interred spirit matures, it is interesting to speculate if such chelated copper can accelerate the formation of esters at the cask wall. Certainly some rapid maturation techniques focus on ester formation, so perhaps a soupçon of copper added to spirit in these systems would yield esters even more rapidly? So, is this bad news for the distiller with a stainless steel still? Not necessarily. There are two aspects to consider here. Firstly, not all distillations require the mediation of copper to attenuate the sensory qualities of their spirits. Redistillation of diluted neutral alcohol should not, at least in theory, require sulfide or cyanide binding. However, some distillers claim that putting a still under total reflux and insertion of copper metal into the column before the dephlegmator can improve the qualities of neutral spirit, especially for vodka production. Secondly, as alluded to above, copper can be inserted into the still, whether it is fabricated from steel or copper. Copper mesh of various mesh sizes, as well as copper netting or even rings, can
provide additional reactive surface area but also more reflux. Depending on the application, additional reflux may not be desirable, especially for applications such as gin production. It is worth bearing in mind that the main products from a distillation in terms of volume are the co-product streams. So, for an initial feed or 8% ABV and a final spirit strength of 80% ABV from the still, a theoretical aqueous co-product stream will be around 90% of what is put into the distillation process. For Scotch malt whisky, for instance, pot ale from the wash still is around 65% of the starting wash and spent lees accounted for around a quarter of the volume. There are various approaches to disposal, but the copper levels, typically 20 – 50 mg/l, cannot be ignored. So if a farmer sprays pot ale on fields to add nutrients to the soil, it is advisable not to graze sheep there as they are sensitive to copper and can readily develop copper toxicosis. The presence of copper in these coproduct streams implies corrosion of the whisky still, so that eventually the copper
thins to a point where it begins to lack physical integrity, at which point the still needs to be repaired. Interestingly it is the acidity of a whisky wash that leads directly to this corrosion. Copper gin stills, such as Tanqueray’s no. 10 still, has been in existence for over 200 years but, because of the neutrality of the still feed, has enjoyed a lot longer life. The use of copper as a material of construction for alcohol stills still has substantial merit, not least because of its largely favorable impact on spirit composition and because ethyl carbamate issues can be minimized. The insertion of additional copper into the still is feasible and employed in practice, but the additional reflux that can be observed by this additional copper may not be desirable in some cases. If in doubt, try it and see!
Paul Hughes, Ph.D. is assistant professor of food science and technology at Oregon State University in Corvallis, OR. For more info visit www.oregonstate.edu or call (541) 737-4595.
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