Explain why does foam form when pouring beer?

Physical process of foam formation

When you pour a beer, you violently mix carbon dioxide (CO₂) with the beer, which had been quietly waiting in the pressurized bottle. The CO₂, suddenly released, escapes by forming thousands of tiny bubbles. These bubbles rise to the surface and stack into a light and airy layer: the foam. Why does it stay stable for a while? Because proteins from the barley cling to the bubbles and help them maintain their structure without bursting too quickly. The richer the beer is in proteins and the more suitable the glass is, the better your foam holds up in your glass.

Ingredients of beer influencing the foam

The foam of your beer largely depends on the ingredients chosen during brewing. Barley, for example, is rich in proteins that stabilize the bubbles. The more protein your barley malt contains, the denser and more durable your foam will be. Hops also play a role: their resins provide bitterness, but also compounds that stabilize the bubbles, allowing this thick foam to last longer without collapsing. Add to the equation certain grains like wheat, which is also rich in proteins and has high foaming properties, and you get an even creamier and denser foam. Conversely, if your beer uses ingredients low in protein or high in fats (oils, fatty seeds like corn), you risk having a less generous or almost non-existent foam.

Effect of carbon dioxide and pressure

The foam in your beer mainly comes from the carbon dioxide dissolved under pressure in the bottle. When you open your beer, the pressure drops suddenly, and the gas escapes, forming small bubbles. These bubbles rise to the surface, bringing with them the proteins and other components of the beer, thus creating a nice thick foam. The more carbon dioxide the beer contains (often related to the amount of CO₂ added or naturally produced during fermentation), the more abundant this foam can become. Conversely, a poorly carbonated or flat beer will inevitably have less foam.

Importance of proteins in foam

The foam of your beer is primarily a matter of proteins. They are the ones that stabilize the small air bubbles by creating a protective film around them. Without these proteins, the bubbles would burst too quickly, and goodbye to the beautiful creamy foam! These proteins mainly come from malt, especially from barley, which contains just the right amount. If the beer is low in proteins, the foam disappears very quickly, while too much protein creates a dense and stubborn foam (sometimes too much!). In short, brewers need to find the right balance to achieve that perfect foam that slightly clings to the edge of the glass.

Influence of temperature and type of glass

A beer that is too cold will limit the formation of foam, as at low temperatures, carbon dioxide remains better dissolved in the liquid and escapes much less quickly. Conversely, a higher temperature will encourage the bubbles to rise quickly and intensify foam creation—but be careful, as the foam will become unstable and disappear faster.

Regarding the glass, its shape and irregularities are crucial. A narrow and tall glass promotes a dense and abundant foam because it concentrates the rising bubbles over a smaller surface area. A wide and open glass generally generates less foam, and it evaporates quickly. As for a ribbed glass or one with micro-defects, it helps carbon dioxide to escape and rapidly boosts foam formation. That’s why some beers are always served in specific glasses!