The Tragedy of a Nice Beer Foam Head

Demonstration Experiment on Video

The Tragedy of a Nice Beer Foam Head

Beer foam has a short existence. It is born, lives, and dies

Peter Keusch

German version

Supermarket products:

2 bottles of weizen beer 0.5 L
fat

Glass wares:

2 wheat beer glasses (weizen glasses) 0.5 L
beaker 50 mL

Experimental procedure:

Two weizen glasses are filled with beer. The inner surface of the top of the first glass is covered with fat. The fat represents the usage of fatty based soaps to clean glasses and the residue that is left behind. The weizen beer is poured gently down the side of the glasses tilted diagonally until a perfect head is created millimeters from the rim.

Videoclip (Download RealPlayer .rm-Datei)

Results:

The foam head in the first glass is smaller than in the second glass. It collapses rapidly.

Discussion and background:

· When beer is poured into the glass sideways less foam is formed. As beer moves down the side of a glass it will spread out and move at a slower rate which causes less disturbance.

· In beer, CO₂ is dissolved under tension in the liquid. While the cap is in place, equilibrium exists between dissolved gas and any bubbles that may be have formed. As soon as the bottle is opened, equilibrium is disrupted due to the reduction of pressure (Boyle's Law) and the flow of CO₂ from solution into bubbles (diffusion).

Before a bubble can grow, it must form a nucleate. In beer, bubbles nuclei form at imperfections on surfaces such as scratches on the glass. After a bubble is released from its nucleation site, it grows as it makes its way to the surface. Bubble enlargement during ascent is caused by a continuous diffusion of dissolved carbon dioxide through the bubble's gas/liquid interface.

Lipid Transfer Protein (LTP1)
· The beer foam consists predominantly of a disperion of CO₂ in beer. The CO₂ bubbles rising through the liquid to the surface accumulate on their way high molecular proteins. These foam active substances cling to the bubbles and coat them with a thin elastic skin. Lipid transfer protein 1 originating from barley stands out as the most important component relevant for the formation of foam. In malt, LTP1 is present in a foam inactive folded form. During the wort boiling LTP1 loses its 3D structure. The denaturated unfolded form is surface active.

· The head of foam is formed when the beer is poured and mix with air. An abundant head shows the quality of the beer. A beer with a good head of foam is a good beer; it is the bloom of a beer. The foam prevents the carbon dioxide from escaping the liquid. The mouthfeel and the creamy sensation of the beer is preserved.

· As soon as it is formed, the nicest beer foam will begin to disappear. This occurs for three reasons. On the one hand gravity causes the liquid surrounding the bubbles to flow through and out of the foam ("foam drainage"). On the other hand a gas exhange takes place between adjacent bubbles. Gas diffuses from smaller bubbles to bigger ones ("coarsening"). In small bubbles carbon dioxide will exert a greater pressure than is found in big bubbles. If a small bubble is next to a big bubble the gas contained within the bubbles will try to reach equilibrium. The result is that the carbon dioxide dissolves through the wall of the small bubble into the larger bubble. The small bubbles therefore disappear and the larger bubbles continue to expand. Their membranes thin until they reach a critical thickness. Collapse occurs at the crown surface by rupture or by diffusion of CO₂ directly to the atmosphere through the CO₂ permeable bubble film. This phenomenon happens more quickly at higher temperatures, but is reduced if the gas pressure above the foam is increased. Next time you have a beer you can test this out for yourself by covering your beer glass. You should find that the foam lasts longer. This is often claimed to be the reason why German beer steins have lids.

· Soap, detergent, grease and wax residues will kill foam formation and retention actually attack the foam on a head of beer. Fatty substances are attached to the surface on the bubbles. The surface tension on the bubbles is lowered. They will burst. As a result the foamy head disappears, causing the beer to look and taste "flat". For these reasons, do not use regular liquid household dish washing detergents for glassware. They are fat-based and will leave a slight oily film on the glass. This causes beer to go flat quickly. Use a detergent designed specifically for beer glass cleaning. It must be low-suds, odor-free and non-fat. After washing, thoroughly rinse beer glasses and, if possible, air-dry them.

References:
Vivienne Baillie Gerritsen One beer please
Charles W. Bamforth Beer: An Ancient Yet Modern Biotechnology.
Richard Kingsley There is plenty of physics involved in opening a bottle of beer.
Wolfgang Kunze Beer Foam

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