Wine Production : Vine to Bottle - Vinum Vine

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9.3 Fermentation, temperature control and
extraction
As we have seen, the process of fermentation results in the conversion
of sugar by the enzymes of yeast into alcohol and carbon dioxide.
9.3.1 Fermentation
The fermentation of red wine takes place with grape solids present,
in order to extract colour from the skins. Initially the fermentation
can be very tumultuous, but as more sugar is converted, the rate
slows down. In the majority of cases the fermentation is continued
until the wine is dry or off dry, and depending upon the richness of
the must, the final alcohol concentration is generally in the range
11% to 14.5% by volume.
CHAPTER 9
9.3.2 Temperature control
The fermentation process is a turbulent one and creates heat
naturally. During red winemaking, fermentation may begin at about
20 °C, but temperatures may rise to 30° to 32 °C. Yeast ceases to
work if the temperature rises above approximately 35 °C. Therefore
some form of temperature control may be necessary, especially
in warmer regions, to prevent this happening before the sugars
are fully fermented. It is only in the past few decades that winemakers
have had the equipment and ability to be able to have real
control.
One hundred grams of sugar in a litre of juice has, when fermented,
the theoretical potential to heat the juice by 13 °C. If grape must of
11.1° Baumé commences fermentation at 15 °C, the potential temperature
increase could be to 41 °C. If fermentation of must with a
Baumé of 14.5° commences at 20°C, the potential temperature
increase is to 54 °C. Of course, some of this heat is naturally dissipated
during the period of fermentation through the walls of the
vat and with the rising carbon dioxide, through the juice to the
surface.
Good colour extraction requires warm fermentations. However,
cooler fermentations aid the growing of yeast colonies and give higher
alcoholic degrees. The warmer the temperature, the less time the