Lesson 3 - Beer Foam SAQ

** Describe the formation of beer foam in a glass and factors leading to its collapse.

Response:

To form foam in the glass, we need two conditions:

1. A supersaturated gas

2. Nucleation sites on the glass

The initial foam formation will also depend on the vigour of the pour, which will encourage gas breakout and the beer temperature.

The foam head is sustained by beading, which is a rapid evolution of bubbles from a nucleation site typically on the base of the glass. The size of the subsequent bubble is determined by the nucleation site diameter, and smaller bubbles from small diameter nucleation sites (or N₂ gas) are more foam stabilising.

The initial foam is termed ‘wet foam’. There is a considerable amount of beer containing foam stabilising compounds surrounding the bubbles, namely glycoproteins, hop acids and divalent cations which bind bubbles together. Almost immediately beer begins to drain, washing out the stabilising substances. This causes the bubbles to agglomerate as a weaker ‘dry foam’ in a 3-D honeycomb lattice.

Foam stability is supported by beading that replenishes the foam on an ongoing basis.

Foam collapse is a result of drainage, lack of beading and a phenomenon known as disproportionation and coalescence. Disproportionation occurs when small stable bubbles with higher internal gas pressure lose their CO₂ to adjacent larger bubbles with lower pressures. This leads to the disappearance of the small bubbles and enlargement of the larger bubbles. Surface tension is increased, causing the bubble to burst and foam to collapse.

Coalescence describes the effect of foam-negative agents such as lipids and detergents, which migrate through the boundary layer between bubbles. It disrupts the stabilising agents and causes similar size bubbles to merge, leading to rapid collapse.