Whisky is essentially distilled beer. Many of the production steps for whisky are very similar to those for beer, like malting, mashing and fermentation. Distillation is the step that turns low-strength beer into an alcoholic spirit, so it is fair to call this step the heart of whisky making. Unfortunately many books and websites explaining whisky making reduce distillation to the act of separating the alcohol from the rest of the beer.
Of course this is not incorrect, but there is a lot more happening during whisky distillation than just alcohol evaporating faster than water. When the mash is brought to boil, a lot of things are going on inside the still, and alcohol evaporation is only one of them.
It is often cited as a difference between beer and whisky making that for beer the wort is cooked – often in a copper pot – while for whisky it is not. While this certainly is true for the wort, whisky makers cook their wash after fermentation inside their copper pot stills, so the similarity between beer and whisky making is even greater. Naturally, whisky makers keep the temperature of the still below the boiling point of water so distillation can work at all while brewers bring the wort to a full boil.
Whisky distillation is often simplified as a process that concentrates the alcohol present in the wash. But during the boiling phase complex chemical reactions occur between the many components. Have you ever tried fermented wash in a whisky distillery? Have you had the chance to compare it to newmake spirit? Wash usually has a rather musty tang to it, some cabbagey, baby-vomity notes which are not entirely pleasant. Usually the disilled spirit is free of this, on the other hand newmake has distinctive fruity flavours – typically pear and raspberry – which are practically impossible to detect in the wash.
The fruit aromas are tied to esters, chemical compounds formed by alcohols and acids. Different kinds of alcohol and different acids give different esters and subsequently different fruity flavours. Yeast fermentation is not simply a conversion of glucose into ethanol with carbon dioxide as an exhaust gas. The metabolism of the yeast is enormously complex, so we end up with a mixture of different alcohols and organic acids that recombine to fruity esters during distillation because such reactions are promoted by heat.
Those unpleasant musty aromas of the wash are caused by sulphur which is naturally present in any organic material. While sulphur compounds can enhance the aromas of whisky – actually some distilleries try to maximize sulphur in their newmake – , too much is too much. And this is essentially the reason why pot stills are made from copper: Sulphur reacts with copper to copper sulfate which is practically insoluble, so it is removed from the spirit. Copper contact during distillation can be controlled by the shape of the still, the method of condensing (worm tub or column condenser) and also by the utilization rate of the still since the copper needs to oxidize in between distillation runs to effectively remove sulphur.
Sulphur removal and ester formation are the main processes going on inside a copper pot still, but there are even more reactions happenening. The wash is a complex concoction of sugars, starches, fats, alcohols, acids and proteins; so we also have caramelization to a certain extent (especially in directly fired stills) as well as the massively complex Maillard reactions between starches and proteins. and that is certainly not the end. Not all of the reaction produducts will make it into the newmake spirit, but if you have ever seen a wash still in full action you will not doubt that some of these complex substances can find their way to the spirit receiver.
And there is yet another fascinating thing about pot still distillation. Foreshot and feints are removed from the spirit because they contain substances that are unhealthy or unpleasant. But in best Scottish tradtion, instead of simply discarding them, distillers save and reuse both for the next distillation runs. Naively, you would expect foreshots and feints to accumulate with time, but no matter how often you distill, their amount stays the same. So evidently some of them will change into more desirable components that make it to the precious middle cut in the next distillation.
Because of the highly complex chemistry of such a rich mixture of substances scientists still don’t fully understand what is going on inside a copper pot still during distillation. Maybe we will never know exactly. But honestly, isn’t it a nice thing that there still is some whisky magic left?