wine, which is
considered a fault by consumers. To avoid this haze formation, white wines are
