Bubbles in Beer

During the nearly five years that I have been preparing this column, the most difficult question I've tried to answer is one posed by Glenn Estabrook of Fairbanks: "In a carbonated beverage, why do the rising bubbles always seem to stream up from individual spots, as though emanating from distinct localities on the surface of the container?"

For more than two months, I've pondered the question and searched scientific literature for the answer. Finally, I found a partial answer in an enjoyable book Butter Side Up written by Dr. Magnus Pyke.

As Dr. Pyke notes, most of us, at one time or another, have watched bubbles rise up in beer or other carbonated beverages such as pop without noticing or thinking about the point Glenn Estabrook has raised. Dr. Pyke goes on to say that it is those who do notice and question why such things happen that are likely to make scientific discoveries.

It is true that bubbles do not appear at random within a glass of carbonated beverage. Instead, they tend to form at particular locations, usually at places on the inside of the glass. Dr. Pyke states that these locations will be found, upon close examination, to be points of irregularity on the bottom or sides of the glass. Also, if a grain of sand or other similar small foreign object is dropped into the glass, bubbles will be seen to form at the sand grain and to rise in a line above it.

So much for what happens; the next question is, why do the bubbles form at places of irregularity? This is a question Dr. Pyke does not answer, but there has to be a reason. The answer to this question has to do with the surface tension within a liquid or gas in contact with another substance, such as the grain of sand or the glass container.

Prior to the formation of carbon dioxide bubbles in a carbonated liquid, the carbon dioxide is contained throughout the liquid. It will collect together to make bubbles in the liquid wherever the pressure within the liquid is lowest. The pressure in the liquid is abnormally low near the boundaries of the liquid because of the surface tension of the liquid. Where the boundary has the most curvature--as on the sand grain or on the irregularity on the glass container--the pressure reaches its lowest. Hence it is precisely here where the bubbles are most likely to form.