Derbyshire single malt whisky and rum; thoughts on fermentation and distillation.

In May 2020, after over 12 months of research, we finished distilling the first batches of Derbyshire rum.  Using traditional pot distillation we’ve created a couple of styles (neither of which use added sugar) which is an exciting development on our journey.

We went back to basics and created rum from scratch, which meant we had to take what we knew about whisky-making and in particular revisit two important areas – fermentation and distillation.

In short, fermentation is the stage where the alcohol is created and distillation is the process by which the alcohol is concentrated (and certain flavour components are selected or removed).

Fermentation involves adding yeast to a vessel full of sugary wort, a liquid which is the result of mashing barley grains (in the case of single malt whisky[1]) with hot water. The yeast goes to work on the wort and converts the fermentable sugars into alcohol.  Generally speaking, the creation of alcohol starts to slow down after approx. 36 hours of fermentation.  Most whisky distilleries stop fermentation after 48 – 60 hours.  Allowing the fermentation to go on longer encourages certain bacteria to build-up which can help to develop wild, fruity flavours that will carry across to the final distilled spirit. Fermentation is one area where the new wave of craft distilleries are trying to make their mark.

For Wire Works whisky, we run a long, 96-hour fermentation which is one of the elements that contributes to our unique spirit.  When we started talking to craft rum distilleries, it became clear that 96-hours for rum was considered to be a short fermentation, with some rum producers running upwards of 2 weeks (depending on season). Fermentation has been one key area of difference between whisky-making and rum-making where we had to do lots of research and trials. We’ve settled on a regime of 10 – 14 days depending on what rum style we’re making – so it is quite different to our whisky!

There are various ways that distillation can affect the concentration of alcohol and flavours from fermentation; type of still (we use a traditional copper pot still), the shape of the still, the material the still is made from, how ‘hard’ you run the still i.e. temperature (alcohol boils at circa 78C) and probably most importantly where you make your ‘cuts’ during the final distillation (the second distillation in our case (and in most of the UK)).

The cut points are what separates out the ‘heart’ of the spirit which ends up in the cask and ultimately the bottle. The alcohol which flows first from the final distillation contains water soluble oils and higher alcohols which are called ‘foreshots’, made up of things such as acetone, acetate, acetaldehyde and higher strength alcohols. These chemicals have a fruity quality and add desirable flavour and complexity to whisky, but only in small quantities. It is the job of the distiller to determine how much of the foreshots to keep and how much to ‘cut’ out from the heart of the spirit.

Similarly, the distiller has to decide when to stop collecting the heart of the spirit, as the later flavours flowing from the still are heavier having metallic notes and solvent-like aromas, called ‘feints’. Every distillery has its own cut points, governing the amount of foreshots and feints included in the cask.

For our first rum distilling trials, we started off using our whisky cut points and soon discovered that these did not produce the desired results!  Many trials and discussions later, we have discovered the cut points that we think make a great rum that’s full of flavour.

Most of the rum we produce is finally laid down in oak casks to mature for at least 12 months.

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[1] In the case of rum, instead of barley the mash is made with a form of sugar cane juice or molasses.

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