The 'wort' from the mash tun is cooled to 20C, 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 fermentation, the temperature will increase by approximately 10C to 15C, depending on the ambient temperature.
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 is switched on, and spins round to reduce the bubbles. This is called 'switching'.
After at least 48 hours 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. The process is similar to brewing, apart from the fermenting 'right out' of the 'wash', so there is no yeast left. In brewing, the fermentation is usually halted half way through, which produces a 'wash' of approximately 4% abv.
Glenfarclas has 12 stainless steel fermentation vessels, or 'wash backs', each with a capacity of 45,000 litres. Traditionally these were made from pine.
Average setting gravity: 59 - 60 Final gravity: 2.5 – 3.0
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.