Agriculture Reference
In-Depth Information
achieved. Various effects of freezing and homogenising the
sample with various durations and tools has been reported
recently (Cynkar
et al
. 2004).
was shown recently to increase polyphenols in wine grapes
(Botondi
et al
. 2011); this effect was associated to a partial
dehydration. Furthermore, polyphenolics are substrates of
enzymes that may alter wine quality (Flanzy 1998). Another
aspect, linked to the food science, is the optimisation of poly-
phenol extraction in the case of red wines using various
processes that will not be detailed here.
Sugars
Sugar level at harvest depends on the region of production.
For classical dry wines, a global figure of 200 g of reducing
sugars/kg of fresh grapes can be given (Jackson 2000), this
will roughly give a 12% alcohol wine. But cool climate
areas will produce grapes which are usually less sweet than
warmer and sunnier areas. Moreover the type of desired
wine will also influence the date of harvest (thus the sugar
level), for example late harvested grapes (with sugar levels
above 300 g / kg) will be used to produce sweet wines.
An optimal harvest date is critical to achieving the right
sugar level.
There are pre- or post-harvest practices known to
increase the sugar levels like leaving the grapes on the
vines for one or two months beyond the optimal harvest
date, thus leading to natural sugar enrichment by berry
desiccation. The vineyard has to be in a suitable climatic
area (dry afternoons avoiding strong rot development).
Aroma potential
Aroma potential depends on several compounds which
vary according to the grape cultivar. Jackson (2000) cited
the terpenes and the glycosyl-glucose contents, as being
among the markers for aroma potential which have received
attention. Their assessment by simple means is still a mat-
ter for research and development (Flanzy 1998). A suitable
harvest date and good postharvest management are critical
to optimise this potential (see 'Harvesting and post-harvest
management; below). Some other aromatic componds, like
methoxypyrazines that are typical of green pepper, are
already present in grape berry tissues in the aromatic form
(Allen
et al
. 1990), but rarely desired in the resulting wines.
However, in white wines, there are also some aromas
produced during the fermentation, that are derived from
precursors, whose levels in grape depend on cultivation
conditions as some volatile thiols in Sauvignon blanc
(Gachons
et al
. 2005).
Acids
The acid level varies according to the region and type of
wine, but as described in
Berry Development
(acidity) it is
inversely proportional to the sugar level i.e. as the acid
content drops, the sugar content increases. To give an idea
of the range of acidity found in musts, one can cite average
acidity of Cabernet Sauvignon in Bordeaux over 20 years:
100 ± 20 g of meq. / L (Ribéreau-Gayon
et al
. 1998).
Jackson (2000) states that must pH should be below 3.3 for
whites and 3.5 for reds. The acid content is important in
various ways: it affects sulphur dioxide efficiency, the
freshness of the taste and the ability of the wine store well,
among other factors. An optimal harvest date is critical to
achieving the right acid level.
Phytopathogens
One of the main post-harvest problems due to grape moulds
and
Botrytis cinerea
in particular is the oxidative action of
fungal laccase (Nair & Hill 1992; Jackson 2000). This
extracellular enzyme induces browning of white musts by
oxidising polyphenols and causes off-flavours in red wines
(Rankine 1997). Laccase activity in musts can be assessed
by various automated systems which are already in use in
many wineries (Ribéreau-Gayon
et al
. 1998). Good
pre-harvest and post-harvest control of fungal infections is
critical to managing this problem. Laccase has been used
as a marker for successful disease control in grapes (Dubos
et al
. 1996). Recent efforts have been made for the
assessment of gluconic acid as an indicator of rotten grapes
(Crachereau 2004).
Polyphenols
Polyphenols are of great importance to wine quality and
can be assessed using various methods (see
Berry
Development
:
phenolic components
). However, rapid and
accurate measurement of polyphenol berry content is still
difficult (Jackson 2000), and a matter for further research.
An optimal harvest date and good post-harvest manage-
ment are critical to achieving and preserving the polyphe-
nol content. There are very few studies on the polyphenol
changes over the post-harvest period; one report least shows
a global decrease of most of them during this period (Borsa &
Di Stephano 2000), but an ethylene postharvest treatment
Wa xe s
Waxes from the berry epidermis are the primary source of
waxes in wine and may contribute to colloidal turbidity of
wines (Rosenquist & Morrison, 1988). The waxes offer
anchoring points for large numbers of spores of various
microorganisms (Zahavi
et al
. 2000) that will influence the
post-harvest life of the berries and the wine quality.
