Basics of Foam and Head Retention
Foam is a dispersion of a relatively large amount of gas in a relatively small amount of liquid. It doesn’t happen spontaneously—it requires some energy by either agitating the beer (e.g. shaking or stirring) or creating a nucleation site (e.g. scratch on a glass or an engineered device) that allows bubbles to form and rise in the beer, sometimes referred to as “beading.” Carbonation is only one aspect of a good, frothy head. Why doesn't carbonated water have a head? Or cider or champagne? Beer is a biochemically complex beverage. Within it are compounds that promote beer foam formation and stability and others that detract. Beer foam chemistry and physics can be disturbingly complex, so keep the following in mind. If the promoters win, you have foam. If the detractors win, you have flat beer. There are two aspects of good beer foam: formation and stability. Beer foam is formed by the interaction of various malt proteins, isomerized alpha acids, metal ions, and carbon dioxide (and beer). The three main proteins (actually polypeptides) are protein Z, hordein species and lipid transfer protein 1 (LPT1). If you think about the structure of foam being like building a skyscraper, protein Z and hordeins are the girders, LPT1 are the struts, isomerized alpha acids are the connecting plates and metal ions are the rivets. In general, the foam of beer brewed from highly modified malts benefits from high mash temperatures. When the malt is mashed in at temperatures above 150°F (65°C), a greater proportion of foam-promoting hordeins survive into the beer. The iso-alpha acids and metal ions act to connect the proteins and bind the structure together. Keeping good head is known as head retention. One of the most important parts of maintaining foam is beer viscosity. Viscosity describes the speed of flow of a liquid. For example maple syrup pours slowly, so we consider it a very viscous liquid. When a beer is first poured, the foam is considered to be "wet" and contains a relatively high proportion of beer. As the beer drains from the foam, the foam dries out, and the bubbles begin to collapse or coalesce. The coalescence of the bubbles is driven in large part by the surface viscosity of the beer, which is partly driven by alcohol content and the type of gas in the bubble. Highly stable foams will exhibit more lacing on the sides of the glass. The stability of the foam depends upon the relative amounts and types of the various components listed above, the surface viscosity and on any foam inhibiters present. The primary foam destabilizers are lipids. Lipids are a broad class of fat-soluble natural compounds, including fats, oils, waxes, sterols, glycerides and fatty acids. If you are wearing lipstick or eating a bag of chips, the head on your beer will dissipate quickly. Gas in an unopened bottle of beer is in equilibrium. This means that at any given temperature, the gas pressure in the headspace and the amount of gas dissolved in the beer are constant. When you open a bottle of beer, you create an imbalance between the amount of CO2 in the beer and the amount of CO2 in the atmosphere above the beer. The amount of CO2 in the air is only about 0.2 percent of the total atmosphere, compared with 98 percent in a beer headspace. To restore equilibrium, the CO2 bubbles will leave the beer until balance is restored. That is why an open beer will always go flat over time.
Choose the Right Malt: Malts high in proteins and dextrin enhance the body and head retention of beer because the proteins act as a structural component in foam. The malt-derived proteins are typically hydrophobic (water-hating), causing them to move up towards the foam where they encounter other positive foam stabilizing substances, like those from hops. However, high levels of proteins and dextrins can interact with tannins and compromise clarity, provide more nutrients to spoilage microorganisms, and mean less fermentable extract per pound of grain. Finding a proper balance is the challenge. Examples of foam-enhancing malts include crystal malts (e.g. Carapils, Carafoam, Caramel malts), as well as wheat malt. There is also some belief that dark malts (e.g. Chocolate) help improve foam stability because of their high levels of Melanoidin, a protein polymer which is formed when sugars and amino acids combine.
