Icing on the Cake, Pt. 1

Welcome, Children of the Beer,

We usually discuss what’s in the glass, or how that stuff got there, but there’s a character we’ve been omitting in this story somewhat: the Haberdasher.

Indeed, when was the last time you saw a freshly poured beer without a hat? Hopefully just about never, but that humblest of wool caps, the foam or “head” atop our beers, deserves respect and contemplation.

So this week we’ll be taking a look at how that foam got there and how it stays there, the various cultural thoughts about the ideal texture and dimension of this Floofy Halo, and some delightfully extreme examples out in the Beer Wilds.

Did I say this week? Well, it turns out that I like writing about the science of bubbles, so this is actually part one of two, namely the technical half - if you’re into this, you’ll love part two, and if you hate it, great news! Part two is all about culture, and probably a somewhat easier read.  But give this one a shot!

How the bubbles get there

It doesn’t take a physicist to suss out the obvious truth that foam is just a big ol’ mass of bubbles. That hasn’t stopped numerous physicists, biochemists, etc, from trying to suss out what the formation of bubbles in beer looks like, what factors affect these bubbles and the foams they create, and how we might brew beers that have better foam characteristics.

Bubbles, it turns out, are fairly interesting! This article goes through a play-by-play of how bubbles are formed in beer, and it’s pretty wild. The short version: microbubbles form on tiny imperfections on the inner surfaces of the beer glass (where, you know, the beer is), and eventually merge together into a bubble large enough to rise through the beer, siphoning off a bit more CO2 from the surrounding liquid as it goes.

(A note: it may have been prudent to start with what a bubble is, and perhaps what boiling is, since that’s a highly related concept, but that strikes me as far too technical for the tone of this blog, so we’ll just take it as granted that a bubble is more or less a sphere of CO2 inside of liquid beer, which is a liquid in which we’ve dissolved some gaseous CO2 according to this law)

So that’s how the bubbles form and evolve; once they’re in that foam layer on top, why do they stay there, as opposed to, say, Coke’s foam, which dissipates radiply? And what’s the deal with Nitrogen?

Fighting off Entropy with Science

Foam’s stability turns out to be fiendishly complicated and only pretty well understood, so I’ll resort to listing out some factors that affect stability, in effect painting the picture by describing how you might make the brushstrokes and choose the colors.

Good things

• Protein

• Since proteins are, take my word on this, important for the stability of beer foam, picking malts that are high in protein in general is one way to boost foam

• Homebrewers will often add wheat to a recipe to boost head retention, since wheat is famously high in protein vis a vis brewing grains, though some foam-promoting malts may be...not really all that helpful!

• Yeast Health

  • You generally want to keep yeast happy when brewing, but, shocker, failing to do so may hurt head retention as noted by that “take my word on this” link above, presumably since their metabolic pathways get more complicated if they’re less happy/they clean up after themselves less completely. Even that part of this story is several books, so...moving on!

• Clean Glassware

  • Glassware cleanliness is a generally good thing, but fun fact: beer foam hates lipids of any sort, so dirty glasses tend to zap beer foam with a vengeance

  • Try adding a single drop of olive oil to a glass of beer if you’d like to give this a shot - I haven’t tried it, but I think we both know what would happen

• And some other stuff

  • High-ABV beers evidently burn off their foams faster, the use of Nitrogen in the blend of gas that you use to push kegged beer through draft lines can extend the lifetime of foam, the degree of carbonation is obviously a major factor, and, as we’ll see shortly, pouring technique is massive

• Hops, right?

  • There’s one last anecdotal detail I’ll throw in: I was in the audience of a little Sixpoint sales pitch some years back, and the rep had an interesting story, in which, since it’s apparently somewhat common knowledge that really hoppy beers have generally beautiful lacing, the brewers at Sixpoint were sampling an early version of their quite excellent Bengali IPA, when, halfway through a glass, one of the brewers exclaimed something to the effect of, “boy howdy, this IPA has stripes like a Bengali Tiger!” Hence the name. Whacky stuff.

Speaking of Nitrogen

You may have had your fingers crossed that the Nitrogen line was a throwaway, but let’s talk about it a bit more, since Beer Gas is such a big deal in the industry.

The atmosphere we live in is, say, ~70% Nitrogen and ~0% CO2 (in relative terms), so might the gas with which we carbonate beer affect the way in which bubbles burst over time? Absolutely - on p172 of George Fix’s excellent Principles of Brewing Science, he explains (pictorially - the explanation, based on the law of partial pressures, comes in the preceding twenty pages or so) that a beer bubble consisting of pure CO2, like that of a naturally carbonated beer, varies greatly in content from the atmosphere, and will hence experience significantly more pressure (read: bubble-destroying force) than a bubble consisting of beer gas which, at roughly 70% N2 and 30% CO2, say, much more closely resembles the atmosphere.

The basic takeaway there being that bubbles, being somewhat unstable, are doomed to return to liquid beer and the gaseous atmosphere, and by minimizing the difference between the inside of the bubble and the outside, we buy more time by giving nature less of a reason to interfere. And that may seem like voodoo, but it’s actually a reasonable description of the Entropic nature of Thermodynamics (I’m pretty sure this paper explores that connection - don’t @ me).

Note to self: where does chaos theory intersect with thermodynamics? That sounds interesting.

Oh and why are Nitrogenated beer bubbles so small? In a word: surface tension. The longer explanation has to do with an interesting fact: Nitrogen basically doesn’t dissolve at all in beer (related to intermolecular forces, hard pass), which is why you have to really pressurize the heck of out Nitrogenated beers, and why, under the foam, the beer isn’t super textured, etc.

I think, then, that the cascade for which Nitro beers are so famous is the migration of those tiny bubbles (why are they slow? Stokes’ Law!) into a tight head, which happens a single time since there’s no Nitrogen dissolved in the beer, merely colloidally (?) dispersed in the beer when it’s in the keg and lines. Like beer mayo. Maybe? I think?

Conclusion

I feel like I’ve struck a reasonable balance between on-topic and reckless meandering, and I hope that as a result, some of these points have stuck, and that you could at least bluff your way through a conversation that, unfortunately for all involved, centers around the inception and maintenance of beer foam. 

And if you started skimming a few paragraphs in, well, next week we’ll talk about my favorite non-MacLeod brewery, Bierstadt, and this absolutely insane thing called “milk pours”

Cheers,
Adrian “Look, I Might as Well Use This Degree Once in a While” Febre