The Crown of the Pint: Unlocking the Secrets of Foam
Ah the mesmerizing cascade in a freshly poured pint of Guinness, the massive rocky head on a Belgian Golden Strong, and the tight fleeting white collar on top of a cask ale. All so different, but the ties that bind…… Foam.
Beer foam is so important, yet so misunderstood. I’m sure we’ve all been served a glass of draught beer with a good 3-4cm of head and felt ripped off. You paid for beer, not air, right? How about when someone opens a beer and pours it ever so gingerly down the side of the glass, raising nary a bubble. Of course there is the careless hard pour in an irresponsibly small glass, resulting in a glass full of foam and the desperate search for paper towels. Neither are ideal. I’ve had occasion to witness, more than onceI might add, a person slide a finger down the crease of their nose and insert it into the foam like some type of wizard, to eradicate the offending head. The oils from your skin will disrupt the surface of the bubbles by the way. It works, but it’s madness.
The great foam divide does not reside in the mind of the drinker alone either. Different beers will raise a different head, and different beers in different regions have different customs. In Belgium, a huge rocky head of foam is the standard for many beers, and the glassware is designed to accommodate. In North America, a 2.5 cm cap is normal, but subject to so much complaint that a 1cm dusting of bubbles is not an abnormal sight. Some Czech beers are served with a “side-pull” faucet, allowing the server to control the foam for different pours. The Snyt pour has a 3:2 foam to beer ratio, the Mliko is about a 3:1 foam to beer ratio, while the Hladinka is about half and half. That’s a lot of foam!
Don’t leave with the impression that these divergences are constrained by political borders either. In the north of England for example, a larger head is preferred, whereas in the south, a much more subtle collar of foam is the norm. It’s called the North/South divide. Seriously.
However you choose to drink your beer is your business. I do however want everyone to have the best experience possible though. The foam atop a well poured beer goes far beyond beautiful vista. Foam improves the mouthfeel of a beer, and helps trap in some of the volatiles, allowing a slow release as it collapses, forever refreshing that wonderful bouquet aromas.
So what is foam?
Foam is essentially gas trapped inside beer. That gas is CO2. In nitrogenated beers, like Guinness, both CO2and N2 (nitrogen) bubbles form. The structure of the bubble is paramount, and it consists of a matrix of proteins and alpha acids, along with some metal ions. The hydrophobic portion on the inside, the hydrophilic portion on the outside. Hydrophobic means afraid of water and will not bond with it, like oil and vinegar. Hydrophilic meaning it loves water and will form hydrogen bonds, like sugar. The stronger this matrix, the more resilient the bubble and the more stable the foam. The proteins involved are Protein Z, LTP1 ( Lipid Transfer Protein 1) and hordein. All three are found in malt. The alpha acids, specifically isomerized alpha acids come from the hop additions. Metal Ions can be found in the water, the malt, the hops, or added by the brewer. Yeast can also release certain trace metals during fermentation.
Protein Z
Protein Z is an albumin, meaning it is a water soluble protein. It has the highest surface viscosity and elasticity potential of all the malt proteins. The maillard reactions during both malting and the boil tend to glycosylate this protein, which means it has a sugar unit attached. This glycosylation serves to further enhance the elasticity properties. Most modern, high diastatic malts contain high amounts of protein Z, making up around 1.5-2.5% of total weight. The actual amount is, to a point, dependent on the barley variety, but the degree of modification is the major determining factor. Malt with a Kolbach Index, (the ratio of total soluble protein to total protein), of 40% or higher tends to have high levels of protein Z. Inversely, malt with a Kolbach Index of lower than 40% tends to have less protein z.
Hordein is the major storage protein in barley and its levels are fairly consistent across most varieties. Hordein is fairly insoluble in its native form and needs to be broken down via hydrolysis by various enzymes. Some hordeins can construct foam with more ease than protein Z, but by sacrificing stability. A mash schedule that includes a protein rest will accomplish higher hordein levels, although it is prudent to note that the foam forming hordeins are the same size as haze forming hordeins. Attempting to promote hordein specific foam will also result in more haze in the finished beer. Not Ideal if a clear beer is desired.
