We’ve all had the experience: You open a bottle of that beer you have been cherishing – saving for just the right moment – and an instant after you pop the cap, your only thought is, “Why is it raining beer from my ceiling right now???”
OK. Perhaps you have been lucky enough to not have that experience. But, in a milder form, have you ever opened a bottle and had it gently spew foam over the top? You think you’ll just wait for a second and everything will be fine, but the foam just keeps on coming…. and coming… and coming….. In desperation, you pour in the gentlest fashion you know and you get a glass full of…. foam?
What the hell just happened to that beautiful bottle of double imperial, oak aged, Brettanomyces fermented, bottle conditioned piece of heaven that you waited in line for at the bottle release party?
Congratulations (in the worst possible way). You have just experienced gushing. In its severe incantation, it’s the phenomenon that turns a bottle of beer into a miniature version of Old Faithful in your kitchen – the kind of experience that leads you to discovering that you can mop your ceilings just as effectively as you can mop your floors. (Pro tip from personal experience: This works great to remove smoke residue after a kitchen fire just as well as it works to remove IPA after a gushing incident. Don’t ask, but do check your stove before leaving the house for several hours. Just sayin’.)
In its more mild incantations, gushing is a phenomenon that converts a bottle of beer to a pile of foam as you sit there, powerless, watching it happen – not even able to get a glass of the beer you have been waiting so long to try. In it’s more major form, it creates a beer bottle version of Old Faithful, which is not a national treasure or geologic wonder.
So why does it happen and what can be done about it? This is wasted beer we are talking about! This is a crisis!
Well, the good news and the bad news is that there is not a damn thing you can do about it. You didn’t mis-treat that bottle, and it’s not your fault. Sightly less comforting is the fact that your favorite brewer didn’t either. So, don’t hold it against them. Please. It was nothing they did, nor anything they could have fixed. So, who is to blame?
Barley growers. Those bastards. They caused us to lose a whole batch of beer. (Rest assured, it was not just your beer that foamed all over the place. Every bottle produced from that batch of barley did the same thing. Probably.)
Now, in all fairness, it’s not really the barley growers ‘fault’. It’s just that that is where the problem started. There’s not a ton they can do about it either, but everyone downstream from them was hosed before they even started making their product from the raw barley.
They are fungi of several species, and they (and their spores which are pictured above) produce several proteins known collectively as “Hydrophobins’. Those proteins are the direct cause of uncontrolled foaming in beers. (OK, it’s also possible you can see that from beers that are infected with various spoilage organisms like Brettanomyces, Lactobacillus, and Pediococcus. But, at the pro-brewer level, those kinds of infections are rare for any established brewery, so we won’t consider them here. At the homebrew level, those are MUCH more likely causes than the fungi I mention above. So, if a homebrewer friend of yours gave you a bottle that just exploded in your kitchen, ‘thank’ them with a bottle of sanitizer and a suggestive look.)
So, what is the link between those fungi and gushing/explosive beers? Well, it’s not the direct one you might think. For the homebrewer, infection with Brett, Lacto, or Pedio means that there is an organism in their bottles that can ferment sugars that the ‘normal’ brewing yeast cannot. Hence, those sugars produce more carbon dioxide (among other things like sour flavors) and the beer becomes grossly over carbonated. Pop the cap and, BAM! Geyser.
But, the fungi I mentioned above do something entirely different, and very cool – at least to a brewing biochemist…..
They don’t ferment sugars left over after terminal gravity is reached. They don’t over carbonate the beer once in the bottle. What they do is turn the dissolved carbon dioxide into tiny little carbonation bombs. I kid you not, these are referred to in the brewing literature as “nanobombs”. This is not a nanobomb…
Some nerdy biochemistry and physical chemistry is coming at you right now. If you get lost or bored, jump to the big summary at the end. It’s worth it, I promise!
OK. Here’s the deal, When your beer is bottled, it is ‘force carbonated’. That means that the CO2 dissolved in the beer is at a concentration higher than the level of saturation. (You might remember the concept of saturation from the somewhat controversial post on the myth of 100+ IBU beers.) What that means is that the CO2 was forced into solution under high pressure. Even if the beer was bottle conditioned, this is still true, though it is done more slowly as yeast produce the CO2 rather than the giant tank on the side of the brewery providing it.
Once you pop the cap, the pressure is released and that CO2 comes out of solution – slowly. That’s what gives you the nice head on your beer and that’s one of the reason the head persists as you enjoy that nice pint you just poured. Over the course of the time you drink, more and more CO2 bubbles out of solution and replaces all the bubbles that are popping in the nice foam pile atop your beer.
But, what would happen if all that dissolved CO2 were to come out of your beer at once? Beer-splosion!
Or, more scientifically, gushing. That is the cause of that beer fountain that just shot out of your bottle.
That, of course, leaves a major question: Why do most beers slowly release their dissolved CO2 while others let it all hang out in the blink of an eye?
Hydrophobins and nanobombs.
One key concept in chemistry is “like dissolves like”. That is, compounds interact well with other compounds that have similar chemical characteristics. This is why, for example, sugar dissolves well in water, but oil does not. Water is polar, sugar is polar, oil is nonpolar. As it turns out, carbon dioxide is a fairly nonpolar substance as well. It can dissolve in water, which is a ‘polar’ solvent, and does so reasonably well – for reasons I won’t talk about here. But, if it is given a chance to interact with something nonpolar, it much prefers to do that.
Enter the hydrophobins.
These are proteins produced by our trio of gushing producing fungi. These proteins can basically collect the CO2 out of solution, forming a nanobomb. The nanobomb is a bunch of CO2 stuck to a hydrophobin. In the picture below, the little triplets of spheres are carbon dioxide. The faint grey outline represents the surface of the hydrophobin and the twisty ribbon-like things are a way biochemists show the shape and organization of a protein. Take home message: That protein is covered with carbon dioxide molecules just waiting to explode forth from your beer. Danger, Will Robinson!!!
(This is a nanobomb.)
Gushing, Mechanism Underpinning Primary. “Combined modeling and biophysical characterisation of CO2 interaction with class II hydrophobins: new insight into the mechanism underpinning primary gushing.” J. Am. Soc. Brew. Chem70.4 (2012): 249-256.
But, this only works under high pressure. So, when the bottle is opened and the pressure drops…..
(For those who skipped the science, rejoin here.)
Those nanobobs basically release all their collected CO2 at once and it all tries to escape the bottle at the same time.
If you are a major sports figure who just won a prestigious Formula 1 race, some alcoholic geysers are OK. If you are chilling with your beer buddys and just cracked that bottle you have been lording over them, it’s not…
So, what to do about this? Here’s the most depressing part of this whole post… Nothing. There’s not a damn thing you can do about it. The infection occurred in the barley field and everyone downstream of that got hosed.
So, here’s the big take-home message. If you bought a bottle of beer from a brewer you love or a trending brewer that you have been dying to try, and if that bottle explodes upon opening, don’t immediately blame the brewer. It may it be their fault. If you taste the tablespoon of beer left over after the foam sprays everywhere and it tasted sour, thin, and insipid, then it’s probably an infection and it’s the brewer’s fault. Think twice before buying their stuff again. Or, better yet, let them know in a kindly e-mail. They might replace the bottle for you, and I’m sure they’d like to know if they have a problem with their sanitation protocols.
But, if you taste that remaining few drops and it still tastes good (if a bit flat), then it wasn’t the brewer’s fault and you should definitely give them a second shot. They had no control over that outcome and are probably just as horrified as you are about what happened.