This is a guest post by Michael, author and founder of Robust Kitchen.
Beer is awesome. Declare it from the rooftops!
Or maybe that’s old news to you. If you’ve ever enjoyed downing a cold glass next to me at the local pub, then you know how much I appreciate properly poured beer. The sight of that golden manna from heaven with its floating layer of fluffy foam is enough to make a grown man “giddy”. Not even the snooty wine connoisseurs can deny the smooth, effervescent, texturally-layered advantage of a good beer.
But there is one quality above all that makes beer, by far, the most visually appealing beverage on our planet:
The fat layer of foam on top
Have you ever thought about why beer has this odd layer or why it stays there for so long? Logically, you might think carbonation, and you’re not entirely wrong. Carbon dioxide is definitely a critical part of the beer foam equation.
But aren’t most beverages carbonated? Why doesn’t soda or kombucha—or champagne for that matter—retain a layer of foam for more than 10 seconds?
While the general answer may be simple, the countless tweaks involved in the fine-tuning of this variable in beer are exceedingly complex.
The goal of this article is to expose the mysteries of beer foam, and to inspire appreciation for this timeless beverage.
What is Foam?
Not surprisingly, foam is just a conglomerate of bubbles—particles of gas caught in spheres of liquid. Everyone already knows that, but what they don’t know is how this gas gets into our drink and why it only comes out when the drink is agitated.
How Does Gas Get into Beer?
Brewers have different ways of incorporating carbon dioxide (CO2) into beverages. The problem is that CO2 is only moderately soluble in water—they can’t just shove the gas into the liquid, so some crafty measures have to be taken.
There are two major strategies to carbonate beer:
The first is called “natural carbonation” or, if carbonated in a bottle, “bottle conditioning”. It involves inoculating a fluid-filled, air-tight container with a CO2 producing organism and culturing it with the correct food. Specifically, yeast in a high sugar environment will convert sugar into alcohol and CO2 (amongst other biproducts). The CO2 will eventually build up and saturate the space above the solution, and once a certain pressure is reached, diffuse into the liquid matrix.
The second strategy, which is much more common, is to skip the natural yeast process and artificially pump stored CO2 from a tank directly into the liquid. Continually pressurizing a sealed vat of liquid with CO2 will, over time, force carbonation. This is primarily what non-alcoholic beverage manufacturers do.
Many of the traditional, old-school brewers will carbonate with natural carbonation only. The modern-day craft beer industry, however, usually uses a mixture of the two. They’ll start with the yeast producing, natural carbonation and make the final tweaks with an external CO2 tank.
To help force CO2 into solution, many brewers use what is called a carb-stone, or carbonation stone. A carb-stone is a hollow tube with a fine mesh screen surrounding it. As CO2 is fed into the hollow tube, gas escapes through the tiny mesh holes into the liquid. The resulting CO2 bubbles are smaller, and thus, have less difficulty getting into solution.
Why the Bubbles?
As stated before, CO2 is not very liquid-soluble. Even after it bonds with the water molecules, it’s still in a delicate state. Any turbulence, temperature fluctuation, or pressure change can cause these interactions to break and release the CO2 back into the atmosphere.
The reason that the gas remains trapped in the beer, however, is because of the cohesive nature of water.
Water is a unique and fascinating compound. Within each water molecule, there is an unequal distribution of subatomic particles called electrons. The absence of electrons causes a positive charge in some areas of the molecule, whereas the presence of electrons in other areas causes a negative charge. The differently charged areas then attract and lock tightly into each other like a magnet, thus trapping the CO2 gas and preventing it from leaving.
The cohesion of water can be demonstrated when you fill up a glass to the brim. You’ll find that you can continue to pour even after the water has reached the edge—a little bulge or mountain will form at the top. Another demonstration is to let a paper clip float. Even though the paper clip should sink because it is far denser, the strong surface tension created by the interlocked water molecules will hold it up (as long as there is minimal turbulence).
We now know how beverages become carbonated and why the gas stays trapped in the liquid, but it still doesn’t explain what makes beer foam so distinguished. Why does beer have a thick layer of foam whereas soda pop only has tiny, thin bubbles that dissipate in seconds?
One word: protein!
Proteins are the building blocks of life. They are big, bulky strands that have the incredible ability to form complexes with one another.
In certain cases, proteins are also water soluble.
Beer is one of the few drinks that is both carbonated and high in soluble protein—there really isn’t anything like it in existence. For example, the most well-known high protein drink is milk, but the thought of it being carbonated is frankly quite disgusting. It’s the smooth texture of milk that makes it so attractive, and adding carbonation would only kill this defining feature.
So beer is unique in that it carries both protein and carbonation in substantial quantities (most beer for that matter).
Protein and Beer Foam
Suppose you’re a CO2 molecule in beer. The beer has been shaken, and your delicate chemical bonds with water are rapidly disintegrating. You begin your journey up towards the outside air, but you notice something unusual: you’re not moving as fast as you would suspect, and there’s a mild drag underneath you.
