Yeast is added to the wort as it is pumped from the cooler into the fermentation vessels, or 'washbacks'. The tun room at The Balvenie Distillery contains ten wooden washbacks, which are made out of Oregon Pine.

Fermentation causes the temperature to rise significantly and, as carbon dioxide gas is produced, the liquid foams up to the top of the washback and has to be kept down with revolving blades. Some two and a half to three days later the liquid in the washbacks, now called wash, with an alcoholic strength of around 7-9% alcohol by volume, is ready to be pumped through to the stillroom.