At the 2016 New Zealand Homebrew Conference, I gave a talk on beer foam. Here’s that material, reworked into an article.
Why would anyone give a talk (or write about) beer foam? Besides the fact that I find foam interesting, there are a couple reasons. When we are served a beer, our first impression is visual — we notice the color, clarity, and the character of the foam in the glass. Secondly, and more importantly, good foam is a partial indicator of beer quality. As we will see, a number of things need to go correctly to get a nice foam stand. So, the presence of nice foam indicates the brewer has (at a minimum) taken sufficient care with his or her process to produce that foam.
What is Foam?
One definition I read in a brewing text stated that foam is a colloidal dispersion with a discontinuous gas phase and a continuous liquid phase. I think this definition can easily be replaced with “foam is a mass of bubbles,” and not lose any critical information. I’m going to start be explaining the general principles of foams in carbonated beverages — beer, sparkling wine, sodas, etc. — then move onto the specifics of beer foam.
How is Foam Formed and Lost?
Foam rises. Foam falls. The entire “arc” of foam in a carbonated beverage is determined by a few processes. These are bubble formation, drainage, coalescence, and disproportiation. The first of these builds foam, while the other processes contribute to its collapse.
Bubble Formation and Ascent
Bubbles form when gas congregates at a nucleation point. Small imperfections in a glass or dirty surfaces become nucleation sites. Eventually, the bubbles grows large enough that it’s buoyancy is enough to break off from the seed bubble (which remains at the nucleation point). Once this happens, the bubble rises through the liquid. As it rises, it collides with dissolved gas molecules and grows larger. As it does, the gas pressure inside the bubble decreases. This is a simple consequence of the surface-to-volume ratio of bubbles (or other roughly spherical objects). Larger bubbles have less surface area per unit of volume, so the pressure the liquid exerts on the gas in the bubble is less. (This has consequences later.)
As the bubble rises, it also encounters surfactants. Surfactants are “surface active molecules,” or molecules that are attracted to gas/liquid interfaces. Generally, surfactants are large molecules that have a hydrophilic section and a hydrophobic section. In these terms, the prefix “hydro-“ means water and “-philic” and “-phobic” mean loving and hating. In other words, surfactants have a section that dissolves easily in water and a section that dissolves less easily in water. These molecules tend to stick to places where liquid and gas meet — such as the surface of bubbles — because the hydrophobic end sticks into the gas and the hydrophilic end remains dissolved in the liquid.
When the bubble reaches the surface, it’s top surface is exposed to the atmosphere. If the surfactants on the bubble give it enough surface tension, the bubble remains (at least fairly) stable. If another bubble pushes up the first bubble from below, foam is formed.
So, bubbles form, rise, and get coated with surfactants. The greater the surface tension on the bubbles, the larger and more stable the foam is. In the next installment of this article, I’ll cover the processes that cause foam to collapse before getting on to the important foam positive and foam negative elements in beer.