Once you make it to the top of the glass, the water layer around you is becoming much denser and stronger. In fact, as you try to break through, a thick mucous envelops you, and for once, you can’t move. This mucous is actually the strong matrix of interwoven protein chains, and it’s got you stuck. There’s no way out!
Pretty soon, more of your CO2 buddies pile up all around you, trying to break through the reinforced strands of protein-laden water. While a single bubble might not be able to penetrate through this shield, thousands of them will eventually force their way out. Nevertheless, it’s a veritable traffic jam at the top, which is basically the foam that you see when you pour a glass of beer.
Proteins vary in size, and some will withstand breakage better than others. The main takeaway here is that while the interaction between neighboring water molecules is strong, the molecules themselves are very small. On its own—or even with sugars involved—water usually cannot hold the gas for more than a second or two. The addition of large proteins promotes intermolecular forces that make it harder for CO2 to break free from the solution.
But then again, why does Coors Light have such little foam compared to Sierra Nevada? Or why does Budweiser have such wimpy bubbles compared to Guinness?
The answer is head retention.
Foam Positive and Foam Negative Character
While every beer has a certain degree of foam, it’s blatantly obvious that some have it more than others. Head retention is the beer’s ability to maintain a head of foam after the beer is poured and at rest.
Head retention is a balancing act: a fight between qualities that promote foam (foam positive) and those that discourage it (foam negative). Brewers are modern-day alchemists, trying to find the right recipe in order to turn stale beer into a foaming, fizzing wonder. The ingredients that they have to work with are as follows:
Foam Negative Qualities
Ethanol—an alcohol in itself—is the major ingredient in alcoholic beverages. It is strongly opposed to the foam layer. Ethanol disrupts the surface tension of water and weakens the bonds between water molecules, thus inhibiting a beer’s ability to form that thick layer that traps CO2 as it makes its way out. Unfortunately, ethanol is the one variable that beer can’t do without, so to compensate, brewers have to include foam-positive ingredients that can overcome the foam-negative influence of ethanol.
While beer has relatively low concentrations of lipids or fats, they can still wreak havoc on head retention. Lipids disrupt the foam-stabilizing interaction of iso-alpha acids (hop bittering acids) and protein. Without this interaction, CO2 bubbles merge and coalesce, making the gas particles larger and stronger and thereby allowing them to push through the surface and escape more easily.
Lipids mostly come from the type of barley malt used, with some contributions coming from hops. It is for this reason that many brewers will take great pains to find malt with exceptionally low levels of lipids. Those who use high-lipid malts will try to compensate with foam-positive brewing techniques.
There are surely countless more variables that are not included here, but suffice it to say that these two factors are the main reasons why many beers have low head retention.
Foam Positive Qualities
As mentioned above, proteins are the major contributors to foam and foam stability. Without them, the inherent foam-negative characteristics in beer would surely win out.
While barley contains dozens of types of proteins, only a select few make it all the way to the finished beer. Many of the proteins are insoluble and are filtered out during the brewing process.
To avoid overcomplication, these are the proteins that have the biggest known impact on foam stability:
Lipid Transfer Proteins (LTPs)
Lipid transfer proteins do exactly what their name implies: they transfer lipids. In the body, lipid transfer proteins are critical in transporting lipids where they otherwise could not. They are a necessary component of our ability to absorb nutrients.
Found in the original barley, some of these proteins can make it all the way through the high temperature soaking procedures. LTPs, however, aren’t conducive to head retention until they are irreversibly denatured or unfolded in the boil kettle. In exchange for its nutritional component, brewers gain an advantage of increasing the foam at the top of the beer.
Hordeins are the major storage proteins found in barley. They are usually not water soluble, and therefore, find their way filtered out before the finished product. Some, however, are broken down by proteases (protein degrading enzymes) along the way, making them suitable proteins for head formation and stabilization.
Protein Z is a major protein constituent of barley malt. There is little evidence to show that this protein, by itself, has a noticeable effect on head retention. There is, however, a good amount of evidence supporting the notion that these molecules work synergistically with hordein and LTPs to have an amplified effect on beer foam retention.
In the world of beer, bitterness is a critical component. Not only do the bittering properties of hops function as a preservative, but also as a necessary flavor balancer. Too much sweetness in a beer is overbearing, and too little makes the beer practically undrinkable. Hop bitterness is the ultimate balancer of malt sweetness and It is a constant battle to get the right amount of pleasure for the taste buds.
When added for bitterness, hops are placed somewhere early in the boiling stage of the brewing process. Acids (primarily alpha acids) are released from the hop resins and isomerized. Alpha acid isomerization is a minor, but impactful, shifting of the molecule’s orientation, morphing it into a more water-soluble, bitter version of itself. We call these isomerized alpha acids, or iso-alpha acids for short.
These iso-alpha acids are thought to promote foam stability by crosslinking with nearby foam-positive proteins, reinforcing the matrix formed around CO2 bubbles. This bond can become even stronger with the addition of cations (positively charged salts/metals) like magnesium, zinc and calcium.
