DistillationDistillation is the process by which lighter and heavier molecules are separated by means of temperature manipulation, this is typically achieved by heating, though freeze distillation is also possible. Distillation is generally used for the purposes of purifying or concentrating the desired molecules from the undesired, simply put because heavier molecules have lower freezing and boiling temperatures than lighter molecules it’s possible to either freeze the lighter molecules and remove them or vaporise and re-condense the heavier molecules leaving the lighter ones behind. As freeze distillation is illegal in most countries, due to difficulties in separating out the now concentrated toxic properties found within the alcohol, distillation is generally understood to refer to the heated method – the English word distillation is actually taken from the Latin de-stillaire or “trickle down”.
From Beer to WhiskyFor the first few days of production, with the exception of hops, beer and whisky production is virtually identical. The grain, malted barley in the case of a single malt, is dried and ground into ‘grist’ in a malt mill before being transferred to the mash tun where hot water is added converting the starch into sugars. The resultant liquid now referred to as the ‘wort’ is combined with yeast and allowed to ferment, the process by which sugar is broken into alcohol, in a washback. As the yeast breaks down the sugars in the wash the alcohol concentration of the liquid increases until it kills off the yeast cells, this generally occurs between 7%-10% though higher limits around 17%-18% are possible. The resultant liquid now called “wash” is ready to for distillation.
Pot Still Distillation
While the design of the pot still used today varies wildly, from the “curiously small stills” used by Macallan coming in at just over 12 feet to the 26 feet and 10 inch variety used by Glenmorangie, these all operate on the same basic principle being akin to a giant kettle which is heated from beneath. In Scotland the method of heating has changed considerably over time, from coal in the 1970s to indirect steam heating with a number of distilleries moving towards biomass boilers regardless the basic principle remains virtually identical to the distillation of Alexander of Aphrodisias in 200AD.The “wash” is transferred into the pot still (shown to the left) where the alcohol which vaporises at 78ºC (or 173ºF) is separated from the water (which vaporises at 100ºC or 212ºF). The spirit vapour travels through a water cooled copper pipes or ‘worms’ and re-condensed.
Double or Triple DistillationThe first run is conducted in a larger wash still which produces a highly impure low alcohol liquid of between 10%-20% the so called low wines, this is then transferred to another smaller spirit still for further distillation to produce whisky. Irish whisky is typically distilled three times before being matured, while in Scotland the whisky is generally done only twice, though this varies from brand to brand. Bruichladdich X4 is the only quadruple distilled I’m presently aware of.
Although a still can technically be made from any number of heatproof non-porous materials such as aluminum, iron, brass, or stainless steel whisky stills are almost exclusively made of copper to reduce the naturally occurring sulfur compounds such as DMS, DMDS & DMTS which are formed during the heating of grain mashes. Although the process is not well understood contact with copper results in the formation of less odorous compounds. Further information on the role played by copper can be found on Whisky Science.