How is Scotch Single Malt Whisky Made?

Fermentation

The wort is cooled and passed to a huge tank called a washback. These can be metal, usually stainless steel, or wood. Yeast is added and begins to feed on the sugar – this produces alcohol and carbon dioxide. Liquid yeast is commonly used, which speeds up the initial time for this process to begin.

Fermentation is the first part of the process that can have a real and influential effect on the final spirit. The yeast’s job is done after 48 hours with all sugar turned to alcohol. Many distilleries take the liquid at this point, which is now called wash, to distil. However, many more leave the wash for longer – this creates extra depth, complexity and fruity notes as the yeast cells die and fall to the bottom of the washback. The wash is 7-8% ABV and reminiscent of a Weissbier.

Distillation

In Scotland, the wash is traditionally distilled twice. Each time a spirit is distilled it becomes more pure and higher in alcohol strength. Single malt is made in batches and through copper pot stills. These have a bowl-shaped pot at the base, a neck and a lyne arm coming off the top of the neck that is connected to a condenser. Copper is great at purifying alcohol vapours.

Like with fermentation, the stills are a key point where a distiller can control and influence the flavour and character of their spirit. The shape and size of stills are a major factor in this – simply put, tall stills make light and delicate spirit while shorter stills produce heavy and oily spirit. Across Scotland there are stills of all shapes and sizes, which provides huge diversity.

First distillation takes place in the wash still. This is normally the larger of the two still types. The wash is pumped into the pot and heated. Alcohols begin to evaporate at a lower temperature than water (around 65-70°C) and vapours rise up the neck and down the lyne arm. Once they hit the condenser, which is a series of copper tubes surrounded by cold running water, the vapours return to liquid form. This is now called low wines and has an alcoholic strength of approximately 25% ABV.

Some distilleries operate with a wormtub condenser system. This is a traditional apparatus that has largely been replaced with modern tube condensers. A wormtub is a large tank on the exterior of the distillery into which the lyne arms of the stills are connected. This is filled with cold water and the lyne arms coil sharply down through the water. This resembles a worm, hence the name. Again, once the alcohol vapours hit the cold copper in the tank they condense back to a liquid.

Second distillation takes place in the spirit still. The low wines are heated in the pot and different alcohols rise at different stages. Foreshots come first – these are very high ABV, pungent and full of undesirable compounds. Next come the hearts. These are the alcohols that are collected as new make spirit and go to be matured in oak casks. They come off the still as a transparent liquid at approximately 65% ABV. Finally come the feints – these are weak, cloudy and contain undesirable compounds.

Maturation

The hearts from the second distillation are the only part that will eventually become whisky. To legally become Scotch whisky the new make spirit must mature in oak casks in Scotland for a minimum of three years. The most common types of oak used for casks are from America or Europe – American oak (Quercus Alba) and European oak (Quercus Robur).

During maturation the spirit breathes in and out of the wood, extracting natural compounds as it does. This is aided by toasting or charring the inside of the barrel prior to filling, which invigorates these compounds and caramelises the natural sugars in the wood. It also cracks the oak to give a larger surface area to present to the maturing spirit.

The cask is said to give up to 75% of the flavour to a whisky, plus all colouration. Over time the level of liquid within the barrel will drop – wood is porous and as whisky breathes in and out of the wood, some will naturally evaporate. This is called the angel’s share. In Scotland the average is about 1-5-2% per year. The alcohol level will also drop by approximately 0.5% ABV annually.

The location of a warehouse that a cask is maturing in will also have an impact on flavour and character. Factors such as temperature and humidity will be different in different places – by the coast versus inland, in a valley versus on a hill, in a town or city vs on a remote island etc. As the whisky breathes the natural surroundings will add influence. This is why some people say that coastal whiskies have a distinct salty tang.

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