Open up a fermentation tank and it will look like a boiling pot of grape juice. This is the other by-product of the fermentation process, carbon dioxide, escaping into the open air. At this stage the winemaker will be keeping a close eye on what’s going on inside the vessel.
In some high-tech wineries it is possible to weigh the vessel at different times to check how much Co2 has been lost, and thus work out how much sugar should remain. The most common technique used is the measurement of density. Hydrometers can be calibrated to determine density and provide a reading of the sugar’s percentage in weight, however, this becomes less accurate the more alcohol is produced because alcohol is less dense than water.
Knowing when fermentation has finished is crucial. If fermentable sugar remains, the wine becomes susceptible to bacterial attack and may need to be treated, so many wineries now take measurements using paper strips or hand-held density meters. For producers of bulk wine, devices can be fitted within the vessel and the beginning and end of the process can be decided by a computer. The result is consistency, even if one might say these wines are characterless and one dimensional, consistency is what their target market demands.
Red wine is usually fermented within four to seven days, white wine takes longer as it is fermented at a lower temperature, often a few weeks. The temperature, the type of yeast, the aeration of the must will all have an impact on how long the process takes. Over time, experienced winemakers will find a formula that suits their style. For example, they might rely on a certain yeast for a particular wine and based on previous experience they will have a good idea of how long the fermentation will last. For the big producers looking for a quick turnover, the speed of the process is often hastened to free up vessels for more juice.
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