8 • THE KNOWLEDGE The quest for whisky enlightenment continues
FEEL THE CHEMISTRY
WORDS: RICHARD CROASDALE
To the malt whisky lover, a traditional copper still is an object of singular beauty. From its bulbous pot to the tip of its elegant neck, there are few sights so evocative of the craft of whisking making. But the shape and configuration
of a whisky still is much more than tradition; it is a delicate and complex feat of chemical engineering, whose every slant, curve and surface plays a part in shaping the character of the mature whisky.
PURITY OF SPIRIT The flavour and aroma compounds in a bottle of whisky make up only a fraction of 1% of its total volume – far less than the principal ingredients, alcohol and water. Many of these compounds originate in the cask, but a significant amount come out of the distilling process itself. Precisely which compounds end up in the new make spirit depends on whether distillation strips most of them out – creating a ‘light’ or pure spirit – or whether some are deliberately retained – producing a ‘heavy’ spirit.
MAKING CONTACT The main mechanism by which distillation removes impurities from the liquid being distilled is known as ‘reflux’. As the liquid in the still is heated, it gradually evaporates into the neck, coming into contact with the cooler inside surfaces.
Glenmorangie’s very tall, narrow
swan necks have more reflux and produce a much lighter spirit than Benrinnes’ squat, wide stills
Compounds with a low ‘volatility’ –
such as alcohol – will continue up the neck and into the lyne arm leading off the top of the still, while water and ‘heavy’ compounds will condense to liquid and drop back into the pot. Crucially, the more contact the
vapours have with the inside surfaces of the still, the greater the opportunity for heavy compounds to condense and be removed. So, Glenmorangie’s very tall, narrow swan necks have more reflux and produce a much lighter spirit than Benrinnes’ squat, wide stills.
MAGIC COPPER The material properties of the still itself also play a key role. A tremendously reactive metal, copper interacts with the liquid and vapour to remove certain compounds. Sulphur, for example, reacts with the walls of the still to produce copper sulphate, which will be left behind in the spirit still (see The Knowledge, Unfiltered April 2012).
COOL IT Temperature influences the reflux process in several ways. First, heating the still contents quickly will blast the vapour up into the lyne arm,
leaving little opportunity for contact and creating a heavier spirit. Slow heating allows the vapour to roll around gradually, providing plenty of contact and giving a light spirit. Vapour leaving the still travels
down the lyne arm and to the condenser, where it is cooled back to a liquid. Again, distillers aiming for a heavy spirit tend to use a worm tub condenser, which gives the vapour little chance to come into contact with the copper sides. Lighter spirits are instead passed through a shell and tube condenser, where the vapour is condensed slowly and flows around vertical copper tubes.
SO WHAT? This is all very well, but what does it have to do with your dram? Many of the classic characteristics of your favourite whisky are entirely down to the distiller’s decisions about which compounds to remove and which to allow through. The final piece of the puzzle is the
spirit safe, where the distiller diverts away the first and last portions of each distillation – known respectively as the ‘foreshots’ and ‘feints’ – keeping
THE SCOTCH MALT WHISKY SOCIETY
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