Apparent attenuation much higher than expected, why?

I recently made a sorta-kinda-clone of Raging ***** Belgian IPA. The apparent attenuation came out around 10% higher than expected, at 85%.

OG: 1.073
FG: 1.011
AG mash temp: 152F
Apparent: 85%
Actual: 69%
Yeast: Wyeast 3944 (attenuation listed as 72-76%)

I believe the attenuation listed on the spec sheet is supposed to be apparent, correct?

My total efficiency is usually right around 69%, if that factors in at all. I also use temperature control for fermentation. This batch was fermented at 65F for its first week, then at 72F.

Having my apparent attenuation come out 10% higher than the expected value has happened to me a few times, though I haven't yet had a bad batch of beer yet.

If there are no other critters fermenting, I assume this is some kind of measurement problem?

Is it possible that your thermometer could be off? A lower mash temp would really in more fermentable sugars, therefore a higher attenuation.

Also, if your hydrometer were off, it could just be a measurement error.


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My thermometer is a Thermapen, and it's been recently verified in boiling water so that probably isn't it.

I've also had the problem across 2 hydrometers--though I have to admit I have never calibrated either of them. I'll check that before next brew day. (I do use a refractometer for pre-fermentation readings, and it's calibrated.)

What's your mash process? I think that could be affecting it.

I mash at 152F for 60 minutes, always.

(Until I was researching this topic today I didn't even realize that mash temperature altered the fermentability of the sugars in the wort... pretty cool.)

Did you use any simple sugars? Since those are 100% fermentable that will raise your attenuation. In my IPA's I use at least a pound of sugar to lower the FG.


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Nope, no added sugars.

I guess I will just double-check all my instruments and try not to worry about it. The beer's fine.

Horseflesh said:

(Until I was researching this topic today I didn't even realize that mash temperature altered the fermentability of the sugars in the wort... pretty cool.)

Click to expand...

...and mash time. Also depending on the generation of yeast attenuation can be affected. I get better attenuation out of generations 3-5 of my yeast when reharvesting.

And finally, there are a couple of ways to calculate attenuation. Apparent attenuation and actual attenuation. I am pretty sure the numbers you see for the yeast potential attenuation are actual attenuation numbers (but can't remember for sure). To get your actual attenuation you need to account and make an adjustment for the alcohol in your FG hydrometer sample reading.

i've found those listed attenuation values to be quite inaccurate. what you got seems right to me, i'd say your instruments are fine

What mash thickness were you using?

DCP, glad to hear it. Like I said the beer tastes fine so maybe it isn't a problem. I am not trying to hit numbers for a competition.

I'm using BeerSmith's default values for grain to water... something like 1.25 qt/lb? I honestly can't recall at the moment.

Are you heating up your wort as soon as you run it off? Letting it sit while you wait for the entire volume to run off out of the mash tun will lower the temp, and that lower temp will create more fermentable sugars. I just recently discovered this problem with my process.


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mash thickness has no effect on attenuation

TastyAdventure said:

Are you heating up your wort as soon as you run it off? Letting it sit while you wait for the entire volume to run off out of the mash tun will lower the temp, and that lower temp will create more fermentable sugars. I just recently discovered this problem with my process.

Click to expand...

AFAIK, this is not entirely true. while dropping to a lower temp will allow beta amylase to hang around longer (given it's not denatured yet), the lower temp isn't now going to make it more active. the limited extended time should have minimal effects on the fermentability.

again, 85% is what i'd expect out of that yeast, but feel free to give this a read if anything stands out to effect your attenuation: http://braukaiser.com/wiki/index.ph...ity_and_efficiency_in_single_infusion_mashing

I do let the wort sit in a pot at ambient temperature as I finish with the mash.

I'll be honest, I realized this before I brewed my last batch. But my last batch was a Saison, so I didn't worry about the temp lowering while lautering bc it would increase fermentability (good for Saison).
But next batch I will heat as I'm running off, and hopefully that will end my very high attenuations.


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