Production

Throwing copper

Ian Wisniewski investigates the effect this enigmatic metal has on the finished product - whisky
By Ian Wisniewski
A mellow, gleaming colour that exemplifies ‘industrial aesthetics’ is an initial, superficial evaluation of copper. Now, let’s slip into our anoraks and take a closer look. Being highly malleable, copper is a perfectly compliant medium, however idiosyncratic the shapes and dimensions of pot stills stipulated by distilleries. As copper’s thermal conduction properties exceed every inexpensive metal, stills can be heated very quickly, while also offering the opposite quality, being able to cool rapidly. Moreover, copper is the best option for dissipating heat at a uniform temperature, preventing ‘hot spots’ from accumulating (always a concern with direct-fired stills). But such assets are only supplementary benefits when compared to copper’s essential role during distillation, and subsequently maturation, which assists malt whisky in attaining its distinct, characteristic flavour. During distillation copper absorbs sulphur compounds, converting them into other, less organoleptically active compounds (ie. less sulphur character), while also acting as a catalyst helping to manipulate the ester character. As sulphur compounds feature a distinctive line-up of notes, ranging from struck match, sulphurous, rubbery, meaty and sweaty socks, to cabbage and vegetal, they can easily dominate and ‘conceal’ other characteristics within the new make spirit. While a certain level of sulphur character can be highly desirable, depending on the house style, lowering the level of sulphur compounds allows the congeners, including esters, to show more readily. The full extent of reactions taking place within the still is unknown, though it’s clear that any action only takes place when congeners touch the copper surface, either as vapour or liquid. (There again, as vapours are hotter than liquid, each reacts slightly differently, though being able to define that difference is another matter). The common denominator is that
components such as organic acid react with copper to create copper salts (verdigris is one example) on the surface of the still neck, and the condenser or worm. Less than 50 sulphur-bearing compounds (ie. in which sulphur is only an element of the total) have currently been identified, and it remains to be seen how ongoing research affects this total. Some of these compounds have a surprising repertoire, bearing citrus and floral characteristics. As these were only identified a couple of years ago, exactly how they are affected by copper is not yet fully understood. However, it is clear that they only contribute to citrus, floral notes, which are primarily created by esters. The desirability (or not) and the level of sulphur character required obviously varies considerably between distilleries, and various approaches can promote or discourage the level of copper influence. The degree of reflux within the still seems like an initial consideration. A higher degree of reflux means more copper
interaction, and so the potential for more copper dissolving into the condensate. However, while vapours presented for a second or third time to the copper surface creates a greater separation of lighter and heavier flavour compounds, and promotes the passage of esters, the crucial question is how much of the dissolved copper actually carries over into the condenser. The answer is a minimal amount. Most copper evident in new make spirit is actually collected after the vapours have passed over the neck of the still. Consequently, most of the copper dissolved within the still returns to, and remains in, the boil pot. This also means that the level of copper in effluent is such that it requires thorough treatment before being released by the distillery, in order to fulfil eco-regulations. Presenting ‘fresh copper’ during each distillation run, by allowing the still to ‘breathe’ is another consideration – depending on the character of the new make spirit required. Allowing air into the still (once it has cooled) oxidises the copper surface, which effectively ‘refreshes’ it. If the still was not allowed to ‘refresh’ in this way, the activity of the copper surface would be different, which in turn would influence the character of a new make spirit. As ever, it’s a case of a consistent regime ensuring a consistent spirit.The most important consideration, however, is the type of condenser used. A more modern shell and tube condenser, comprising numerous copper pipes, provides a greater surface area of copper, and consequently an increased degree of copper contact compared to the more traditional worm (a coiled copper pipe of decreasing diameter, set in a worm tub with cold running water). Now, spot the difference. The copper content of new make spirit using worms is typically around a quarter of that achieved using condensers. The subsequent challenge lies in controlling the level of these sulphurous characteristics, in order to produce malt whisky with a particular character.The influence of copper within the new make spirit continues during maturation, as it reduces the level of sulphur compounds and sulphuric pungency within the cask. The level of copper similarly diminishes during this process, with copper believed to get off to a quick start in the cask, achieving significant results within the first three months. In fact, copper’s role is thought to be completed essentially within the first year of maturation. That’s why the subsequent rate of decline is minimal, and accounts for an insignificant difference in copper levels between a 10- and 21-year-old whisky (other changes to the sulphur character during maturation also occur, but these are not necessarily mediated by copper.) Having performed its essential role, a certain percentage of the ‘spent’ copper becomes attached to the charred layer within the cask, while also forming insoluble salts. Some copper salts remain in the barrel, with any remainder caught during routine filtration prior to bottling. A level of copper remaining within bottled whisky is obviously safe to consume. In fact, as a trace element some copper is essentialin our diet, as it forms a component of various enzymes, including an anti-oxidant enzyme. On the other hand, copper deficiency leads to a diseased state, whereas a high intake of copper is toxic. Among foodstuffs shellfish and liver have the highest levels of copper, while meat, bread and vegetables are also a source. As a metal, copper degrades at varying rates in different parts of a wash and spirit still. In a wash still the highest wear occurs where the vapour of the wash turns to liquid, at the junction of the lyne arm and condenser. Being the narrowest section of a still, this is subjected to the highest concentration of the hottest liquid within the smallest area. Being more volatile, the first distillation attacks copper in the neck, and beyond, most of all. This means the neck of a wash still can last around 8 to 10 years, and the condenser perhaps 7 to 8 years. Some condensers are ingeniously designed to be reversible, which can extend their life-cycle up to 10 to 11 years. This advantage does of course entail paying a price premium. The boil pot, which is far thicker, can wave good-bye to all the other sections of a wash still, remaining in service for up to 25 years. This chronology is reversed in the spirit still, with the boil pot having a life expectancy of 8 to 10 years. However, as vapours rising through the still are altogether less aggressive than the first distillate, the neck and condenser can last for around 25 to 30 years. Beyond counting the years, the most accurate assessment of a still’s condition is the equivalent of an annual ‘MOT’ conducted by a leading practitioner such as Forsyth’s (see Issue 22). Copper needs to be replaced when it reaches half the original thickness, which typically starts at 4 to 5 mm in the head and neck, 6mm for a steam heated boil pot and 12 to 16 mm for a direct fire boil pot. Traditionally, tapping surfaces with a hammer and listening to the ringing tone was the method used to judge the thickness of the copper. Meanwhile, the past decade saw the advent of ultra-sonic machines that provide an exact measurement in terms of millimetres and fractions thereof. It’s unusual to replace an entire still, with a company such as Forsyth’s generally replacing sections (of a pot or whatever) during a distillery’s silent period. Apart from financial considerations, such smaller changes also ensure that it’s less of a ‘shock’ to the equilibrium of a seasoned distillation system. Nevertheless, a new or even ‘patched-up’ still may require a week or more of distillation runs to season it. Seasoning is vital as dissolved copper levels are higher in a still that has been resting, affecting the usual character of the spirit. With consistent distillation runs the flow of vapours becomes more regular, and copper salts accumulate on the surface of the still. Meanwhile, the level of copper dissolved into the vapours/liquid stabilises at the usual level, ensuring a supply of consistent new make spirit. And then everyone’s happy.