Cornish rum distillery

Fermenting & Distilling Rum


Naturally occurring wild yeasts or secret cultured strains? Open or closed fermentation? 24 hours or three week ferments? Dunder and muck, or keep it clean? 

The great thing about rum is that there are so many varieties and options to make your own special spirit including Honey Rum.

Most distilleries choose to use sealed pressurised vessels for fermentation and blocks of baking yeast to start fermentations.

We wanted to create a more wild rum that was influenced by local wild yeast strains and bacterias. Getting this right required a lot of studying and research, but luckily enough, brighter minds than mine had done this. A lot of these old papers had slid into obscurity until recently when they had been translated from Spanish to English.

Our 1600 litre fermenter is a re purposed open top dairy tank that sits open for any wild yeast strains that fancy a nibble on our rum washes.

Fermenting and distilling use vast amounts of water and to mitigate this we try and recycle both heat and water whenever possible. One example of this is with the need for hot water to mix our molasses with to create our rum wash.

We also produce gin and when running the still you require cooling water continuously. The cooling water is pumped through the still and back into the fermenting tank on a loop, and eventually by the end of the gin distillation run we have a fermenter filled with warm water ready for the food (molasses) and the yeast to create some delicious rum wash for us!

Once all the molasses is mixed into the water and ph and sugar tests are complete, we add our first yeast. A Norwegian farm house yeast that is something of a monster when it comes to fermenting. It gives fruity flavours and adds so so much to our rum.

If we chose just to use a simple bread yeast, we could make alcohol but the flavours wouldn't be there. We would have to take a very narrow cut when running the still to make sure the rum did not pick up any untoward flavours and this would make a very neutral spirit, lacking in all the fun flavours you can get with playing with different yeast strains and bacterias

Another way to add extra flavours in rum is to add something called dunder (think of it like a bacterial soup) filled with wild yeasts and bacterias. Doesn't sound too tasty, but it would be similar to kombucha, kimchi, cheese making or sour beers. This added liquid competes with the original yeast for eating the sugars in the molasses. 

With the tank being open, the rum wash also picks up natural wild yeasts that are in the air and on the breeze, throughout the fermentation period.

After three to four days our rum wash is ready. The yeasts and bacteria have eaten all the sugars and the alcohol content is at about 9% ABV  and it is time to pump the wash to our still.

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