Adjust Your Mash Schedule: Head retention depends on the level of proteins in your wort. So, any step in the mash that breaks down these proteins will negatively affect your beer’s foam stability. For example, the typical protein rest at 120 – 130°F (49° to 54°C) is used to break up proteins which might cause chill haze and can improve head retention. However, this rest should only be used when you use moderately-modified malts, or fully modified malts with over 25% of unmalted grain (e.g. flaked barley, wheat, rye, oatmeal) because it will break down larger proteins into smaller proteins and amino acids, thereby reducing foam stability. In contrast, fully-modified malts (most of what you’ll buy at a homebrew shop) have already made use of these enzymes and adding a protein rest will remove body and head retention. To improve head retention, you would want to favor a full bodied, higher temperature mash, with a main conversion in the 155 – 160°F (68 – 71°C) range, and avoid intermediate protein rests.
Hops: The bitter substances from hops, isohumulones (a form of alpha acid), will help hold the bubbles together. These hydrophobic substances help form the framework for head formation. The iso-alpha acids are thought to facilitate cross-linking of the malt proteins via hydrogen bonds. However, this interaction doesn’t happen right away. You’ll notice when you pour a beer, the beer foam is wet and sloppy but changes to almost solid over a few minutes, in which the foam can adhere to the glass surface, otherwise known as “lacing.” In other words, the longer you wait to drink your beer, the better your beer foam and lacing on the glass. Overall, highly-hopped beers should have better head retention, but remember to maintain a malt-bitterness balance. Isohumulone is a much more effective foam stabilizer than isocohumulone, and so it stands to reason that today's high alpha hops, which were bred for low cohumulone percentage of alpha acids, have better foam stability than older hop varieties. Ironically, the light-stable hydrogenated pre-isomerized alpha acid extracts have the greatest foam stability by far. The use of these extracts in beers will yield whipped egg white type foam that will last until it is time to wash the dishes. Metal ion hydrides form hydrogen bonds between hop acids and proteins and stabilize the molecular structure. Zinc additions of as little as 2 ppm have been shown to have significant increases in foam stability. Unfortunately, most of the metal ions in the wort are lost to the trub during wort boiling, and the only ion that doesn't impart off-flavors in the list above is calcium
Nitro Mix: Since it is not very soluble in liquid, N2 tends to leave the beer and go directly into the foam. However, keep in mind the idea of equilibrium. The atmosphere is 75 percent N2 as opposed to 0.2 percent CO2, so the N2 gas in the beer foam is not escaping to the atmosphere at any great speed. Consequently, a nitrogen foam is happiest just sitting on top of the beer. However, nitrogen will alter the character of the beer, giving it a creamy, thick mouthfeel and will also take away from some of the beer’s bitterness. Also bear in mind the percentage of each gas will depend on the style of beer you’re serving. Make sure to check what percentages work best with a particular style.
Glassware: The glass you choose can influence head formation and head retention. A tall, narrow glass is a good choice because it minimizes exposure to ambient air, and reduces the ability for CO2 to escape. If you have a large opening on your glass, there is more air exposure, which allows CO2 to escape more easily. Covering the beer allows for a buildup of pressure within the headspace in the glass and prolongs the head. For example, many Bavarian wheat beers and Pilsners are served in tall narrow glasses to maintain head formation, retention and overall beer presentation. This might seem obvious, but many beer drinkers forget about glassware. Your glassware should be “beer clean.” Follow proper cleaning procedures for beer glassware and avoid oil or grease. These substances will occupy space on the surface of the beer and prevent bubbles from forming. So next time your munching down a burger or putting on lipstick, make sure to wipe it off! Temperature: Also, anything that increases the viscosity (thickness) of the beer should prevent foam from disappearing. Since viscosity increases as temperature decreases, colder beer has better foam stability. So make sure you pour your beer at a chilled temperature.
