The Appeal of Beer Foam23/12/2021
Have you ever kicked back with a beer and pondered the science behind a beautiful head of foam?
Many of us take the foam on our favourite beer for granted. We drink beer with our eyes before our mouths, so foam possesses an aesthetically pleasing nature and is a definitive characteristic of beer.
I have recently thought deep about my foam preferences and decided I have a love for foam. In my opinion, it is unappealing to be drinking a flat beer, utterly devoid of foam. I strongly believe that dissolving gas into a fermented libation makes for a delightful drinking experience.
What Then is Beer Foam?
The foam on a beer is a marriage of two main beer ingredients, malted grain and hops. When referring to beer foam I am referring to a gas encased in a bubble. This bubble is comprised of proteins bonded to iso-alpha acids, each connected by a cation (a positively charged ion). Using quality malted barley adds all the needed proteins while using hops with any amount of alpha acid would provide enough iso-alpha acids to create consistent and stable foam.
Iso-alpha acids are hydrophobic (or water-hating) making them a foam positive beer component. So, to create a bubble, proteins and isomerized alpha acids migrate and bond to form a hydrophobic sphere. Parts of each of those bonds are connected by a cation, commonly Zinc (Zn2+). This makes zinc not only an important nutrient for yeast growth during fermentation but also an important part of foam formation.
Beading and the creation of nucleation sites are both important to a stable and consistent beer foam. A gas, usually CO2, is dissolved into beer and must find its way out to produce foam. The spontaneous formation and release of gas bubbles, or nucleation, will lead to beading and finally to foam. Nucleation occurs on particles, scratches, or indentions in a drinking vessel. This means having an unfiltered beer or an emblem etched into the bottom of a glass can provide more nucleation sites and more opportunity for the gas to bubble out of the liquid, creating foam.
Components to beer that are good for foam (referred to as foam positive) are the obvious: protein, alpha acid, and zinc; but some less obvious are the beer’s pH, the wort boil, and the beer widget (commonly found in Irish Stout Ales).
A lower pH will encourage a dissociation of hop alpha acids and proteins aiding in their migration and bonding. But before these components can bond a vigorous wort boil is critical because it isomerizes the hop alpha acids and denatures proteins, bringing out their foam potential. Then there is the nucleation widget. These can be found in nitrogenated stout beer cans and works as a large nucleation site promoting bubble creation.
Wheat is also a foam positive component used in some beer recipes. It has proteins that perform better than barley proteins in foam formation, stability, and consistency. Though it may be good to keep in mind that wheat proteins can lead to beer haze too.
Of course, there are foam negative components like alcohol, lipids, detergents, and adjuncts.
Lipids and some detergents are absolutely foam damaging. They are similarly hydrophobic and will disrupt the bonding interaction between the proteins, hop acids, and cations.
Many adjuncts (for example added sugars) don’t provide the protein required for ample foam production, so when a brewer adds a large volume of simple sugars there are fewer available proteins around for making foam. This can result in a beer with inferior foam quality.
Alcohol is the number one foam negative ingredient, and high alcohol beers can have poor foam production. Though some beer drinkers prefer this, personal foam preferences vary as much as people vary. Some of us prefer a thick foamy Hefeweizen, others like a slight fizz from a boozy barley wine, and some of us enjoy both.
Each has a visual appeal that enhances their drinking experience. Enjoy that wonderful marriage between chemistry and aesthetics and appreciate that beer foam.