Beer Foam | Part 2

Following the last article, we will keep talking about beer foam. I hope you enjoy it!

In the previous article, we learned how proteins, iso-alpha acids, and lipids affect head formation. If you didn’t read it, click here!

Bubbles

When the beer is contained in the bottle, the pressure keeps the CO2 solubilized in beer in the form of carbonic acid (H2CO3). After the pop of a bottle, the pressure is reduced, and the CO2 starts to ‘separate’ from beer becoming insoluble (and hydrophobic). This process is encouraged by nucleation sites, or irregularities in the glass, where the bubble forms around this point and starts to grow. As the bubble reaches a size where the buoyancy is strong enough, the bubble leaves the nucleation site and rises. Pouring out the beer adds air bubble in the beer, serving as nucleation sites.

The bubble has a spherical shape because sphere has a lowest surface area between the shapes. The size of the bubbles depends on the surface tension, temperature, pouring techniques, glass type (and how clean it is), beer characteristics, and gas (CO2 or Nitrogen). As smaller the bubble is, the slower it will rise, giving the bubble more opportunities to interact with surfactants.

Foam Formation

Surfactants are compounds that have a hydrophilic and a hydrophobic end. The hydrophilic end bonds with the bubble. The surfactant increases the surface tension, making it more stable.

When the bubble comes to the liquid surface, it touches other bubbles, forming the foam layer. At this moment, the layer is wet; however, the liquid between the bubbles will drain, and the bubble wall becomes thinner. If the surfactant force is not strong enough, the bubble collapses, leading to foam with poor retention.

Many proteins, such as protein Z, hordein, and LPT1, are amphiphilic, meaning they have both hydrophilic and hydrophobic ends and can act as surfactants. These proteins bond in order to form a double layer, called foam lamella, where both the interior and the exterior of the bubble are hydrophobic, with water in the interior of the wall, on the hydrophilic side.

Foam Retention

Therefore, as more surfactants are better for foam, right? Not exactly.

When more than one surfactant is present, they can reinforce one another, or have a negative effect. Some surfactants stick to each other better than with the bubble; in this case, they provide worse head retention. Traces of detergent in beer or greasy substances resulting from incomplete glass washing are foam killers. This happens because the hydrophobic part of the surfactant sticks better with the oil than with the bubble.

The iso-alpha acids have an important role in foaming; they interact with proteins, improving head retention and preventing the bubbles from leaking gas.

What Improves Foam:

• Presence of hydrophobic proteins.
• Wheat malt.
• Addition of hop bitter compounds.
• Lower pH grants less acidic hop compounds are ionized.
• Nitrogen gas enhances foam, avoiding the migration of gas out of small bubbles.
• Some studies point to the use of a mash filter instead of lautering improves foam.

What Kills Foam:

• Presence of detergent, fats, and oils.
• High alcohol levels.
• Oats.
• High gravity wort followed by dilution before packing.

I hope you enjoyed it! Please comment with the topic you want to read in future articles.