Typically, the greater the influence of copper the more elegant the resulting spirit, while less influence promotes a fuller-bodied spirit. During distillation the charge (ie. alcoholic liquid) is heated and begins to vapourise. As vapours ascend the neck of the still the temperature becomes relatively cooler, causing vapours to condense when they meet the copper surface. Meanwhile, heat from these vapours also transfers to the copper surface, increasing the temperature of the copper. This results in the liquid re-boiling and vaporising, in a virtually instantaneous process. Vapours then rise from the copper surface and continue ascending the neck, where the temperature is (once again) relatively cooler, and the process of condensing, re-boiling and vapourising repeats. This process may occur numerous times before the vapours reach the top of the neck, where a pipe conducts them to the condenser (to condense into liquid). Each time vapours are in contact with the copper surface various reactions occur, which influence the character of the resulting spirit. However, the full extent of these reactions is still being researched.
"A popular view is that copper acts as a catalyst to produce esters (fruity notes). I think the copper surface provides a site where various compounds meet, including acids present in the liquid being distilled, and reactions between the copper and various compounds lead to the formation of esters," says Dr Bill Lumsden, Director of Distilling, Whisky Creation & Whisky Stocks, Glenmorangie Company. Copper also has the ability to absorb pungent and assertive sulphur compounds from the vapours. "We think the surface of the copper has a 'matrix' which holds the sulphur compounds, though the exact science is not known. This also means there's a finite amount of sulphur compounds that can be held by the copper surface, and when reaching this limit we have to disengage the sulphur compounds from the copper surface to expose fresh copper. All we do is open the door of the still to allow air in, with 15-20 minutes usually sufficient, perhaps once a week. This results in the attached sulphur compounds flaking with the copper surface and falling to the base of the still, from where they can be discharged with the residue liquid remaining in the still after a distillation run," says Douglas Murray, Diageo's process technology manager.
Sulphur compounds include rubbery, meaty and vegetal notes which are only present in tiny quantities (measured in parts per billion), but they are very assertive and 'mask' various other characteristics. Consequently, lowering the level of sulphur compounds 'reveals' lighter notes such as esters (fruityness), as well as sweetness and richer characteristics including cereal notes. "Lowering the level of sulphur compounds is the biggest influence of copper contact, and one way of controlling the degree of copper contact is the rate of distillation," says Kirsteen Campbell, master blender for the Edrington Group.
Heating the still more gently gives a slower rate of distillation, which means a relatively cooler temperature in the still and lower density of vapours ascending the neck. This optimises the process of vapours condensing and re-boiling on the copper surface, and so promotes greater copper influence. Applying greater heat to the stills and raising the temperature increases the rate of distillation and density of vapours ascending the neck, which also means some vapours reach the condenser without any copper contact. Another key factor is the size and shape of the stills, which varies enormously among distilleries. For example, a longer, wider neck provides the vapours with a greater surface area of copper to interact with than a shorter, narrower neck. "The level of copper contact is a fundamental factor in the character of the new make spirit, and consequently the differentiating factor between malt whisky from one distillery and another," says Stuart Watts, Dufftown site leader, Wm Grant & Sons.
The question of copper contact also applies to the condenser. The standard choice is a shell and tube condenser. This is a 'chamber' containing numerous copper pipes through which cold water is conducted. Vapours from the still condense on these pipes, forming a 'film' of liquid all around each pipe, which descends along the pipes and drains from the base (copper has the same influence on liquid as on vapours, just less intense).
The other option is a worm, a coiled copper pipe of decreasing diameter set in a worm tub with cold running water. The vapours, which condense within the pipe, are initially in contact with the entire surface area, but after condensing the liquid forms a small stream along part of the pipe, so only some liquid is in contact with copper. Consequently, shell and tube condensers promote more copper contact and a greater reduction of sulphur compounds, resulting in a more elegant spirit, compared to worms which promote a fuller-bodied spirit.