| Science of Wine
Wine is an alcoholic beverage made from fermented grape
juice. Wines made from other fruits are always named accordingly. A glass of wine is a
mixture of chemicals, and drinking it stimulates your body and mind. The chemical
composition of wine is about 87.7% water, 11% alcohol, 1% acid, and 0.2% tannins.
There are many steps in making and producing the wine. Firstly,
the grapes are allowed to ripen in the vineyard until they attain suitable sugar content,
which is about 18% or more, and acidity. During ripening in the vineyard, grapes may
become infected by molds, yeasts, and bacteria. These infections generally destroy desired
flavours and colour and add undesired acetic acid and oxidized flavours. However, the
infection of white grapes with the rot fungus called Bortrytis cinerea is very
useful. Infection of white grapes with this mold leads to the concentration of the juice
in the berry and also gives a characteristic odour to the wine.
The second step in the making of wine is the fermentation of the
grapes with various yeasts and lactic acid bacteria. Grapes can be fermented by adding
selected wine yeast to dominate the yeast that derive from the vineyard (grape surface,
leaves, and stems) and the winery environment (tanks, barrels, hoses). The addition of a
selected yeast culture ensures a complete fermentation without the loss of odours and
produces a wine of consistent flavour quality. Fermentation temperature and
characteristics of the selected yeast determine the amount and type of flavours produced.
During spontaneous fermentation, a range of different yeasts grow at different stages of
fermentation. Therefore, a winemaker must carefully guide spontaneous fermentations to
reduce the risk of spoilage by unwanted microorganisms. Successful spontaneous
fermentations can produce very flavourful wines with a range of odour and texture derived
from the juice flavours
Followed by yeast fermentation is a second fermentation by lactic
acid bacteria. This fermentation is called a malolactic fermentation and during this
process, lactic acid bacteria convert malic acid to lactic acid and CO2 , which
results in a lowering of the acidity of the wine. The bacteria also change the fruit
flavour of wine and add some flavour compounds from their metabolism. Temperature, pH, and
availability of other sources of energy affect the rate of malic acid utilization.
After completion of fermentation, wine is clarified by filtration
and stabilized. Wine flavours can continue to change while the wine is stored in wooden
barrels, stainless steel tanks, and in glass bottles. During this development of wine,
various yeasts and bacteria can be present and further modify wine flavours. These yeasts
are generally considered spoilage yeasts. Depending on the type of microorganism and on
the extent of growth, desirable fruit flavours can be lost by unpleasant odour and taste.
Different wines benefit from longer or only short aging. |
