Lesson 2 - Beer Foam
In this lesson we will discuss the physics and mechanics of foam formation and collapse, along with the chemistry of the compounds involved with beer foam.
Foam is affected by the brewing of raw materials and by wort and beer processing within and outside the brewery. The measuring of foam will be discussed and we will end the lesson by looking at gushing.
Preface
The presentation of a well-poured beer with a tight dense foam delights the customer and provides anticipation as to what will follow. For good reason, the ritual of pouring a beer with a good foam has gone viral on the internet. Beer foam provides a sensory experience through its visual appeal as well as a mechanism for delivering the aromas from the liquid.
Good beer foam is defined differently according to the individual consumer. Some expectations may include the size, colour and density of the foam head, its rate of collapse and its lacing or cling ‘tide marks’ on the side of the glass.
Beer foam involves some complex physics and relies on bubble formation and release from the supersaturated carbon dioxide (CO2) in the beer. This does not happen on its own but requires a nucleation site in the beer or its container, which can be from a particle or an imperfection in the glass surface.
Beer glasses are often etched on the base for draught beer dispense. This increases nucleation sites and promotes fine CO2 beading and continuous replenishment of the head. The result gives a visually appealing appearance to the product.
The released bubbles travel to where the air and liquid meet. When the liquid begins to drain, and the smaller bubbles begin to lose their CO2 to the larger bubbles, the latter grow in size until they become unstable and burst. New bubbles replace these from the nucleation sites in a synchronous effect known as beading.
The faster the beading, the more layers of small bubbles there are at the surface and the more stable the foam.
Beer foam collapse can be accounted for by the combination of liquid drainage and a mechanism called disproportionation (more on this later). The gas diffuses from small higher-pressure bubbles into a larger lower-pressure bubble. This causes the small bubbles to shrink and disappear, leaving larger unstable bubbles.
We know that the bubbles from carbonated water do not form a foam so there must be more at play in beer. The answer to this lies in the bubble wall; this is where there is an interaction with the components present in beer that are derived from the raw materials. These either increase foam stability or protect the beer from foam-negative (foam destroying) material. In beer, there are several foam-positive substances that stabilise beer foam. However, other components tend to be foam-negative and destroy beer foam.
Later, we will look at how we can promote foam-positives and minimise foam-negatives but let us first explore the physical world of beer foam.