Most home brewers encounter the problem sooner or later: some beers come out excessively foamy. You pour it carefully, only to end up with a glass full of foam, or the foam literally climbs out of the bottle when you open it! So what causes this problem, and what can we do about it?

The physics of foam

Beer foam is more than it appears to be. Some brewing scientists hold one or more doctorates on beer foam physics alone! Fortunately we don't have to delve quite so deeply, because the principle is fairly simple. In a nutshell, beer foam in regular beers is the result of three main factors.

First of all, beer is fizzy. This is the result of the carbon dioxide gas that is dissolved in the beer. While the beer is under pressure (typically n a closed bottle or keg) the carbon dioxide gas stays in solution. When the bottle is opened or the beer is tapped from the keg, the pressure is greatly reduced, which causes the carbon dioxide gas to break out of solution and form gas bubbles that rise to the surface. Because cold liquids are better at retaining carbon dioxide than warm liquids, cold beers tend to be less foamy when poured than warm beers, since the latter simply let go of their carbon dioxide gas more quickly.

Secondly, the properties of the liquid play a role. All fizzy drinks have carbon dioxide gas dissolved in them, but not all carbonated beverages are foamy. Soda water, for example, typically contains a much larger amount of carbon dioxide (and is therefore much fizzier) than most beers. Yet soda water does not have a head of foam when poured. This is where the second factor comes in: the capability of a liquid to trap the gas as it rises to the surface and form bubbles there. Some liquids (such as water) simply part and let the bubbles through so that they escape in the atmosphere, but others (such as soap) cling together in a thin film that traps the gas as it rises, fills up and thus forms a bubble.

The third factor is the presence of "nucleation sites". When a carbonated liquid such as beer or soda water comes into contact with a rough surface, the carbon dioxide tends to cling on to these surfaces, which in turn causes them to come out of solution. For a demonstration of how this works, simply put a teaspoon of sugar (the crystals of which are full of rough surfaces) into a glass of Coke. Keep a mop handy.

The problem with nucleation is that microscopic particles may have quite a rough surface when you view them under a microscope. While you can't even see these contaminants, they may be enough to cause the beer to foam. Beer poured into a glass that has just been rinsed will generally foam less than beer poured into a dry glass. This is the result of microscopic dust particles present on the inside of the glass. Rinsing the glass removes most these particles, but in a dry glass they may be present in sufficient quantity to cause the beer to foam a lot more.

Excessive foam

While the above applies to regular beers that are not overly fizzy, we also encounter the odd "problem beer" that, no matter how we pour it and how cold it is, turns a simple pint into a complete foam party. There are a few possible factors that can lead to these excessively foaming beers. In the discussion below we will assume that, as a home brewer, you will bottle condition you beers, i.e. you bottle them with a measured amount of priming sugar which ferments in the bottle to make the beer fizzy. (If you keg your beer and force carbonate it with pressurized carbon dioxide gas from a cylinder, you can simply use a little less gas pressure to solve the problem!)

1. The beer was bottled to early.
Beer forms carbon dioxide gas during fermentation. In the fermenter this gas escapes (typically through an airlock bubbler) but in the bottle the gas cannot escape; it stays in solution to make the beer fizzy. If the beer is bottled too early, part of the fermentation that should have taken place in the fermenter now takes in the bottle, which raises carbon dioxide levels in the bottle to beyond what was intended. In extreme cases the bottles may even explode.

2. The wort contains complex sugars that are slow to ferment.
This is most commonly the case with beers brewed from dry malt extracts (DME) and with dark beers such as stouts. Both contain complex sugars, either from the DME (as a result of the spray drying process which is a rather brutal on the sugars) or from the dark roasted malts, which contain lots of caramels. These complex sugars will eventually (!) ferment out to a certain extent. How much and how far will depend on the yeast and the actual sugar profile, but the long and the short of it is that this may take anything between three weeks and three months, and much of that will take place in the bottle, contributing to carbon dioxide pressure..

