Carbonation Size

I have to admit that I really have come to dislike questions about beer foam because the more I think I learn about foam, the less I think I know about foam! I studied beer foam in graduate school and, in 1994, went away thinking I had a pretty good handle on the subject. I then began my life as a practical brewer and 20 years into this phase of my career am often flummoxed about foam.

When it comes to foam, brewers lump things into two broad groups: Foam positive stuff and foam negative stuff. I use the term “stuff” to account for compounds and actions. Examples of foam positive stuff include certain proteins, hop acids, metal ions, propylene glycol alginate (an emulsifier derived from kelp), carbon dioxide, and nitrogen. These foam positive compounds can be further grouped into those compounds that reduce the rate of foam collapse and into those compounds that promote foam lacing.

Foam negative stuff is the grouping of enemies. Lipids and detergents top this list, especially when left as deposits on beer glassware, and we all know that oil from greasy foods, like potato chips, and films left by detergents and sanitizers, like quaternary ammonium compounds, are particularly foam negative. Other foam negative things include adjuncts (they dilute malt proteins), over-modified malt (excessive protein degradation), high alcohol beers, and foaming during the process (foaming removes foam positive compounds from beer).

The maddening thing about this is sometimes a beer is brewed that should have a great frothy crown of foam when poured, yet instead has a sad, floppy cap that quickly fades into oblivion. Add texture into the desirable qualities we want in our beer foam and things become even deeper!

The analytical side of my brain tells me that foam texture should be influenced by those compounds in beer that stabilize carbon dioxide bubbles. And carbon dioxide bubbles only exist when dissolved carbon dioxide, in the form of carbonic acid (H2CO3), is released from the beer during pouring. Mechanical action during dispense has a real and apparent effect on foam formation and foam texture. Rough pouring may produce more foam, but the foam is often coarse, visually unappealing and less stable than foam that is formed through more controlled pouring methods. And if the pouring method brings air into the foam, for example sparkler taps that are designed to literally suck air into the beer stream during dispense, nitrogen from the air makes the texture creamier and enhances foam stability. This argument is all based on pouring.

But the reality, at least what I have come to accept as reality, is that the texture and appearance of beer foam is not just about pouring, and it’s not just about what is in the beer. It is related to how the beer was carbonated. An extreme example of this is when a keg is carbonated using a carbonation stone, and in the process of carbonating the beer foams.

This process results in the loss of foam positive compounds and the resulting beer has less stable foam than beer carbonated in a slower manner. In fact, the loss of foam positive compounds can be seen under a microscope as “bubble skins” in beer; bubble skins are like deflated balloons floating aimlessly in beer, never again to be inflated with carbon dioxide and able to participate in beer foaming. This extreme example is obvious and is why brewers want to force carbonate in a manner that prevents foaming.

A related, but different example of the same thing is when naturally conditioned beer is compared to force-carbonated beer. It is frequently the case that naturally carbonated beer has a tighter foam texture and better foam stability than their force-carbonated kin. This could be due to foaming in the process, but many brewers truly believe that there is something about the natural conditioning process that results in a very different foam. Sparkling wine producers know that wines carbonated through natural conditioning in the bottle have smaller, more persistent bubbles than wines carbonated in the tank.

I will avoid speculation about why this is the case and merely state that I believe that natural conditioning makes a real difference, even when force-carbonation is only used to “touch up” the carbonation in beer that has primarily been naturally conditioned. If you are looking for fine bubbles and a creamy foam, begin by brewing beer that is rich in foam positive compounds and deficient in foam negatives. Use good cellaring practices so that you bottle or keg beer with a minimal amount of yeast and trub solids, and add the right amount of priming sugar so you do not end up with over-carbonated beer (excess carbon dioxide causes foam problems of its own, including large, funky bubbles).

 

Response by Ashton Lewis.