I have found that my stouts produce much more foam during fermentation than other beers do. On the Mr. Beer website it says: Plenty of foam, although messy, is not a bad thing as it indicates healthy yeast and a strong fermentation. Excess foaming is more likely to occur when using ale yeast with darker brews and higher fermentation temperatures." (Emphasis mine.)

This confirms my own experience. Question: why is it that ale yeasts fermenting dark styles are so prone to producing more foam in the fermenter?

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    I think that rule of thumb is a bit misleading, at least in terms of dark beer == higher krousen. IMO yeast strain + temperature is the real factor. However, a bit ironically, I think you have previously answered your own question! – rob Aug 19 '20 at 14:22
  • @rob I do believe the issue is related to wort viscosity, but that in turn begs the question of why stout worts tend to be more viscous. Or rather: do dark grains cause more wort viscosity and if so, why? – Frank van Wensveen Aug 20 '20 at 9:40
  • I've never noticed this. For a set temperature, I think it's 99% the yeast strain. Fermenting a hefeweizen (light German Style Ale) produces a truck-load of foam, more than anything else. – Kingsley Aug 22 '20 at 1:08
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    @FrankvanWensveen I think you're getting into a land of really interesting beer schooling, which is part science, part qualitative perception. What's the difference in dark beer and lighter styles? Most would simply say, higher unfermentable dextrin chains. As brewers, we accept that more dextrins == higher viscosity (we just say mouthfeel though). The science there itself is a bit..lacking. This blog has some decent research into (albeit outdated) research on the process, however I find the experiment at the bottom to be a bit lacking. – rob Aug 24 '20 at 13:08

I have finally managed to discuss this with someone who has forgotten more about brewing than I have ever known, and he opines that the main cause of increased foaming in darker styles had mainly to do with the melanoidins that occur in greater quantities in darker malts. Melanoidins are foam-positive.

The combination of high levels of melanoidins and bittering hops also appears to be mutually enhancing, i.e. separately they produce a certain amount and type of foam, but combined they produce more than the sum of both separately.

It's a funny ol' world, innit?

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