Carbonation Bubble Size & Microwave Decoction: Mr. Wizard

How does one control the size of the carbonation? Some beers seem to have very fine bubbles where others have large bubbles.
Mike Bourcier
Mason, Michigan

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).

I do countertop partial mashing, putting 4–5 lbs. (1.8–2.3 kg) of grain in a mesh bag and then into 5–8 quarts/liters of hot water in a beverage cooler. When my temperature is 1–2 °F (0.5 to 1 °C) low i drain a couple cups and heat it in the microwave for a minute, and then stir it back in. That usually raises the mash temperature by 1 °F (0.5 °C), and I repeat once more if necessary. I have never heard this method discussed, but it seems similar to decoction mashing. the heated liquid is not taken all the way to boiling. Is there anything wrong with this technique?
Richard Swent
Palo Alto, California

I really like this approach! This is the first time I have heard of your method and cannot think of any huge problems. It is certainly similar to decoction mashing, except that you are heating the wort phase of the mash, as opposed to the grain phase of the mash. The significance of this difference has to do with where the bulk of enzymes reside, and that is in the wort phase. I want to hold that thought for a second and briefly review decoction mashing.

The decoction method begins by removing a portion of the thick mash (the heavy solids that sinks to the bottom), boiling the thick mash in a separate vessel (the “mash kettle” or “maischpfanne”), and returning the boiled portion to the rest mash (the mash that was not boiled). When the rest mash and the decoction mash are mixed, the combined mash temperature increases. Traditionally, the decoction mash method provided a reproducible way to perform step mashes. It also helped degrade cell wall bits associated with under-modified malts. There are also flavor differences attributed to the method, but the two things I want to focus on are enzymes and cell wall degradation.

As malting barley varieties and malting control have both improved over time, the norm these days is well-modified malt with more enzymes. The practical result has been a reduction in so-called mashing intensity; this translates to shorter mash times with fewer rests, and the virtual elimination of the decoction method from new brewery construction. The bottom line is that decoction mashing does not have a clear use in the production of most beers brewed on today’s global scene.

However, for traditional brewers, homebrewers, and craft brewers, decoction mashing is still relevant because certain beer styles are brewed using this method. When it is used, it is important to be careful to boil the thick portion of the mash to protect the bulk of the mash enzymes that partition into the wort phase. However, today’s malt has a much higher enzyme content than malt of yesteryear and the process is arguably more forgiving with modern malt.

So back to removing a small volume of wort from your partial mash method, microwaving it, and returning it to the rest. The only thing that you need to be aware of is that the wort phase is richer in mash enzymes than the mash trapped in your grain bag. As long as you are only removing a small portion of the total, you should have no problems. Modern malt, even “low enzyme” pale malt, can easily handle up to 25% adjunct. So there are ample enzymes for your method.
The reason I jumped so far down the decoction rabbit hole is that it is possible to use a few small bags of grain in your mash with the express purpose of having a pre-determined thick mash ready to pluck from the pot, boil, either in a small pot or in the microwave, and returning to the rest mash. I have never read anything about microwave decoction boiling and don’t know what flavor effects this method would have, but from a practical perspective it may make the process easy if you ever became concerned that you were denaturing too much enzyme by removing portions of wort, heating, and returning.

Issue: December 2016