I was asked, the other day, as I often am, a fairly random question: “why does the beer always overflow the first time I pour a glass”?
Well the answer is pretty simple but some of the other people crowded around the pub table got very much the wrong end of the stick. “You’re not pouring it right” was the most common suggestion, others added helpfully that you always realise your mistake and that’s why the second glass poured better. Good guess but wrong.
The answer is of course, as it is for so many things in life, dust. Basically the head in beer is formed by trapped bubbles. Bubbles that are free fly off through the top of the beer and off into the atmosphere. But all bubbles are formed around dust. The bubbles are formed around any little particles they can find. If there are a few particles then the gas remains lighter than the water and breaks for freedom. But what if there is enough dust grouped together that the gas is trapped under the surface? Well that’s the head and the more dust there is the faster the head grows. The head has more volume than the liquid and so it takes up more room in the glass, hence the overflowing.
Now I will admit that poor pouring is a factor because it means that more of the dust is touched by the beer more quickly. In fact if you are pouring your beer correctly then you’ll probably end up only touching a small section of one side of the glass. And we now know that the reason the beer doesn’t overflow the second time isn’t because your pouring improved but simply because you’ve drunk all of the dust.
The safest way to be sure your beer pours perfectly is to rinse the glass (no soap) and you won’t have any problems.
By around this time you might be thinking, “urgh I don’t like the fact that I’ve been drinking dust all of these years”. But don’t worry, unless the glasses that you’re drinking from are actually dirty this dust is no different than the dust you inhale through breathing. In fact without it you would be dead. So worrying about the dust is silly, but rinsing the glass before you pour will stop you spilling beer and surely that makes it all worthwhile.