Sour times

This post is aimed mainly at people who know stuff. I did one year of Chemistry at school; we had to take one science for O Level, but I chose Physics on the grounds that my friend was doing it. Kids, eh? Then again, I chose German on the grounds that my mother had talked me out of choosing Spanish on the grounds that my friend was doing it, and that wasn’t much better as choices go. My Spanish is much better than my German these days… sorry, what was the question?

Anyway, if you – like me – know next to nothing about organic chemistry, this post probably isn’t aimed at you. If you do know about this stuff – and brewing in particular – have at it in the comments.

Question 1: Why does cask beer eventually go sour?

We know it does. It’s obviously something to do with yeast, and something to do with contact with the air; that’ll be why keg beer doesn’t go sour – even if it’s got yeast in it. (Or does ‘real’ keg eventually go sour – does contact with oxygen just mean that cask just goes sour quicker?) Yeast plus beer plus time (plus oxygen) equals… what? Something to do with sugar turning to alcohol? Oxidation? Oxidisation? (Are they not the same thing?) Help me out here.

Question 2: Why does (good) bottle-conditioned beer not go sour – or not for a very long time?

I’ve drunk five- and ten-year-old bottles of Chimay Blue; by the time you get to ten years the taste is starting to get a bit thin (no doubt from all that sugar being turned to alcohol), but it’s not sour. Chimay Blue is pretty mellow when it’s brand new, and the older it gets, the mellower it gets. Whatever it is that happens in a few weeks to beer (with yeast in it) in a cask, it doesn’t happen in a decade to beer (with yeast in it) in a bottle from the lads at Scourmont; if anything, the opposite happens. (Other people have reported similar things of old bottles of Fuller’s Vintage and Thomas Hardy ale.) What’s going on there?

Question 3: Why does bottle-conditioned beer sometimes go sour?

I have – to my regret – had bottle-conditioned beer that wasn’t meant to be sour, but was every bit as sour as a barrel end pint: a harsh, battery-acid sharpness, drowning out whatever flavour the beer was originally intended to have. Will a beer like this have gone sour for the same reason that the barrel-end pint would have done, or are there other processes which might lead bottle-conditioned beer to go sour? If it is the same process, what went wrong with those bottles that made it happen, when (per question 2) it’s the opposite of what usually happens?

Question 4: Why gushers?

An easy one to finish with: what’s going on when you open a bottle of bottle-conditioned beer and it gushes like a Formula 1-winner’s champagne? What does that, and how can brewers stop it happening?

(Also, why is the triangle inscribed in a semi-circle always a right-angled triangle? I’d love to see a proof of that one.)

Update Matthew (in comments) explains all. Acetobacter converts alcohol to vinegar (it’s nothing to do with yeast per se); there’s acetobacter in the air, so any alcoholic drink in contact with the atmosphere will eventually get vinegarised. Bottled beer, on the other hand, shouldn’t have this problem, unless… well, unless what?

Let’s promote question 3 (which, to be honest, always was the one I was most interested in). When bottled beer goes sour – as, sadly, it sometimes does – what’s (most probably) going on? A pre-existing acetobacter infection? Some other sort of infection? (I’m not sure I can tell one kind of sourness from another; I’ve seen references to lactic acid as a fault, as well as acetic.) If it is some other kind of infection, what kind? And, if it is an infection (acetobacter or otherwise) what should the brewer have been doing to stop it? Or might it be some kind of problem with the yeast strain?

All suggestions welcome!

(Yes, I do have a specific brewery in mind. Two, in fact.)