I did my first homebrew a fortnight ago, an american IPA BIAB-style on the stovetop. I ended up with a low OG (~1.045 - target was 1.075) and now after two weeks of fermentation a high FG too (~1.026). I have tried readded some dry yeast but a day later the gravity has not changed.

After a little reading it looks like one of two things went wrong: I forgot to shake the carboy to aerate it & maybe I mashed at the wrong temperature - I heated to strike temp OK but then just left the pot (covered) for an hour without any insulation & without checking the temp. Apparently if you mash too low then you can convert the wrong kind of sugar for the yeast, which is ringing true here.

Is there a quick way to check if the beer is full of unfermentable sugars?

As an aside, would you even bother to bottle such a low-ABV high-sugar beer if that is indeed what has happened? It smells like beer mixed with syrup..

  • How are you measuring specific gravity? If using a refractometer, this can cause some issues. Have you calibrated your gravity instrument in plain water? Have you calibrated your mash thermometer in both boiling water and ice water? – dmtaylor Aug 24 '20 at 13:50
  • @dmtaylor yes I am using a refractometer and I am calibrating it with room-temp water before taking readings. I haven't calibrated my thermometer but I left it in during the boil and it sat at 100C whilst boiling so it is accurate enough at that end I should think..? – thisextendsthat Aug 24 '20 at 13:55

Unadjusted readings from refractometers are accurate only with no alcohol present in the sample. So taking a reading on fermented beer will not give you an accurate specific gravity.

Also, as an aside, mashing cooler would cause higher conversion rate (not lower). As long as you had it hot enough (but not too hot to cause the denaturing), then the conversion probably is fine.

So the problem is that you've used a refrectometer and measured a sample that contains alcohol without running the adjustment math. I would suggest that your specific gravity is probably 1.014. This is based on the presumption that your OG was 11.2 brix and the reading after fermentation is 6.6 brix.

brix calculator on northern brewer site


If it really does taste "sweet", then it could be that you mashed too hot, and the yeast are unable to consume the types of sugar produced at those higher temperatures. Alternatively, it could be that the yeast have stalled and there really are fermentables still present. An experienced brewer can easily tell the difference between flavor of unattenuated sugars, and the flavor caused by a higher mash temperature. But the main thing is if you personally like to drink the beer. If so, just sit back and enjoy!

For more on correct hydrometer usage: Can a refractometer be used for testing final gravity?

  • This answer is exactly where I was going with my comments above. Dale just beat me to it! Personally I use the Brewer's Friend calculator which gives the same result, 1.014: brewersfriend.com/refractometer-calculator Cheers! – dmtaylor Aug 24 '20 at 18:51
  • Interesting! I had no idea that refrectometers worked like that. I bottled the beer just now and a hydrometer reading agrees with you both - dead on 1.014. Appreciate the help, I was about to bin the whole batch and now it's sat conditioning instead :) – thisextendsthat Aug 24 '20 at 19:45

This sounds like a simple case of mis-interpreting the refractometer. But the typical way to check for lowest possible final gravity (i.e.: remaining unfermentables) is to do a "Forced Fermentation Test".

Essentially this is taking a small sample of the wort and giving it an abundance of yeast and warmth to see just how low the yeast will take the sample. This typically ferments out quicker than the larger batch of beer (with a normal amount of yeast), so providing an accurate measure of what the limit of the final gravity could be.

Further reading (including methodology): https://grainfather.com/forced-fermentation-test-what-is-it-and-why-do-it/

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