- Non-Starch Polysaccharides (Fiber)
Starches are long molecules of sugar. The initial stages of the brewing process (malting and mashing) are primarily responsible for breaking these long chain sugars into simpler and smaller ones. Small sugar molecules, like glucose and maltose, can easily be metabolized by yeast and turned into ethanol and CO2 for a successful fermentation.
Non-starch polysaccharides (or fiber), on the other hand, are incapable of breaking down without the proper enzymes. Enzymes that are active only at a particular temperature ranges most brewers no longer use in the brewing process. As a result, much of the non-starch polysaccharides are filtered out during the filtering steps, but some make it into the final beer. These long-chained molecules can form complexes with proteins, making for a stronger foam matrix.
Bringing it All Together
Let’s look at a few beers to bring these concepts together.
Foam Negative Properties:
- Ethanol (4.2% ABV)
- Lipid quantity (hard to tell from a consumer perspective. Probably very minute amount)
Foam Positive Properties:
- Protein (Budweiser uses rice as a major part of the grain bill. Rice is significantly low in protein, so not much foam-positive character here)
- Hops (Coors light is a light American lager. Since the beer is already so dry (non-sweet), there really isn’t much need for a balancing hop bitterness. Coors light has approximately 10 International Bitterness Units (Very low).
- Non-starch polysaccharides (fiber) – (Again, hard to tell from a consumer standpoint. Probably pretty negligible amount of beta glucans in the finished beer.
Summary: While the foam negative qualities aren’t staggering, neither are the foam-positive ones. So, while Coors light is heavily carbonated and pretty low in alcohol, there aren’t a lot of foam positive qualities to counteract the inherent negative ones. This makes for a present, but quickly dissipating, foam layer.
Sierra Nevada Pale Ale:
Foam Negative Properties:
- Ethanol (5.6% ABV)
- Lipid quantity (hard to tell from a consumer perspective. Probably very minute amount)
Foam Positive Properties:
- Protein (Sierra Nevada uses practically all barley malt. Barley has approximately 9-13% protein and therefore can have a substantially positive effect on head retention).
- Hops (Sierra Nevada Pale is a pale ale. Pale ales are known for their moderate level of hop bitterness. Clocking in at 38 International Bitterness Units (IBU), the iso-alpha acids can also have a positive impact on head retention.
- Non-starch polysaccharides (fiber) – (Again, hard to tell from a consumer standpoint. Probably pretty negligible amount of non-starch polysaccharides in the finished beer)
While Sierra Nevada Pale has more ethanol (foam negative quality) than the Coors light, it also has far more protein and iso-alpha acids (foam-positive qualities) in the finished beer. Comparing the two beers, Sierra Nevada will have a thicker head that will remain intact for much longer.
The point of this comparison isn’t to diminish one beer or the other, but to reveal the true nature of why beers behave the way they do.
Well, there you have it. The unique and mesmerizing layer of beer foam broken down into its constituent parts. While I admit we’ve discussed some complicated material here, any experienced brewer would agree that I have vastly oversimplified the subject. The real challenge comes with putting this knowledge to work; creating the ideal environment for a head of foam that accentuates the beer style at hand.
So next time you pour a beer into a glass, leave a full inch of foam at the top. Take a look at the size of the bubbles, the thickness of the glossy mucous around each bubble and the rate at which those bubbles disappear. I have no doubt you can make some solid inferences into the ingredients that went into that beer.
Happy drinking folks,
Creator of Robust Kitchen
Special thanks to Charlie Bamforth for his relentless search to uncover the mysteries of this magnificent drink. Earning the worldwide title “Pope of Foam,” doesn’t even begin to illustrate the impact this man has had on the beer industry. Thanks Charlie
Bamforth, C.w. “Progress in Brewing Science and Beer Production.” Annual Review of Chemical and Biomolecular Engineering, vol. 8, no. 1, 2017, pp. 161–176., doi:10.1146/annurev-chembioeng-060816-101450.
Bamforth, C.w. “Brewing and Brewing Research: Past, Present and Future.” Journal of the Science of Food and Agriculture, vol. 80, no. 9, 2000, pp. 1371–1378., doi:10.1002/1097-0010(200007)80:93.0.co;2-k.
Evans, D. Evan, and Charles W. Bamforth. “Beer Foam: Achieving a Suitable Head.” Beer, 2009, pp. 1–60., doi:10.1016/b978-0-12-669201-3.00001-4.
Okada, Yoshihiro, et al. “The Influence of Barley Malt Protein Modification on Beer Foam Stability and Their Relationship to the Barley Dimeric α-Amylase Inhibitor-I (BDAI-I) as a Possible Foam-Promoting Protein.” Journal of Agricultural and Food Chemistry, vol. 56, no. 4, 2008, pp. 1458–1464., doi:10.1021/jf0724926.
Photo on top of post by Michael