LTP1
Lipid Transfer Protein 1 is found in the aleurone layer of the barley during the germination process. In its original form it is quite good at forming foam, but is completely unreliable when it comes to stability. The boil will denature it to some degree, and this denatured form binds easily with protein Z and provides adequate foam stability. In its native form it binds with certain lipids, disallowing them to contribute to foam collapse. A careful balance therefore is ideal.
Other proteins
A few other lipid binding proteins can be found in malted grains, namely the purindolines from wheat and the hordeindolines from barley. Both being highly hydrophobic, they ought to be excellent foam promoters. Sadly they don’t generally survive the brewing process, rendering them absent from the protein matrix. They do however prove their utility by binding with certain lipids along the way, partially removing the threat to the precious foam in the finished product.
Wheat
It is important to talk about wheat. Wheat has a higher protein content than barley, as illustrated by that shiny haze in weissbiers and the like. This haze can be good or bad depending on your intentions, but of course we’re talking about foam, not haze. The proteins in wheat tend to be of a higher molecular weight, also important for foam, and the higher levels of contributed beta-glucans will increase the viscosity of the beer and reduce the rate of drainage from the foam. Interesting eh?
Alpha acids
Isomerized Alpha acids are big time foam promoters. Alpha acids isomerize in the boil, so the early additions are good for foam. There does seem to be a positive correlation between higher IBUs and better foam as well. By way of hydrogen bonding, the iso-alpha acids facilitate the cross linking of the malt proteins. Molecular structure is very important here as Isohumulone is a much more effective stabilizer than cohumulone. Luckily, a higher humulone to cohumulone ratio is found readily in high alpha hops. The pre-isomerized, light stable, hop extracts (the ones that allow beer to reside in clear bottles without succumbing to skunking) contribute the best stability. Be warned however, that sometimes the foam becomes too stable and can result in some peculiar and unappealing visual effects.
Metal ions
A crew of metal ions can positively affect foam. The ion hydrides form hydrogen bonds between the hop acids and proteins, stabilizing the structure even further. The metals in question include manganese, aluminum, nickel, tin, zinc, magnesium, calcium and barium. This is all great, except that most of these precipitate out, and so their contribution is purely academic. Not only that, but some are particularly negative for flavour. Iron and copper are notorious for accelerating staling. They can also give the beer a flavour resembling pennies, or blood. Yum. Aluminum appears to be the best foam stabilizer, but as it is virtually impossible to get into solution, it too remains mostly fantasy. Calcium provides no off flavours and is essential for yeast health. It also contributes to the stability of enzymes in the mash.
Lipids
A good chunk of the short-chain fatty acids found in beer are synthesized by the yeast. Fortunately, these aren’t as detrimental to foam stability as the longer chain fatty acids. The biggest lipo-villains in foam stability are the hydroperoxides which are formed by the enzyme lipoxygenase from linolenic acid, native to malt. Hydroperoxides are highly hydrophobic and will cause the bubble wrapping film to rupture. They accomplish this disruption by competing with, and displacing proteins in the matrix. The lipids have poorer elasticity, breaking more easily and resulting in rapidly collapsing foam. Remember the nose-wizards from earlier on.
Brewing process
Starting with the grist. Barley, wheat and rye all contain foam positive proteins while adjuncts like corn, rice, and various sugars do not. The closer to all malt the grist gets, the better the foam should be. Interestingly, barley cultivated in wetter regions tends to be higher in LTP1. Wetter regions provide for favourable conditions for infectious molds and fungi, and as LPT1’s primary role is protective, it stands to reason. LPT1 binds to lipids, such as those found in cell-membrane sterols of the offending pathogens, guarding against infection, and so barley in a more stressed environment will produce more LPT1.
The malting process also determines foam quality to a degree. Heat denatures proteins, and so high colour malts are not good promoters of foam. The melanodins in darker malt however, are positive influencers in foam stability. These melanoidins, which are the result of a thermal reaction involving sugars and amino acids are a variety of high molecular weight compounds including crosslinked sugars and proteins, polyphenol adducts and nitrogenous polymers. Melanoidins possess both a hydrophobic and hydrophilic region allowing them to attach to the wall of the foam bubble and reinforce the matrix. They can also cross-link with proteins, and bind up lipids. The helpfulness of the melanoidins however, can hardly make up for an extensive lack of protein.
