Continuous Still Vs. Pot Still

The difference of flavour between Malt and Blend whiskies can largely be traced to three factors, the grain used, the way it’s distilled, and the way the alcohol is matured. While the influence of the later is cited as high as 70% (by the Scottish Whisky Association) or around 50% by US Bourbon makers the type of still used has historically, and every distillery places emphasis on the quality of its grain the type of still has arguably changed the global face of whisky more than either. Ironic for a country associated with malt whisky production it was among other political considerations, the early adoption of the continuous still over the pot still, which put Scotch to the forefront of whisky making at the expense of Ireland.

Distillation

Distillation 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 Whisky

For 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 Distillation

The 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.

Copper Stills

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.

Continuous or Patent Stills

The continuous or patent still, sometimes also known as the Coffey has a contentious history, while the invetion is often attributed to Aeneas Coffey who’s patent most closely resembles the continuous still of today in reality this was simply a further evolution of a basic designs already used by French, Irish, and Scottish distillers. In his book A Short History of the Art of Distillation (1970) Robert Forbes attributes the concept of the first continuous still to Edouard Adam & Jean-Baptiste Fournier, the first patent was granted to Sir Anthony Perrier the operator of Cork based Spring Lane distillery in 1922. Perrier’s design was improved upon by a Scotsman, Robert Stein in 1928 who added a still to feed the “wash” through the still while allowing for the transfer of heat from the re-condensing spirit to the remaining wash. At this point Aeneas Coffey added two additional pipes allowing for the creation of higher proof whisky without the need for receiving vessels to be repeatedly changed. His patent was granted in 1930.

Grain Whisky & The Patent Still

While the creation of spirits using grains other than malted barley, such as corn, wheat or rye, has always been possible the lightness of these spirits and the necessity of using the same batch stills meant their was little motivation to do so. The patent still changed this, as Coffey’s refinement allowed for twenty-four hours a day production of considerably higher concentrate (around 190 proof or 95% ABV) and was quickly adopted for the creation of everything from rum, gin, vodka and of course grain whisky across much of Europe and the America’s. The notable exception to its adoption was in Ireland, which in large part lead to the downfall of the Irish Whiskey industry.