3. "Slow yeast".
In some (rare) cases a yeast will go through about 90% of the fermentation at a normal pace, then decide that this is too much like hard work and take several weeks for the final 10% to complete. I have had this problem with WLP504, which is a liquid Belgian Abbey yeast. Most yeasts are fairly well behaved so this is not a common factor, but it is possible. My Belgian dubbels and tripels brewed with WLP504 all came out badly over-carbonated as a result of the yeast's tendency to finish the fermentation at a snail's pace.

4. Too much priming sugar.
Depending on how much carbon dioxide the beer contains when it comes out of the fermenter (which in turn mostly depends on the temperatures during fermentation and at bottling time) the beer coming out of the fermenter will contain a certain, usually rather small, amount of carbon dioxide at ambient pressure and temperature. The amount of priming sugar should be calculated to take that into account, especially if the beer has fermented (and is being bottled) relatively cool. A good online priming sugar calculator can be found at  https://www.brewersfriend.com/beer-priming-calculator/. There are others as well. The amount of priming sugar is important: too little and the beer will be flat; too much and the beer will be too foamy or, in extreme cases, the bottles may explode.

5. Wort/beer composition
Even at normal carbonation levels some beers are much foamier when poured than others. This is a factor of wort/beer composition, especially in terms of viscosity, protein content and other factors. In general: the more protein, dark malts and/or hops the beer contains, the more foam it tends to produce. Everyone who has served even a moderately carbonated a Hefeweizen from tap will know how notorious this type of beer is for pouring overly foamy. Stouts have a tendency to climb out of the airlock during fermentation, and hoppy beers such as IPAs tend to come with a generous head of foam as well. In short, "It's the nature of the beast!"

6. Material contaminants.
The beer may come into contact with particles that act as the nucleation sites (discussed above) that will cause dissolved carbon dioxide gas to break out of solution. One common problem is improperly cleaned bottles. If the effect is limited to some bottles in the batch, this is the most likely cause. If the entire batch is effected, though, this may be the result of brewing malt infected with a fungus that produces a protein known as hydrophobin, which attracts carbon dioxide and causes it to form bubbles that rise rapidly to the surface.

7. Microbial contamination (a.k.a. "gusher bugs").
Beer contains some unfermentable sugars that stay behind after the fermentation is complete. These unfermented sugars contribute to the beer's flavour, mouthfeel and other characteristics. If beer were to contain only fermentable sugars, the end product would resemble carbonated alcoholic dishwater more than beer! However, it is possible for beer to become infected with wild yeasts or bacteria known as "gusher bugs". These microbes may be able to ferment and/or break down the sugars that your brewing yeast will leave alone and thus produce more carbon dioxide gas. This is usually easy to diagnose, since these contaminations typically ruin the flavour of the beer.

Great. Now what?

Dealing with over-carbonation issues is usually more in the nature of diagnosing the problem with the aim to prevent it in future batches. While industrial brewing scientist have recently found that treating the beer with strong alternating magnetic fields of the closed beer bottles helps to prevent excessive foaming when the bottle is opened and the beer is poured, such exotic methods are not available to the average home brewer.

Typical ways to prevent over-carbonation in home brewing is to ensure that the fermentation is fully complete before bottling, working out the proper amount of priming sugar, and observing proper brewing hygiene. But once a beer is over-carbonated, there are only a few things we can do. Simply put, once the beer contains too much carbon dioxide, the only option is to get rid of the excess.

One way to achieve this is to refrigerate the beer as much as possible without actually freezing it. Carefully open the ice cold bottle (cracking the cap is usually better than removing it entirely) and let the beer gradually decarbonate. Eventually it will usually begin to foam, at which point the bottle can be re-closed. If necessary this process can be repeated.

Another and more heroic way is to pour the beer out of the bottle into a sanitized bottling bucket and wait until the foam has subsided. Since typically 1/3 to 1/4 of the foam consists of beer, the foam should not be removed but instead must be allowed to collapse naturally. Then the beer may be re-bottled with fresh bottling yeast and a moderate amount of priming sugar.

Both methods carry a major drawback in the form of a huge risk of contaminating the beer. Therefore this is generally considered an option of last resort if pouring the overly foamy beer down the drain is the only other option.