Now that the distillers got all the sugars from the barley it’s time to use it to produce alcohol. To do so the distillers need yeast to convert the sugar.
After cooling the wort to 18-20°C it is filled in washbacks, traditionally made from wood, but distilleries also stainless steel washbacks for their fermentation.

The distillers add yeast in either powder or liquid form to the wort. The most commonly used yeast is Saccharomyces cerevisiae. Belonging to this type of yeast there are different variations and distillers usually go for a hybrid of brewer’s and distiller’s yeast. The former provides more aromatic flavour compounds while the latter produces more alcohol and a mix of both provided the best results.

When the yeast is added it needs to 8 hours to acclimatise to its environment. After that it multiplies using the oxygen in the wort (aerobic respiration) and producing carbon dioxide and water. When all the oxygen is used up the yeast starts using the fermentable sugars (anaerobic respiration or fermentation) produces carbon dioxide and ethanol alcohol.

This process now takes around 40 hours. In this time the produced carbon dioxide is bubbling up inside and it needs to be stirred so it won’t flow over. Stirring also helps to keep the temperature low, as it mustn’t exceed 35°C (yeast cells die above that temperature.

After 48 hours the alcohol production stops as all the sugars are used up. But distillers don’t necessarily stop here. Often they leave the now called wash for longer in the washbacks for a total of 50-100 hours. Lactic acid bacteria are now transforming parts of the wash into:

      • acids
      • esters: responsible for fruity aromas in whisky
      • aldehydes: can later transform into many flavours when interacting with high alcohol and esters during maturation. It then forms flavours such as vanilla, cinnamon, sweet notes and even smoky and spicy ones.
      • more complex alcohols

They might be few, but make up an important part of the flavours in the future new make and therefore the base of every whisky to be.

The wash now has an ABV of 8-12% and is ready for distillation!