The Mash
The mash is really where the brewer has some control. Using both highly modified malts and a higher mash temperature tends to produce better foam. Hordeins can break down in the mash into smaller proteins, whereas protein Z and LPT1 are unaffected for the most part. At around 65°C more of the hordeins survive. Multistep mashes can equalize hordeins in highly modified malt, but the longer rests can degrade other form stabilizing proteins and produce excess FAN ( Free Amino Nitrogen). Although FAN is essential for yeast health, and its existence alone is not necessarily foam negative, the presence of excess FAN indicates the breakdown of larger foam positive proteins into smaller amino acids and peptides. These smaller molecules, just like lipids, will compete for space in the bubble film and cause the overall structure to be weaker and less elastic. Another consideration in the mash is the enzyme Lipoxygenase. Lypoxygenase is most active between 35°C and 60°C. The enzyme breaks down lionenic acid into di- and trihydroxyoctadecenoic acids, the hydroperoxides discussed above. The tricky part is that high adjunct grists will require a mash at this temperature range to promote sufficient enzymatic activity. In this case it is prudent to avoid aeration, which can result in rapid staling.
The Boil
During the boil a lot of proteins and lipids are removed via denaturing and coagulation, reducing the magnitude, but leaving the proportions for the most part intact. High pressure boils are especially problematic as they can crush LPT1 to unacceptably low levels.
Fermentation
Conditions during fermentation are key. Low levels of FAN (free amino nitrogen), high alcohol levels, high levels of dissolved CO2, and hydrostatic pressure all cause yeast to produce more proteinase A. Proteinase A is an enzyme that will break down larger proteins into smaller ones. Specifically, it will attack LPT1. Yeast that is stressed will also produce more esters, more fusel alcohols, acetaldehyde, and VDKs. These are byproducts we generally want to suppress anyways, but in relation to foam, the fusel alcohols are the main culprit here. Recall that alcohol is foam negative after a point as it reduces surface tension. Fusel alcohols contain more carbon atoms than ethanol, and are therefore even more damaging to foam. The other metabolic byproducts aren’t inherently foam negative, but rather an indication of a higher probability of the presence of excess fusel alcohols and proteinase A.
Nitrogen Gas
Nitrogenated beers have some unique properties when it comes to foam. Nitrogen forms smaller bubbles that are less buoyant. As a result, they rise more slowly, allowing more time for the surface active compounds to crosslink. Nitrogen is less soluble than CO2 and so the gas is in less of a hurry to dissolve back into the liquid.
The Pour
The Czech side-pull as we discussed can generate a crazy amount of foam. Lukr is the company that makes these faucets, but I’m sure there are knockoffs. Beer Engines, which are hand pump mechanisms for pulling beer from cask to glass without the use of applied pressure, have a sparkler integrated into the nozzle, which forces the beer through a series of tiny holes. This serves to drive the CO2 out of solution and to form a foamy head. In the south of England, the sparkler is often absent. Nitro faucets function in much the same way, a restrictor plate in the nozzle forces the beer through tiny holes creating a head of both nitrogen and CO2 filled bubbles.
At home, one can simply hold the glass at 45° angle and pour down the side of the glass until ⅔ full, and maneuver the glass back to upright and finish pouring down the middle of the glass, using height to manage the foam within your preferred range. With canned nitro beers, one can simply invert the can into the glass and “hard pour” it.
So, remember, foam is an essential part of most beers. The foam itself is gas trapped inside a film of beer, consisting of a matrix of proteins, iso-apha acids, and metal ions. Smaller bubbles create more stable foam and slower drainage. Lipids and higher levels of alcohol are bad for foam. For brewers, the grain bill matters, the mash matters, the hop additions matter, the boil matters, the health of the yeast matters, and the level of alcohol matters. Bar managers, please train or have your staff trained in proper pouring techniques. For drinkers, use a clean glass, wash it with lipid free detergent if possible, keep greasy food residues away from your pint, put some thought into your pour, and enjoy the beauty of foam.