Bourbon - Alcoholic Fermentation
After the mash has been cooled down to approximately 25° to 30°C (77° to 86°F), it can be put into a fermenter (along with a larger amount of yeast), where a beer with approximately 9% abv is produced through fermentation.
At this stage also parts of the stillage are added. The stillage is formed at a later stage. It is part of the much-praised sour mash process, which will be explained in detail later.
The size of the fermenters varies between 'large' and 'giant'. Since column stills work continuously and can process large amounts of beer, a steady supply must be guaranteed so they don't run dry.
That's why American distilleries have a 'beer well' into which the fermented content (beer) is emptied. The beer well is usually made of stainless steel and is placed amidst the numerous fermenters. The size of the beer well corresponds to the size of the fermenters. The beer well is usually one third larger than the largest fermenter, so the column stills can continue production even if the emptying of a fermenter is delayed.
During alcoholic fermentation yeasts convert the sugar into alcohol and carbon dioxide while also generating heat. Fermentation usually takes three days. An alcohol content of approximately 8% to 9.5% is reached. Few distilleries (e.g. Labrot & Graham) ferment significantly longer, but they also don't reach higher alcohol contents than 10% to 11%. The result is called beer or distiller's beer.
The larger a fermenter, the hotter it gets, since the surface-to-volume ratio decreases with increasing size and the fermenter can't discharge the heat anymore. If the temperature in the fermenter rises above 35° to 40°C (95° to 104°F), the yeasts start dying off until fermentation stops. That's why many fermenters are water-cooled.
Distilleries such as Early Times or Four Roses already evaluate the result of the fermentation. The smell of the beer tells a lot about the future whiskey. The beer is desired to be aromatic (e.g. having a strong apple aroma). A decrease in the aroma of the beer indicates that the yeast is contaminated, and a new yeast strain is used for the next batch.
After fermentation, the beer is distilled into raw whisky, the 'white dog'.