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The recipe calculator works off the predicted attenuation of the yeast (sorry for the obvious stuff at the start). That's generally from the manufacturer and is for standard gravity (around 5% ABV) blond beers. There's many things that make that number not particularly reliable for dark beers, but the two bigger ones are that crystal and roast malts will make fairly unfermentable wort and they have materials that prevent the yeast working as well as predicted (maillard reaction materials).Thanks Nosy, so you think this is an attenuation issue? 50%ish (realty) instead of 70%ish as defaulted in the receipt creator.
just trying to learn/nail down my process.
A high amount of crystal and roast malts (up to 50%) can drop the attenuation 10 - 15%, medium amounts (around 30%) can drop it 5-10%.
There's more info that you probably want here - https://onlinelibrary.wiley.com/doi/pdf/10.1002/j.2050-0416.2005.tb00648.x
I'd have thought a 60-65% attenuation would have been good with S04, but I haven't used it for dark beers. Someone who's used it more in dark beers would have a better idea. The 50% isn't great, but it's as good, if not better, than I was doing on my early dark beers. I'm still trying to work out if it's healthier yeast, more yeast or yeast nutrient that is more important. Probably a mix of the three.