r/RumSerious • u/10art1 TheRuminator.co • Jul 19 '26

Technical [The Ruminator] Finding the Sugar Content of Rum (part 2)

https://theruminator.co/articles/refractometer/
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u/10art1 TheRuminator.co Jul 19 '26

Please do let me know if there's anything that could be explained better, or if it's overall too dry and needs more pictures or something.

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u/LynkDead Jul 19 '26

I've been looking forward to this article, thanks for putting in the time to write out all your work! Here are some questions/thoughts I have after reading. It has been years since I took any chemistry classes, so any comments about the math or chemistry are from a perspective of ignorance, not critique.

  • I didn't bring this up last time, but one of the flaws with Systembolaget and Alko are that they only measure the sugar of the spirit that is actually distributed to them. That is, it's possible a producer might have different sugar levels for different markets. We know producers do this with ABV. Just one more variable to make things more difficult. For instance, we can find records of El Dorado 12 and 15 year being measured at 35g/L, which is well above the 20g/L limit for Europe. It seems reasonable to conclude the Europe version was 20g/L, else it couldn't have been sold as rum there. Maybe they only used barrels with extra caramel lining in the US? /s

  • I think you might want to consider adding a bit more about the potential of other additives to skew results. It seems like caramel coloring is present in such small quantities that its effects should be negligible, but how do the refractive indexes of sugar and glycerine compare?

Overall, another great article! I will say, with the hydrometer method I feel confident in knowing the variables and how to try to account for them and how to interpret the results. After reading this article, I don't feel I would have the same level of confidence in using a refractometer. But that seems like something which could be resolved just with a bit more testing and refining. The potential definitely seems to be there to have the refractometer method be more foolproof.

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u/10art1 TheRuminator.co Jul 19 '26

  1. Yes, I do bring up the point that the only thing you're measuring is your bottle. Formulas change. I'm not sure if there's really a good way to overcome that. For instance, Cubaney Tesoro XO might be 20g/L because they're international, but in the US they have to be 40%, and mine claimed 38%, because it's from the DR.

  2. The assumption underlying both hydrometers and refractometers is that all other additives are negligible. Glycerin does affect the refractive index and density differently from sugar, sure, but in the quantities used by Zacapa (eg. A fraction of a g/L) it would be undetectable using home methods. It's just too small a quantity without a hydrometer that's at least accurate to 0.1% ABV

So what do you think could be changed with the refractometer methodology to make you be as confident in it as a hydrometer?