This is the easy part of the production process, but also the most crucial. Distillation is a method of separating mixtures based on their boiling point and is used extensively in the drinks industry to make spirits. Let us consider the situation: the distillery has gone through the process described above and have created alcohol through fermentation, and now have a few (hundred) thousand litres of wash at about 7% ABV. In order to make whisky the ABV has to be around 70%, which means that the alcohol has to be separated from the water in the wash.
Water boils at 100 degrees and alcohol boils at 78 degrees, so it is a simple problem to solve using a copper pot still. A still is basically a big kettle. The mixture (in this case the wash) is heated up to above 78 degrees (alcohol’s boiling point), but below 100 degrees (water’s boiling point). The resulting vapour contains more alcohol that the wash and when condensed, through cooling, the new liquid will have a higher percentage of alcohol. Various alcohols boil at different temperatures, and this makes it very easy to capture only the clean ethanol in the middle, or the heart of the run. Very simply, methanol boils off first, hence the heads or fores, and then comes the heart and finally the tails or the fusel oils boil off.
There are two types of stills in use today, the copper pot still and the column or refracting still. If you’ve been paying attention you will know that single malt is made with a copper pot still. Copper removes sulfur-based compounds from the alcohol that would otherwise make it unpleasant to drink. Pot stills generally deliver spirit between 60 and 80% ABV, the rest being made up of water and various flavourful organic compounds, which is why this system is preferred for single malts, cognac and top quality rum and tequila. Column stills are far more efficient systems, designed in the industrial age to refine petroleum amongst other things. These take wash from 7% to above 90% in one go and are used for all clear spirits like vodka, gin, white rum and the non-malt components of blended whisky.
The wash is distilled in two separate batches (double distillation) in order to reach the required strength of around 70%. Two stills are used, a “wash still” and a “spirit still”. During the wash-run the wash still converts the wash into “low-wines” at around 20-25% ABV. This is followed by the spirit-run, where the “low-wine” is then converted into new-make spirit at around 65-70% by the spirit still.
The shape of the still can have an effect on the spirit in a variety of ways. Generally short squat stills make big, oily whiskies, while tall stills make lighter smoother, more delicate, spirits because of greater reflux. Reflux occurs when alcohol vapours condense before making it out of the still. They condense and fall back into the still to be redistilled.
Diagram x shows a typical wash still. The wash is added to the still and heated to a temperature above 78 degrees (alcohol’s boiling point). The resulting vapours are higher in alcohol than the wash and travel up the swan neck, along the lyne arm and into the condenser where the vapour is cooled and converted to liquid, called low-wine, at an average ABV of 20-25%.
The spirit run works on the exact same principle, low-wines are pumped into the spirit still and the process is repeated.
However, the spirit comes out in three parts: the heads, the heart and the tails. The heads have a high methanol content and are dangerous to human consumption (you have heard warnings about cheap booze making you blind… well this is why) and the tails have a high concentration of fusel (German word for “bad liquor”) alcohols, which is a mixture of several alcohols produced as a by-product of fermentation (mainly amyl alcohol) and taste horrid.
What the distiller wants is the heart of the run. This contains the sweet, clean ethanol, the type that doesn’t give you a headache and makes the best whisky. Every distillery has their own cut, some leave it a bit long (tails) to capture some extra flavours from the fusels, but this can have its negatives…