It just wouldn’t be the same without the foam, would it? Beer I’m talking about. You don’t want the glass to be full of it, but you certainly want enough to tickle your lips when you go bottoms up. But if you happen to be wearing lipstick, you may have a problem. The foam may just collapse around your mouth! So what’s going on? The basic reaction in making beer is fermentation. Yeast acts upon the carbohydrates in barley to produce alcohol, which of course is the most desired component of beer. Fermentation, however, also yields carbon dioxide gas, much of which dissolves in the beer. As the beverage sits in the bottle, carbon dioxide gas slowly escapes from the liquid into the airspace above the liquid until equilibrium is reached between the dissolved carbon dioxide and the gaseous carbon dioxide. Now, when the bottle is opened, the carbon dioxide above the liquid is released, the pressure is reduced, and more carbon dioxide escapes from the body of the liquid. Actually, this escape is into tiny air bubbles that became trapped in the imperfections of the glass when the bottle was filled with beer. As these air bubbles are inflated with carbon dioxide, they begin to expand and rise up through the liquid. When the bubbles reach the surface they pop.
That seems pretty straight forward, but it doesn’t explain the foam. What is the foam? Basically, unpopped bubbles at the surface. And why do they not pop? Because the thin layer of water that surrounds each bubble has been strengthened by a network of protein molecules. These proteins stem from the original barley and a particular one, known as LTP1, is thought to play a key role in foam formation. During the brewing process this protein is “denatured,” meaning that its three dimensional structure is altered. The result is that it becomes less soluble in water and begins to accumulate at the gas-water interface. Here it acts as a glue to strengthen the bubble. Remember the story about old horses going to the glue factory? The glue there is a protein called collagen. Now then, why does the foam dissipate if you are wearing lipstick? Because lipstick is full of fat and the LTP1 protein just happens to have a great affinity for fat. Essentially then the protein is sucked away from the bubbles into the fat and the thin layer of water around the bubbles is no longer strong enough to contain the gas and the bubble bursts. This also explains why any soap or detergent residue in a glass prevents foam formation. LTP1 has an affinity for these substances so it is not available to strengthen bubbles. One interesting possibility for the future is to genetically engineer barley to contain less LTP1 protein. That would result in beer with an impressive head of foam but some people may find biotechnologically enhanced beer difficult to swallow.