Whisky distillation, a craft refined through centuries, is an art form rooted deeply in science and precision. It’s easy to get lost in the romance of it all. In how a small detail might ripple into a significant flavour or remain a minor anecdote…
There’s so much to wrap your head around and at the heart of the action are the stills.
Before you get into the intricacies of reflux, lyne arms and worm tubs – let’s just say this. It doesn’t have to be too complicated. In the cold light of day, it’s possible to suggest that pot stills are not much more than glorified copper kettles…
Ok, maybe that’s taking it too far. A kettle is too artless and meek a comparison given their shapes, their sometimes stories-high size in and their significance in the whisky process.
Nevertheless, the beauty of a pot still lies in its simplicity and despite the following article detailing the intricacies of how they work in the context of whisky – it’s worth remembering that.
Whisky Distillation – recap of the basics
Distillation is the process of separating compounds based on differences in volatility (a compound’s boiling point). Water boils at 100°C and pure ethanol has a boiling point of 78.4°C.
Heat up a one-to-one mix of the two liquids and the vapour (the resulting steam) will be a mixture of both, but with a higher concentration of ethanol than the original liquid mixture. This is because ethanol is more “volatile” than water.
As the heady mix makes its way up the still and comes into contact with the copper higher up (or a condenser / a plate etc) the vapours are turned back into a liquid again and fall back into the mix in the pot. This is called reflux.
Through the act of slowly raising the temperatures, and this change of state from liquid to vapour to liquid and back again – an ever-higher concentration of ethanol rises.
Most whisky that is pot distilled requires two distillation passes to produce a decent new-make spirit.
The first distillation takes place in the ‘wash’ still, taking an 8% ABV wort to a 25% ABV “low wine”. The second pass concentrates the low wine to something closer to 70-80% ABV. The second pass is also where the distiller will “cut” the spirit (more on this below).
Wash Still
Wash stills tend to be the biggest stills in a whisky distillery. There is always a link between the capacity of the wash still and the fermentation tank to ensure efficiency. They are often big too! Many Scotch Distilleries typically work with volumes ranging from 15,000 to 30,000 litres.
As described above, post distillation, you obtain ‘low wines’ with an alcohol content of 20-25% . But you also have a residual ‘pot ale’ – liquid that never distilled over. This is a rich source of proteins and minerals and is often used for animal feed.
You can find out more about by-products in our distiller’s guide to Mastering the craft: Pot still distillation in spirit production
Spirit Still
Once made, the low wines are then transferred to the spirit still for a second distillation. It’s a process executed at a slower pace to facilitate finer separation of alcohol from other components.
The distiller must not only be sure of when to start and stop distillation, because different flavour compounds boil at different levels, they need to separate the distillate flowing off the sill too. This is called “making the cut”.
Cuts are where the distiller makes a choice as to which part of the spirit to keep and which to recycle. (Typically, recycled liquid it’s sent back into the next batch going into the wash still)
The first part of the distillate that comes flows from a whisky still is often high in solvent aromas, esters, and aldehydes. It’s not particularly pleasant to smell, nor to taste and this is directed at the Heads (also known as foreshoots).
After these compounds dissipate, the distiller switches the flow to their “Hearts” cut. This is the portion of the distillate they want to keep and that will be matured in casks.
Once the hearts have been running for a while, flavours and aromas turn bitter and less pleasant. This signals to the distiller that there is a need to make another cut to their “tails” (which is often recycled in the next distillation run).
The ABV of the hearts cut collected from the second distillation is around 70-80% ABV.
Pot still designs influence flavour
It’s easy to become obsessed by still designs. What impact does each kink might have on the flavour of the spirit being distilled? What matters and what’s erroneous?
The height and shape of the neck are one of the most obvious areas where the shape influences the flavour.
Taller necks (e.g. Glenmorangie’s famous stills) require the spirit to travel high and therefore causes a lot of reflux (the act of vapour turning back into liquid and falling back down towards the pot). This means that the spirit is likely to be far lighter in aroma and flavour – making it more subtle overall. Conversely, small, squat stills might favour heavier compounds to pass over more readily.
Similarly, the lyne arm (the section of the still between the top of the neck and the condensing column) can either be angled upwards or downwards towards the condenser. If pointing up, more internal reflux to occur while a downward-pointing lyne typically makes for a heavier-flavoured spirit.
Even on the way down as condensed liquid, there are minor details that can make a small difference. Modern shell and tube condensers provide different levels of copper contact compared to the old “worm tubs”. It’s possible that the former therefore aids fruiter lighter spirit and the latter a heavier, oilier one.
Still design and shape clearly have an influence, put flavour needs to be considered holistically
It is very rare for one thing to be the fundamental single point of a distillery’s signature flavour.
As we have discussed in our milling, mashing and fermentation articles – flavour is generated long before liquid enters a still. It’s a combination of many different factors.
That said, between the shape and set up of the two stills, as well as the choices a distiller makes for their hearts cut – the distillation stage will have a huge influence on the spirit’s profile.
Arguably, it also provides the easiest point for a distillery to optimise their infrastructure and their apparatus around the style of whisky they want to make too.
We hope you’ve enjoyed this beginners guide to whisky distillation!
Are you curious to find out more about pot still whisky distillation? Why not continue reading with our more advanced article for distillers and new distillery founders here.
