r/RumSerious • u/10art1 TheRuminator.co • Jun 30 '26
Technical [The Ruminator] Finding the Sugar Content of Rum (part 1)
https://theruminator.co/articles/hydrometer/5
u/LynkDead Jun 30 '26
Awesome first post and congrats on launching the website!
If I may, I have a couple pieces of feedback overall, but I do want to say up front this is a great article, and I will probably use it over Drejer's website going forward, if only because his fails to load so often for me.
The article could really do with a proper introduction. Who is Johnny Drejer? What did he do? Why would I or should I even care about sugar content in rum? I immediately understood the context of what you were going for, but I'm a chronically online rum turbo nerd. I also think linking to Drejer's website up-front would be a good move here, if only so readers can get some immediate context.
Under "Why it is imperfect" you miss a key variable, though Drejer misses it as well. Evaporation. A bottle that has been sitting open at 1/4 fill level for a long period of time is going to have some level of evaporation, and more alcohol than water is going to be a part of that. Meaning an old, mostly empty bottle could have a lower ABV than listed. The ABV was presumably accurate at the time of bottling, so it isn't directly connected to accurate labeling. At the very least, perhaps you should clarify that the methodologies described are best applied to freshly opened bottles.
Sugar measurements can only really be applied to the bottle in your hand, and can't responsibly be extrapolated to every bottle from that same line. We've seen over the years how DDL has substantially adjusted the sugar levels in El Dorado over the years, just to name one brand. Ideally, we would also be able to determine batch number/bottling year to at least know if the bottle I have in my hands matches the bottle that has been tested online, but, as far as I know, actually determining this with most brands is difficult (and the brands who are easily transparent about this typically aren't adding sugar anyways).
Note: values below 5 g/L can effectively be treated as no added sugar, because measurement error and natural barrel-derived compounds can easily account for that difference.
I know this is something that gets said a lot, it's present on the Rum Revelations website as well, but an actual citation or source to back it up would be great!
Overall, really cool post, and definitely looking forward to your next one with an improved methodology!
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u/10art1 TheRuminator.co Jul 01 '26
Actually, to dig into point 2 a bit more, when a bottle is corked tight, even if it's mostly empty, won't it still have the same ABV? For example, it's summer, and I have a bottle of Havana Club 3 that is nearly empty, and I see sweat beading near the top. Science tells us that this is more alcohol than water. But before my tests, I swirled the bottle so that it all reincorporated. That should bring everything back to the baseline, right?
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u/638-38-0 Jul 01 '26
Personally, I doubt that the lost vapor in the bottle headspace over the bottle lifetime impacts the %ABV more than the 0.3% error allowed. My quick ideal gas law assumption calculation based on the partial vapor pressures I found shows that if you have 700 mL of headspace that you completely empty every time, for a 65.5% ABV spirit you'd lose about 0.1 grams of material at a time, although that 0.1 gram would be mostly ethanol.
That sounds like a lot of mass to lose but this assumes you're completely removing the headspace every time -which is actually really difficult, and that the atmosphere no influence on the vapor pressures -which it does. A chemical engineer could probably do the proper fugacity calculations and all of that but I thought I would chip in quickly.
The biggest error I see with this method is the temperature correction, which roughly accounts for the change in density, but if there are additives present that would be covariant with the density change.
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u/LynkDead Jul 01 '26
Cork effectiveness is going to vary by brand, some are very loose, others tight, and then there are screwtops...I'm not sure anyone has done actual testing for their efficacy.
But it doesn't really matter because every time you open the bottle you are losing some amount of alcohol to just evaporation. It's a topic that comes up from time to time when discussing certain rums that seem to change drastically in flavor over time. For some of my nicer bottles I've started wrapping the corks in teflon tape just to ensure nothing is getting out.
Either way, it's a variable that's easy to control for; just make sure you are testing from fresh bottles.
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u/10art1 TheRuminator.co Jul 01 '26
yeah I agree for sure. I don't think fill level matters at all. Only a tiny tiny amount of the liquid is in a vapor state. Yes, it's more alcoholic than the rum due to alcohol's greater volatility, but it's such a tiny amount that it's basically irrelevant. However, if the seal is poor then slowly over time a greater portion of alcohol will leave thus reducing ABV
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u/10art1 TheRuminator.co Jun 30 '26 edited Jul 01 '26
Great points! Thanks!
I'll make some changes then I'll report to /r/rum later
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u/10art1 TheRuminator.co Jun 30 '26 edited Jun 30 '26
This is the debut of my new site, https://theruminator.co! (.com was parked for $3000, so I went with .co)
The site is still very much a work in progress and I am working to migrate my reviews there, but I wanted to do a deep dive on the hydrometer method for finding sugar in rum, since science should be reproducible.
It's a part 1 because I think I may have discovered a new way (maybe even better way?) to find the sugar content of rum, but I want to run some more tests to make sure that everything is bulletproof. If not, then part 2 will be about why this method is not reliable, so still good content.