Calculating ABV after adding yeast starter to a stuck fermentation

[BonJoey68]

Hey guys:

I was wondering something. The OG of this batch I'm brewing = 1.045. My targeted FG = 1.005, but the gravity from my initial fermentation got stuck at 1.014.

I decided to pitch a yeast starter, bringing the gravity up to 1.019. After a week, my gravity is sitting at 1.017. Not sure exactly how to calculate ABV at this point.

Do I just add the ABV from the initial fermentation (1.045/1.014) to the ABV after the second fermentation when I pitched the new yeast starter (1.019/1.017)?

If I want to get that ABV I'm looking for, I may need to add more yeast, but I want to make sure my calculations are correct. Thanks in advance.

 

[dmtaylor]

If your FG changed from 1.014 to 1.019 (when you added the yeast starter), then you added 5 points to the gravity. You can add the same 5 points to your OG, so now your effective OG has gone up from 1.045 to 1.050. From there, the ABV depends on whether the new yeast starter did anything to reduce the gravity further. Looks like the yeast starter didn't help much, ending at 1.017. So okay... your calculation will be:

ABV = (OG - FG) * 131

= (1.050 - 1.017) * 131

= 4.3%

Adding more of the same yeast strain will not help. Adding a different yeast strain *might* help. What yeast strain did you use? What beer style is this? Can you share the recipe?

 

[BonJoey68]

Thanks. That was the calculation I was going with & I'm glad I could at least verify my findings.

I'm brewing a Kolsch from a Northern Brewer all-grain recipe kit. Used Omega OYL-17 for initial fermentation, then used OYL-44 in the yeast starter I used the second time around. Everything I'm reading says to go for an ABV of ~4.8-5.2%.

 

[dmtaylor]

I don’t think it is done fermenting yet. You had 69% attenuation originally, when it stopped at 1.014 with just the OYL-017 yeast. This yeast will typically attenuate to at least 75%, so your 69% wasn’t too far off the mark. Then you added more sugar and OYL-044, which after refermentation ended up at 1.017 with 66% attenuation. Something is not right there… the OYL-017 yeast should still be present and able to attenuate to AT LEAST 69%, but it didn’t yet. My guess is that one or both yeasts are STILL EATING - - one week has not been enough time to complete the fermentation. Also, both strains should easily be able to hit 75-80% attenuation. This makes me wonder if you have a calibration issue, either with your hydrometer, or your mash thermometer, or both. You should check calibrations of all of these. If you mashed too hot, this would explain the relatively low attenuation compared with expected.

You will not be able to achieve 4.8% ABV with this brew unless you can drive the FG down another 4 points. I don’t see that happening here, even with more fermentation time. If you really want the ABV higher, add 250 g of plain table sugar, then wait an extra week. That will do the trick for sure.

 

[BonJoey68]

This is awesome info. Thanks again. It's just weird, because my gravity readings remained the same for a week, plus I saw no evidence of fermentation, either in my airlock or in the vessel itself. And it's funny you should mention calibration. I'm using a Tilt hydrometer and was just thinking that I should compare vs my analog hydrometer. I just want to push it a few points further. Will add sugar and see what happens.

 

[RoomWithABrew]

it's funny you should mention calibration. I'm using a Tilt hydrometer and was just thinking that I should compare vs my analog hydrometer.

There's your answer.

Get a sample and test it with either refractometer or hydrometer.

Floating electronic hydrometers are very good for trend but their readings can't be trusted once krausen, pressure and temperature effect them.

You did take a traditional reading for your OG?

Don't add the sugar until you've confirmed the true gravity.

 

[dmtaylor]

Floating electronic hydrometers are very good for trend but their readings can't be trusted once krausen, pressure and temperature effect them.

Don't add the sugar until you've confirmed the true gravity.

I agree. The Tilt is often in error by several points if it had any hop or yeast residue stuck to it. Perhaps your FG is much lower than the Tilt was indicating, then everything is just fine.

 

[BonJoey68]

So, I tested the gravity with an analog hydrometer and a refractometer:

Refractometer: 1.0207
Tilt: 1.017
Hydrometer: 1.012

Assuming my/your OG calculation is accurate (I only took OG with the Tilt), then I’m looking at 3.85, 4.33 & 4.99% ABV, respectively. The low end is not ideal, but it’s close enough for jazz. Besides, if the refractometer reads higher than the Tilt, maybe it would have read the OG higher too, so it all kind of evens out, doesn’t it?

 

[dmtaylor]

Those readings can't all be right. You should measure plain water with each one to see how far off they are from 0.0 or 1.000. Then report back those results.

Also, the refractometer needs correction when alcohol is present, via this calculator, looks like this (may or may not be correct depending on calibration of your tools):

http://seanterrill.com/2012/01/06/refractometer-calculator/

 

[BonJoey68]

Checked them with plain water and they all read the same: 1.000. Calculating ABV using the refractometer calculator and wort correction factor, the refractometer and hydrometer both give me the same result, around 5%, assuming an OG of 1.050.

Man, I really appreciate the advice. I’m a hobbyist who still has a lot to learn.