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In the last three year I have brewed several beers, both light and dark, all-grain, and I fermented some of them with the same types of yeast.

I find that for my lighter beers, my calculations are always correct (I use Brewtarget), but that for porters and stouts I always get an attenuation that is about 10% to 15% lower than expected, even though the lesser yield of the dark and roasted malts is taken into account in the recipe.

However, I do not find any equivalent information or questions about this on the web.

Does any one ever encountered the same problem, and if so, do you then always take precautionary measures in the recipe and the brewing?

FWIW, I also brewed a beer that was a mix of a porter and a dubbel, and I did the following:

  • Mash low
  • Add yeast nutrient
  • Try to aerate well
  • Use an active starter

And it did work, but for me this is just a single data point.

EDIT: I do not have any problems regarding pH or getting my correct OG.

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    I do find that stouts and porters may slowly attenuate over (say) 6 months conditioning in the barrel, keg or bottle. I have noticed that darker grains seem to under attenuate in primary fermentation but have a lower SG after some time conditioning. Not sure if that is down to very slow but continuing yeast or bacterial metabolism with reduced nutrients or something settling out of solution.
    – barking.pete
    Dec 7, 2017 at 17:58
  • Have you tried to input the same data into different softwares or spreadsheets and see the expected output? I mean, each software maybe has its own way to do some calculations and maybe brewtarget is well tunned for light beers.
    – rondonctba
    Dec 7, 2017 at 21:22
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    @rondonctba: Your comment reminds me also that I have actually two good books to consult too, How to Brew and Designing Great Beers. The fact of the matter is that I have also developed my own model and a spreadsheet for doing my calculations, and this is a clean room implementation. I use Brewtarget to check my figures, and then adjust my hypotheses accordingly. But I have actually always skipped the calculations in the books, because I was more interested in the techniques and the beer styles.
    – chthon
    Dec 8, 2017 at 7:49

4 Answers 4

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I would say its incorrect estimates of unfermentable sugars from roasted / caramelized grains.

Consider using darker more flavorful versions but use less overall.

Don't exceed the recommended specialty / base grain ratio from the grain data sheets.

Yes. I've seen this happen a lot with beers with ratios that exceed recommended %.

For example is a recipe calls for 1lb C-60 but someone uses 3lb of C-20 the color will be correct but there will be 3x more unfermentable sugars.

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    I hesitatingly accept it as an answer, because the software I use automatically adjust the amounts of ingredients based upon the percentage needed and the yield of the grains. However, I have used indeed rather more than normal roasted malts in these recipes, except in the one which gave a correct attenuation. The reason being that I wanted to find out first hand how these tasted (not as bitter or astringent as people want you to believe).
    – chthon
    Dec 15, 2017 at 12:52
  • I also need to presume there is a flaw in my model, being based solely upon the amount of created sugar in a wort and not taking into account any other dissolved compounds which might influence the gravity of the wort. This issue needs to be studied more closely.
    – chthon
    Dec 15, 2017 at 12:55
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Do you check your pH? If it's off you may be getting low conversion which could lead to low attenuation.

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  • Yes, I always check and correct my pH. I always get my OG figures, that is not the problem.
    – chthon
    Dec 8, 2017 at 17:07
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    On the other hand, thinking further, maybe I should also measure for the coming brews, the pH of the wort before fermentation.
    – chthon
    Dec 9, 2017 at 10:29
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While I was searching for "caramel malts and highly attenuated beers", I found a link to a scientific document which contains probably the best explanations for the problem:

Impact of Dark Specialty Malts on Extract Composition and Wort Fermentation

To summarize a little bit:

  • Adding more roasted malts decreases the amount of fermentable material (well, duh)
  • But even accounting for this, a wort with more roasted/coloured malts will ferment more difficult because of
    • having less nutrients
    • more Maillard components, which seem to inhibit the yeast metabolism

Which all seem to indicate that the precautions taken in the one brew that went well are important. It would also seem to indicate to use an active, healthy and large enough yeast starter.

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Never assume that an FG prediction has anything to do with the reality of how your beer will attenuate. It's nothing more than a guess. Sometimes it's right, sometimes it isn't. Don't sweat it.

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  • Well, it does work for me for beers which do not contain roasted malts.
    – chthon
    Jan 4, 2018 at 20:24

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