Yeast is a single cell fungus that feeds on oxygen and multiplies at a fast rate. This is called aerobic fermentation. Yeast is also capable of converting sugars into alcohol and carbon dioxide, which is called anaerobic fermentation.
The anaerobic reaction between yeast and glucose is expressed with the following formula: C6H12O6 (glucose) -> 2C2H5OH (ethanol) + 2CO2. This is the fermentation process that makes whisky. When yeast is added to the wort in the washback, the liquid starts foaming and frothing aggressively and sugars are converted into carbon dioxide and alcohol. A chemical reaction is set off with the acids in the malt, which creates esters and aldehydes. We perceive them as different aromas of fruits and flowers.
The fermentation process differs from distillery to distillery. A minimum of 48 hours is a regular cycle, but 60 to 70 hours is not uncommon. A few distillers will even allow a 100-hour fermentation cycle. The length of fermentation affects the flavours and aromas of the drink, as more esters develop over time.
However, allowing fermentation to go on for too long may cause bacteria to form, which can lead to undesired flavours and aromas like butyric acid. The remaining liquid at the end of fermentation is called “wash.”
Our whisky Q&A is provided courtesy of SMWS ambassador Hans Offringa’s A Field Guide to Whisky: An expert compendium to take your passion and knowledge to the next level