SPIR4080 - Technical guidance: Stages of production: Fermentation

A visual illustration is shown in the mashing and fermentation diagram (PDF).

Fermentation is the process where the sugars in the wort are converted by yeast into alcohol and carbon dioxide. This takes place in a large collecting vessel called a washback (6). The quantity of yeast added to the wort is normally between 1% - 1.5% of the volume, and is usually a mixture of brewers and cultured yeast. Once the washback has been filled to the required level, the volume of the wort collected is measured by dip and the specific gravity (known as the original gravity or 'OG') and the temperature are recorded. This is known as 'setting the back'.

At most modern distilleries yeast is injected into the worts line during transfer to the washback. This early introduction of yeast starts fermentation in the wort before the original gravity can be measured and recorded. To compensate for this fermentation, an addition is made to the original gravity readings - this is known as the 'addition for work'. Notice 39 contains information on measuring gravity.

Fermentation normally takes about 2 days, during which time most, if not all, of the available sugars will be converted to alcohol. As the alcohol is produced, the density or specific gravity of the 'wash', as it is now known, decreases until all fermentation ceases. The specific gravity - known as the 'final gravity' at this point - is then taken. The difference between original and final gravities is the degrees attenuated or the 'attenuation'.

The wash, which is not unlike a strong flat beer containing 6 - 8% alcohol by volume, is then pumped to a holding tank known as a wash charger (7) from where it is transferred to the stills for distillation.