The ‘wort’ from the mash tun is cooled to 23C, before the 100% cultured Distillers’ yeast is added, in a ratio of 1% to start the conversion of the sugars present in the ‘wort’, into ethanol, releasing carbon dioxide and producing heat as a by-product. During the 48 hours of fermentation, the temperature will increase by approximately 10C.
Two stainless steel fermentation vessels, or ‘wash backs’, are used for fermenting the first and second ‘worts’ extracted from the mash tun. With each holding 41,500 litres, the fermentation vessels are two thirds full, allowing one third for expansion. Frothy bubbles begin to rise during the process, due to the carbon dioxide, and at the height of fermentation they reach the wooden lid of the fermentation vessel. To prevent over-flowing, a single rotor blade automatically switches on, and spins round to reduce the bubbles. This is called ‘switching’.
After 48hrs the froth has died down and a malty alcoholic liquid called ‘wash’, has been produced. The ‘wash’ is 8% abv and is often referred to as ‘sour beer’, since it is similar to a malty beer.
During this process, sucrose, fructose, glucose, and maltose are fermented and minerals zinc, amino acids and fatty acids and some malt derived flavour compounds are used by the yeast for growth, producing flavour compounds. These compounds, ethanol, higher alcohols, ethanal, organic acids, esters and carbonyls, all contribute to the individual flavours of the different Single Malt Whiskies. For example, chemical reactions between the various acids and alcohols, produce esters, which have characteristic ‘fruity’ or ‘flowery’ aromas.