[Originally published in Brewers Journal Canada, March 2019. Revised for Escarpment Labs Blog, April 2022.]
Foam is a defining characteristic of beers across the globe, with different cultures embracing different styles of beer foam. Just think of the dense nitrogen-enriched stout heads, frothy German lager coifs, and Belgian beers poured with foam so ample and meringue-like, it persists until the last drop (we’re looking at you, Orval). However, foam is also an often-overlooked property of beer, but foam quality may make the difference between an okay beer experience and a truly great one.
There is a lot of pretty cool science surrounding beer foam, so we are going to walk through some of the highlights and finish up with some practical tips to enhance beer foam.
What we talk about when we talk about foam
Foam is, at its most simple, a mixture of gas and liquid. In the case of beer foam, we are talking about a mixture of CO2 (or beer gas) inside bubbles of beer. Beer foam has two major properties: foam formation, and foam stability.
When carbonated beer is dispensed, foam is produced as a result of CO2 bubbles released by the pressure reduction. The more nucleation points (cracks, imperfections, etchings) in the glass, the more CO2 will be released from the beer. Usually, the CO2 will pick up some surface active compounds along the way (such as proteins, polysaccharides, etc), which help to stabilize the foam.
Up close and personal with your foam. Credit: Eric Saulis
You can’t make stable foam in a glass of water – these surface active compounds are required for foam stability. Both foam-positive compounds and soluble gas are important for the overall foam quality of a beer. In general, the more CO2 in solution in the beer, the greater the capacity for foam formation. This is why bottle conditioned Belgian beers with high CO2 pressure tend to produce more foam than a standard force carbonated ale.
When foam is formed, the inevitable process of collapse begins. However, the rate of foam collapse is highly dependent on the foam’s stability. When a foam collapses, bubbles within the foam burst and are absorbed back into the liquid phase of the beer. Some beer pouring strategies intended to promote stable form involve pouring beer, then waiting a half minute before pouring more beer, to encourage a thicker foam “cap” with less potential for collapse.
A controlled rate of foam collapse is important. Since foam contains beer, and its associated aroma molecules, a stable foam is capable of releasing the flavour of the beer in a more controlled manner, ensuring a better drinking experience throughout the whole pint. We’ll often see the evidence of a good foam on our glass of beer – the beer leaves behind traces of foam called lacing on the glass with each sip.
The frothy details
There are many compounds present in and near beer which can be foam positive or negative to the beer. The main foam-relevant components of beer are proteins, particularly two proteins called LTP1 and protein Z. This sounds complicated, but essentially LTP1 helps to form foam in the beer, while protein Z and other grain-derived proteins help to stabilize the foam once it has been formed. The intricate balance between different proteins and other molecules helps explain why we see so many different types of beer foam, from lacy and delicate to dense and rocky!
All ingredients impact the quality of foam. For example, hop iso alpha acids have a big influence on foam quality. Hop suppliers have noticed this and have even developed specialized hop extract products which can aid foam stability such as tetra- and hexa-iso hop extract. In general, higher foam stability can be achieved from beers with a higher wort protein and hop iso alpha acid content. Other wort components can also play a role in foam quality. Polysaccharides have a role in stabilizing bubble size, while melanoidins can also promote foam stability.
Of relevance to the juicy IPA crowd, there is some evidence that polyphenols can negatively impact foam, since they can bind with proteins that may otherwise be foam-positive. Recent hop research even shows that different hops can have positive or negative effects on foam quality! Foam is a very complex topic, and no single ingredient is a silver bullet for optimal foam.
Foam and our favourite fungus
Even yeast plays a role in beer foam quality. When beer (and the yeast that fermented it) is aged, the yeast can sometimes enzymatically break down LPT1, which can lead to decrease in foam stability. This is especially common with yeast which is experiencing poor nutrition and being starved in the bottom of a big tank. Unhealthy yeast can lead to bad foam!
Beyond unhealthy yeast ruining foam, some yeasts even have the ability to enhance foam. Lager yeasts contain a gene called CFG1 (Carlsbergensis Foaming Gene). CFG1 codes for a mannoprotein (sugar-containing) on the surface of the yeast cell, which can “stick” to bubble surfaces and prevent them from draining, helping to stabilize foam. There are a couple other similar proteins in ale yeast, but not much is known about them (yet)!
We have checked all our Lager strains for the presence of the CFG1 gene using PCR and can confirm that all of our commercially-available Lager strains are certified foam-positive.
You lost me, just tell me how to make nice foam
I understand, this stuff gets complicated. Brewing science is a rabbit hole! Ultimately, we can boil all of this down to factors which are foam-negative, and those which are foam positive, admitting the current limitations of science to explain everything that is happening.
- Over-modified base malt
- Too much trub in fermentation (more fatty acids)
- Protein rest too long
- Poor yeast health, leaving finished beer on yeast
- Dirty glassware
- Shorter mash times/rests
- Higher protein content (e.g. wheat malt, flaked barley, etc)
- Clearer wort or trub settling
- Higher hopping rates
- Dry hopping (usually)
- Bottle conditioning
- Scrupulously clean glassware
- Nitrogen dispensing
If we want to maximize foam quality in the brewery, we can take all of what we’ve learned as an example. We could use a small amount of flaked grain in an all-malt grist, mash using a step mash in short steps (just enough to achieve conversion). We could also use a good dose of hops to improve foam stability.
As you can see, this regimen can be quite flexible as to the beer style – this would work equally well for a saison as it would for a lager! In fact, it might be even better with a lager yeast, since these yeasts naturally aid foaming due to the CFG1 gene.
Photo Credit: Corey Fairs