Fermentation: If we assume that healthy yeast produce a healthy fermentation and thereby produce a typically high quality foam, all else being equal, then factors that stress the yeast will also affect the foam. Yeast produce a variety of byproducts and waste products, including lipids, during fermentation, and produce even more when they are stressed. Common examples of stressed yeast byproducts are elevated levels of fusel alcohols, esters, acetaldehyde and vicinal diketones (VDKs). Yeast also excrete an enzyme called proteinase A, which particularly effects proteins around 10,000 Daltons in size, including LPT1. It has been shown that stressed yeast produce higher levels of proteinase A and lower levels of foam stability as a result. Factors that are known to contribute to increased proteinase A secretion are low FAN levels, and high levels of alcohol, dissolved carbon dioxide and hydrostatic pressure. Proteinase A will continue to be secreted by yeast after fermentation is complete, and will remain active in the beer, even if the yeast is filtered, unless the beer is pasteurized. Experiments have shown that foam active proteins will continue to be degraded in the bottle and that the enzyme is more active at room temperature than at refrigerated temperatures. So store your beer cold if you want to preserve the foam. Finally, if the yeast are stressed to the point of autolysis, the rupture of the yeast cell will release a variety of foam degrading lipids and enzymes into the beer, greatly impacting foam stability and flavor.
Better Beer Foam Tips:
- Get your carbonation right.
- Cover your drinking glass
- Choose malts with high protein levels (e.g. wheat malts).
- Avoid low-protein adjuncts (e.g. corn, rice, sugar).
- Wheat malts and flaked barley will increase head retention.
- Increase your viscosity – mash higher and use dextrin malts
- Bittering hops help with head formation.
- Sanitize and rinse your equipment well.
- Depending on the grain, mash at high enough temperatures.
- Nitrogen- CO2 gas mix can help with foam stability.
- Heading compound can increase beer foam.
- Avoid fats and oils. Watch out what is on your lips or the lip of the glass!
- Make sure glassware is beer clean.
- Carefully measure priming sugar.
- Serve beer chilled.
- Don't use protein rests on well-modified malts.
- Avoid overlong boils and avoid excessive thermal loading.
- Malt extract brewers should avoid concentrated boils and diluting in the fermenter. (Use the "extract late" method instead.)
- avoid stressing the yeast during fermentation
Strategies for the right foam
Awesome Foam -- for the brewer in search of drop-dead head
- Brew all-malt beer (no adjuncts such as rice, corn, or sugar); made from all-grain brewing.
- Use foam-building ingredients such as wheat or unmalted barley.
- Carbonate to a slightly higher level or use a nitrogen stout gas.
- Use scrupulously clean glassware.
- Leave no chemical residue on any brewing equipment, beer bottles, beer glasses, or body parts.
- Foam suffers from the thought of some chemicals.
- Brew hefe-weizen, wit beer, and stout (dispensed through a stout tap). These three styles have naturally awesome foam.
Good Foam -- for the brewer who wants a good head without jumping through hoops
- Brew all-malt beer (no adjuncts such as rice, corn, or sugar) made from all-grain brewing. Extracts lose much of the foam-building proteins during processing.
- Use a generous addition of hops. Hop bittering acids help foam cling.
- Properly carbonate so the foam has enough gas to form correctly.
- Thoroughly rinse your equipment but don't obsess -- no-rinse sanitizers are acceptable for the brewer in search of good foam.
Bad Foam -- some of the most common culprits to avoid
- Excessive use of adjunct ingredients. Ever see an American-style lager with awesome foam?
- Very, very little hops. Light beers with low hopping rates have bad foam!
- Fats and oils -- oats, coffee, chocolate, potato chips, and the like all contain fats and oils. Avoid the use of such ingredients if you want good foam. If you want chocolate porter more than foam, then don't worry -- you may get lucky and have good foam.
- Foaming cleaners and sanitizers. Detergents destroy beer foam even though they are foamy themselves. These compounds must be rinsed off of all brewing equipment.
- Foaming the beer before drinking. Once a foam forms, the foaming compounds do not form foam a second time.
- Flat beer doesn't foam unless it is dispensed using special taps (beer engines for example).
The information here was gathered and collated from multiple online sources. Several BYO articles and AHA articles: