You may remember my issue with Batch 41 of Aberlour A’bunadh. Here is a little experiment I did with some of that whisky.
Since that batch showed pretty nasty sulphury aromas, I filled some of it into a sample bottle and added a few copper wires in an attempt to remove the sulphur. The wires were salvaged from a piece of standard power cable by removing the insulation.
It turned out that even after a few weeks nothing had happened. The whisky tasted as nasty as before and looked the same. I left the bottle on my shelf and forgot about it. But now, more than three months later, I noticed that a strange transformation has taken place in the sample bottle:
The copper really must have done its work here. And I have to admit that my curiosity was too big to leave it untasted. The whisky now has a straw colour and essentially tastes like a halfway decent refill bourbon cask dram.
The goo should consist of ‘sherry and wood extract’ that somehow has coagulated through the help of the copper. Well knowing that copper compounds are not exactly healthy I could not resist to at least taste a tiny drop of the suspension that remained after careful removal of the clear whisky with a big pipette. And indeed, the taste was a mixture of sweet fruit and adstringent tannins, with a very long metallic aftertaste.
Why did it take so long until something happened? The wires were blank copper, but copper works best in removing sulphur from whisky when oxidized (2 CuSO4 = 2 CuO + O2 + 2 SO2). This is the reason why distilleries who specifically go for a sulphury newmake character like Benrinnes or Mortlach try to use their stills as continuously as possible, so the copper cannot oxidize through air contact in between distillation runs. So the copper wires had to oxidise first in order to have a significant effect, and indeed the wires were black with a copper oxide layer.
And yes, the nasty sulphur aromas of the original whisky were gone. This means the process really works, but is also removes all sherryness, so it can’t be considered a real option for getting rid of sherry sulphur…