The barley is soaked in warm water for two to three days before it is spread on the floor of a malting house. Steeping the barley will help to convert starch into sugar. The barley needs to be turned regularly by hand and with small machinery to maintain the right temperature and to germinate evenly. This takes around five days depending on the time of the year. There are still some distilleries that do floor malting (although only partly); nowadays this can be done using large rotating drums, which work better for large-scale whisky production.
Once the barley has started to sprout and the starch has turned into sugar, the process needs to be stopped. This is done by drying the barley with hot air. Some distilleries add peat into the ovens to add a smoky aroma to the barley.
The malt is milled into a flour before being added into a mash tun together with hot water. This part is called mashing. During mashing the sugars dissolve and are drawn off through the bottom of the mash tun. This process is normally done three times with the water temperature increasing each time, starting from 65°C and with the final process at just below boiling point.
The liquid with the sugars is cooled down to 20°C before yeast is added. Together they create a liquid called wort. This is stored in large wash backs until the fermentation is finished, usually two to four days. The resulting liquid, known as wash, is like strong beer and is around 8% ABV.
There are actually two types of copper pot stills used in whisky production. A wash still is used for the first distillation and a spirit still for the second. The wash is added into the wash still, which is heated from below. The alcohol vaporises and goes up the still. When the steam reaches the neck, it turns back into liquid. The shape of the still will have an impact on the flavour of the new-make spirit.
The first batch of alcohol is called low wines, and it’s around 20% ABV. It will go through the spirit still, where the alcohol is split into three batches: foreshots, heart and faints. The first and last batches are redistilled with the next batch of low wines. The collected heart has 65–70% ABV. If a third distillation is used, the final spirit is purer and stronger at 75% ABV or more. A third distillation is not that common in Scotland.
The spirit is added into oak casks, which are stored in large warehouses. Two of the most common cask types are ex-bourbon barrels and sherry butts. To be able to call it scotch, the spirit must mature for a minimum of three years. The same cask normally has up to three fills over the years. The flavours of the spirit will blend with the aromas coming from the wood. Many single malts are a blend of several casks. The final whisky can be diluted with water before bottling or bottled at cask-strength.