Why does beer foam up more when you add a tea bag to it?

Answer

Dear Sarah,

Beer foams through the CO₂ which is dissolved in it. This CO₂ mainly comes from the brewing process, where yeast sugars from the wort (actually a sugar-rich liquid made with water and barley) are converted into ethanol and CO₂. The beer often contains a supersaturated amount of CO₂, causing it to spontaneously want to exit from solution. Sometimes extra gas is even added to the tap.

However, in order to form a gas bubble, a “seed” or nucleation site is needed: a pre-existing gas bubble that can grow with gas from the liquid. Those germs cannot arise on their own, because then they must be able to overcome an infinite pressure.

Beer can therefore only foam if such germs present can grow. When filling a glass, for example, impurities and irregularities in the glass cause your beer to foam, because there are many germs present. The same principle applies when you add something to it. The tea bag will bring air into the beer and those are germs to start gas bubbles.

Two great examples to illustrate this:

You will always see an ascending stream of gas bubbles in a glass of Duvel. If you look closely there is a ‘D’ (van Duvel) engraved on the bottom of the glass. These small irregularities in the glass create gas germs, where gas bubbles can grow. This way, the beer collar at Duvel is preserved.

You can see another example in the attached video (link below). This isn’t about beer, it’s about water. Naturally, water (without fizz) does not contain as much gas as in beer. When you heat it up, on the other hand, the water will want to evaporate, because the water changes to the gaseous state. When you now heat this water in a very pure and even container and with very pure water (without irregularities), there are no germs where the water can start bubbling. The water can then heat up to even > 100 °C (superheated), without boiling. When you add something, such as a lump of sugar in this video, you suddenly do introduce gas germs and the water can start boiling very spontaneously (and explosively). Don’t try this at home ;-).

I hope this answered your question.

Greetings!

Dries

Photo © Martin Garrido cc 2.0 on Flickr.