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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
    Commented Aug 19, 2020 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
    Commented Aug 20, 2020 at 9:40
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    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
    Commented Aug 22, 2020 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 This blog has some decent research into (albeit outdated) research on the process, however I find the experiment at the bottom to be a bit lacking.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
    Commented Aug 24, 2020 at 13:08

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There are multiple reasons why you will have different levels of krausen in different beers. As @Frank van Wensveen pointed out, melanoidins are one source. These are produced by Maillard reactions during the malt kilning process (i.e. making dark malts) as well as during any similar heating process, such as a decoction mash (where a portion of the mash is removed and boiled in a step-mash process).

Other sources of high krausen include: using ale yeasts instead of lager yeasts (clearly a bottom fermenting yeast will produce less krausen than a top fermenting yeast), hop oils, proteins, vigorous yeast, and wort density, to name a few.

The mere fact that a beer has dark malt is not determinative of krausen level. By way of example, measure the krausen height of a dark ale you brew. Then make a simple blonde ale using K-97 kolsch yeast (a yeast with a notoriously thick, persistent krausen) and measure which is taller. Alternatively, you can view pictures of K-97 krausen for comparison here (or on any similar forum via a simple internet search): https://www.homebrewtalk.com/threads/safale-k-97-questions.586790/

At the end of the day, the amount of krausen is due to many factors, not simply malt lovibond.

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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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