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I have been letting it sit for 10 days and it is still not 40%. I would like to know how to raise alcohol content in my rum while it is still in the process of fermentation. There are bubbles rising from the bottom and I am not sure what to do.

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    Are you aware that distillation is required to achieve 40% alcohol? Most fermentations will quit anywhere from 5-15% alcohol. From there, you need to distill the ferment to concentrate the alcohol. Otherwise without distillation you are just making a sort of unhopped ale-beer. – dmtaylor Oct 30 '19 at 10:42
  • Not necessarily related - but sort of....when the rum producers in the Caribbean Islands produce rum that is a bit low in alcohol they use a product of Carriacou called Jack Iron. Which is "rum" that is 99% alcohol. They just add as much as is needed to their batches to get it up to 80 proof (40%). Not that this is an answer for you but thought some might find it interesting. I don't believe it is legal in the states but is rather inexpensive on Carriacou where it is made. (Like the most expensive part was the bottle to put it in) – K4 Nerd Oct 31 '19 at 16:12
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    @K4Nerd actually the reason that is done is to preserve flavor. If you continue stripping alcohol out of a wine, you get pure white rum or vodka which is above 90% ABV. White rum or vodka is then watered down to 40% ABV for drinking. And you all know what they taste like because that’s what you buy in the store usually. What you described is creating a low wines from a sugar mash, which is full of flavor, and adding high proof rum to bring the abv up to 40%. It’s easier than trying to force exactly 40% out of the pot still in one production run. – Escoce Nov 1 '19 at 13:58
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You will not achieve 40% using fermentation as the only technique. Yeast have a certain alcohol tolerance that is usually between 14% and 18% (wine yeast), above that percentage, yeast will stop working.

Depending of your yeast strain (check the yeast pack for this information), it should stop working around the alcohol tolerance (more or less).

To raise the alcohol content, you need to either mix another rum with yours, or use distillation. I cannot provide any information regarding distillation, there are risks if not done properly and depending of where you are from, it might not be legal at all.

Maybe next time, you can try to use the yeast that has the highest alcohol tolerance.

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    White Labs has a yeast that can ferment up to 25%: WLP099 Super High Gravity Ale Yeast. – Philippe Oct 30 '19 at 13:21
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    Under the right conditions, meaning it needs the correct temp, pH, and the right balance of nutrients. You may also need to feed the sugar in as the ferment progresses. A light sugar syrup (which is what you have when you put in 1:2 sugar:water), prevents the yeast from hydrating properly. – Escoce Nov 3 '19 at 16:46

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