Agriculture Reference
In-Depth Information
commonly treated with a volcanic clay mineral (bentonite) that is efficient in ex-
changing cations such as sodium and calcium ions for the positively charged sites
of proteins. Once attached to the bentonite mineral the suspended bentonite settles
and the wine is decanted from the bentonite/protein sediment. Alternatives to ben-
tonite have been sought, as high rates of benonite can be of detriment to sensory
features of the wine, and the use of bentonite requires a variety of processing steps
for the removal of the deposited sediment. One recent approach has been the use of
a specific enzyme for the degradation of targeted proteins that have been identified
as key instigators in the haze formation (Waters et al.
1995
).
Another detrimental phenomenon that may occur during the bottle aging of wine
is the precipitation of salt crystals. The most common precipitate in this regard is
due to potassium hydrogen tartrate crystals. The formation of such crystals does
not tend to impact the organoleptic qualities of the wine but they are considered
highly undesirably by consumers. To prevent the crystal formation during bottle ag-
ing, the crystal formation is induced during the wine production process. The most
common approach is the chilling of wine to 0-4 °C or lower, with the addition of
potassium hydrogen tartrate crystals to aid crystal growth, for extended periods and
then removal of the resulting precipitate from the wines. Such a chilling step can
contribute significantly to the energy costs of wine production and consequently al-
ternative treatments have been proposed, including techniques such as electrodialy-
sis, ion exchange and reverse osmosis (Low et al.
2008
). Although these alternate
techniques can significantly reduce energy costs, it ultimately comes at a cost of
some other production parameter (i.e., additional processing steps, increased labour
costs, elevated water usage and/or excessive wine losses (refer to Low et al. (
2008
)
for further comparisons)).
Stability against oxygen during wine storage is of crucial importance to red and
white wines. The dissolved oxygen in wine reacts with phenolic compounds to gen-
erate products capable of changing the colour and flavour of wine. Sulfur dioxide
is the key preservative to prevent the detrimental reaction induced by oxygen, as
well as its ability to inhibit microbial growth. Sulfur dioxide, equilibrates between
molecular sulfur dioxide (SO
2
), hydrogen sulfite (HSO
3
−
), and sulfite (SO
3
2 −
) at
wine pH, with the hydrogen sulfite form dominating (i.e., 80-90 %). All these forms
of sulfur dioxide in wine are termed 'free sulfur dioxide', whilst the component
that reversibly binds to aldehyde and ketone compounds present in wine is termed
'bound sulfur dioxide'. The free sulfur dioxide in wine can efficiently scavenge the
oxidation products formed by molecular oxygen and phenolic compounds before
any significant impact occurs to wine. The ability of sulfur dioxide to perform this
role in red wine is complicated by the ability of the anthocyanins in red wine, re-
sponsible for the red colour of grapes and young red wines, to reversibly bind to
sulfur dioxide. Consequently the concentration of sulfur dioxide in red wines cannot
be maintained at concentrations as high as in white wines without the removal of
significant colour in red wines. During time the anthocyanins in red wine become
incorporated into polymeric phenolic compounds (i.e., via reactions with other phe-
nolic compounds) and as a result they become less reactive to sulfur dioxide.
