The finished malt is milled to flour. This coarse flour is called grist and is mixed with hot water in the mash tun. If the grist is too coarse the sugar can’t be fully extracted. If the grist is too fine it sticks together, and the sugar can also not be extracted completely.
The malt is mashed three times before the sugar solution is cooled in a cooler. In the first run, the water has a temperature of about 65° C; in the second run, the temperature of the fresh water is increased to 80° C. For the final run, the water is heated nearly to the boiling point (95° C). During this third run, only so little sugar is extracted that this weak sugar solution is cooled down and used for the first run of the next batch. The remaining mash is brought to specialised plants where it is dehydrated and the residue is processed into animal feed. The exhaust air of these plants can be smelled for miles.
The sugar solution must be cooled down to 20°C since the yeast wouldn’t survive higher temperatures. 50kg of yeast are added to 15,000 litres of sugar solution.
The resulting liquid is called wort. It is stored for two to four days in the wash backs until fermentation is finished. During alcoholic fermentation the yeast strains convert the sugar into alcohol and carbon dioxide (CO2), an odourless and colourless gas. Beer breweries and large grain distilleries collect the CO2 for industrial use. Malt whisky distilleries are usually too small to do that, except for Tomatin, who used to collect the CO2 from their more than 20 pot stills.
The wash backs are covered with lids so no vinegar bacteria can enter and the wash back doesn’t boil over. In addition, the wash backs have a horizontally rotating blade that continually cuts the foam. The wash backs are usually made from Oregon pine or cypress wood, which is especially resistant to fungi. Recently also stainless steel has been used, since it doesn’t have to be impregnated with chemicals or cleaned so much.
Fermentation is finished after approximately 48 to 96 hours. The “beer” – the Scots call it wash – then has an alcohol content of 8 to 9% and is ready to be filled in the stills.