A stable crown of rocky foam is a signature hallmark of many a beer style, and brewers go to great lengths to promote a healthy head on each and every pint. From adding a dash of wheat or dextrine malt in the mash to monitoring the cleanliness of glassware, brewers are obsessive when it comes to foam. But what is beer foam and why is it so important?
What Foam Is
Foaming is a complex phenomenon involving the interaction of numerous chemical species. But first you need some bubbles, and to get those bubbles, you need carbon dioxide, or CO2. Before you open a bottle (or pull down the faucet handle), CO2 is dissolved in the liquid beer. But as soon as that beer hits atmospheric pressure, CO2 wants to escape. In fact it has to.
Henry’s Law requires that any gas in solution remain in equilibrium with the gas in the headspace. If you think about it, it makes sense: Imbalance will result in gas diffusing into or out of solution until equilibrium is established. Atmospheric CO2 only exerts about 0.006 pounds per square inch (40 Pa) of pressure at sea level, which is nothing compared to the 12 to 30 psi (83 to 207 kPa) typically dissolved in beer. So carbon dioxide will rush out of a newly opened beer until the amount of CO2 in that beer is accordingly as low as it is in the atmosphere.