“Where do the bubbles in beer come from Rob?” An almost child-like question in it simplicity, but one that can produce a rather in-depth answer. One which I decided not to explain at the time. Mostly because I was unsure of most of the facts at the time, and partly because of the seemingly high chance that the questioneer would be too ‘lubricated’ by said substance to fully appreciate the information I’m sharing with you here.

So, where to begin. The bubbles in beer, and in fact all ‘bubbly’ drinks is carbon dioxide. Thus the name ‘carbonated-drink’. Where does this CO2 come from?
Well in the case of beer it comes from yeast. When yeast grow they consume complex sugars. Their primary method of doing this involves the use of oxygen (no need to go into this further I feel, if you’re interested check the links at the bottom of the page). When the oxygen runs out, the yeast uses a different method to extract energy from sugar, the by-product of which is alcohol and carbon dioxide. Coincidently the carbon dioxide given off by yeast is what makes bread rise. So, being in a sealed environment, the CO2 given off by the yeast is held in the beer as a solution.

Why does this CO2 get released in the glass or after the bottle top has been popped?
Well this is all to do with pressure. The decrease in pressure allows the gas to essentially boil away (bubbles!). The last point to cover is the ‘dots’ on the glass where the bubbles seem to magically spring from. These are called nucleation sites, these have in fact been already touched on in this blog when I was talking about super-heated water in the microwave. Nucleation sites are usually defects in the glass which cause low pressure due to surface tension. Essentially (as far as I can make out) the sites are ‘mountainous’ enough to cause the beer problems in properly filling in all the gaps.
This reduced pressure enables the CO2 to congregate (it is evenly distributed throughout the liquid) until it has enough mass to survive without being re-absorbed in the liquid. The it releases (being less dense than the liquid) and rises. This bubble does in itself act as a nucleation site during it’s rise, which is why it grows during the assent.

Thatโ€™s why beer has bubbles ๐Ÿ™‚ I look forward to the next question ๐Ÿ˜‰

Alaska Science Forum
The straight dope
Stanford Report