The Science Behind Beer Foam
Whether you refer to it as foam, head, kräusen or the junk on top of your beer—love it or hate it—the foam that develops on top of most craft beers is impossible to ignore. But what is it, exactly? How can foam differ from glass to glass and beer to beer? Why is your beer foamy, but not the one served to the person sitting next to you?
Behind beer foam’s mysterious curtain there is actually a lot of science!
When beer foams, it is obviously due to the creation of bubbles. This phenomenon is referred to as nucleation.
The physics of nucleation as a whole isn’t entirely understood, and there are a large group of proteins and smaller polypeptides (additional proteins) that can act as a group and individually as foam positive agents. One particular protein naturally found in barley is Lipid Transfer Protein 1 (LTP1), and it plays a large role in a beer’s foam.
LTP1 has a high rate of “hydrophobicity” (meaning it doesn’t like water), so in order to help alleviate its predicament, it grabs hold of a bubble of CO2—produced in fermentation as well as introduced in bottling/kegging—and then rises to the surface alongside the CO2 bubble (or N2, for nitrogen beers…but more on that in a bit).
Once at the surface, the LTP1 protein forms a coating on the bubbles which helps maintain the foam. Hops also come into play as the hydrophobic polypeptides derived from grain cross link with bitter iso-alpha acids from hops to render foam more ridged, stable and clingy.
There are dozens of variables that impact the differences in head-foam quality, texture and retention, but quantities of LTP1 in a specific batch of beer can be traced back to factors like how wet or warm the climate was that produced the barley.
Foam’s Effect on Flavor
A beer often tastes different when it’s topped with head of foam, and this is due to surface active compounds that move into the bubble walls as they percolate to the top of your glass. Foam also carries a profound trigeminal sensation—that is, “taste” effects which are actually perceived physically. Think of the “cool” sensation of mint, or the “hot” sensation of chili peppers. Neither is delivering an actual thermal load, but rather they cause a physical perception. The creamy, fluffy feel of foam can dramatically alter the perception of any given beer by “softening” the overall palate.
It’s also important to remember that our senses of taste and smell are intimately interwoven. In fact, many times a specific characteristic that a drinker may describe as ‘taste’ is actually detected in their nasal passage. Foam brings more odor compounds to the surface of your beer, kind of like un-stuffing your nose and opening up the full range of flavors.
What is Lacing?
The LTP1 proteins mentioned earlier, which form a coating around every foam bubble, interact with other compounds that also happen to rise to the top of your glass.
Once these proteins and compounds begin to interact with one another, they become denser, undergoing a textural transition, and begin to stick to the sides of the glass when left alone for a bit. This is why a beer consumed slowly will accumulate much more lacing than its guzzled counterpart.
Why Does My Beer Have More Foam?
There are a myriad of answers to this question, but here are a few common (and interesting!) variables to think about:
- “Beer clean” glassware: Detergent or other cleaning agents that may be invisibly residing in a glass can decrease the formation of foam. Your glass should be thoroughly rinsed before being filled.
- Etched glassware: Some glassware is etched on the bottom to create an additional nucleation site. Bubbles cling to the etching and accumulate until they’re buoyant enough to break free and rise to the top of your beer, replenishing the head.
- Lipstick or chapstick: Certain waxes and compounds in lipstick and chapstick can block protein interactions and/or poke holes in the bubbles’ protective protein skins, killing beer foam.
- Greasy or fatty foods: Much like lipstick, fats from foods that are on your lips can pose a real detriment to your foam’s stability and overall length of life.
- Alcohol content: The alcohol (ethanol) in beer actually acts as a foam deterrent. After one percent ABV, ethanol’s ability to deter foam progressively increases with the alcohol content.
- Temperature: Temperature not only affects your perception of taste, but also your beer’s overall attractiveness. The process of ‘disproportionation’ is when smaller bubbles become absorbed by larger ones to create a spotty, bladdery effect. This happens at higher temperatures, creating poorer foam in the glass, as well as more overall foam in kegs.
- Nitrogen: Nitrogen has the unique and wonderful ability to produce a creamy, “rich” mouthfeel and a thick head which produces a wonderfully drinkable and enticing experience. I mentioned disproportionation above and its relationship to temperature, but it also has to do with the solubility of a gas through liquid. Nitrogen gas is not very soluble, so it produces many small bubbles that create a very creamy, stable head. This also explains that wonderful, cascading “reverse waterfall” effect as the bubbles rise. Learn more in “Good Beer Gas: Nitro Beers Explained” by John Holl.
As you can see, there is much more to foam than meets the eye, which can also be said for other aspects of beer. All of these factors can add or subtract from your enjoyment of your next pint.