Yeast is a marvellous, single-celled plant, which lives off sugar and, as a by product during fermentation, produces alcohol and carbon dioxide (CO2).
This process also makes small quantities of other volatile compounds, such as esters, ketones, and aldehydes, all of which add to the aromas and
flavours of a wine.
Once the wine has reached about 15 percent alcohol by volume, or all of the sugar has been consumed, the yeasts will begin to die and settle on the
bottom of the fermentation vessel as sediment or "lees."
Once at the winery, the grapes are pressed. Treading by foot is the best way to do this, although it is very rarely used today. The foot is hard
enough the split the skins of the grapes but soft enough to prevent damage to the seeds, which would release bitter oils into the wine, or the stalks,
which would add unwanted tannins. However, costs and the sheer volume of grapes that need to be pressed quickly in order to avoid oxidation damage
mean that mechanical presses have become the norm.
BTW All text extracted from: Wine, A Basic Course by Tom Forrest; Thunder Bay Press 2003
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