Foam and Head Retention: The Science of Why Some Pours Behave Better

A head holds because beer is full of dissolved protein, and bubbles rising through it collect that protein on their surface and bind to it, which is why a beer built on a high protein malt keeps a foam while a stripped lager does not. Everything that survives contact with a glass decides how long that foam lasts, and the variables are few enough to hold in your head, namely the height you pour from, whether the glass is clean, how warm the glass is, and what you ate before you drank. The most reliable of them is also the one most people ignore, since a glass washed with detergent and not rinsed under the tap will never hold a foam no matter how well the beer is poured.

Why A Head Exists At All

Foam is not carbonation itself but a structure built out of it. When a bubble forms on the inside wall of a glass, it rises, and the rising surface collects dissolved high molecular weight protein from the beer, which unfolds at the gas to liquid interface and re-folds into a film. That film is what stops the bubble from merging with its neighbour, and a bubble that merges becomes a bigger bubble, which rises faster, and the loss of the film is the point at which the head collapses. The other half of the structure is hop resins, since the alpha acids in the beer are surface active and get dragged to the interface along with the protein, and this is the mechanism behind the well known observation that a heavily bittered beer holds a head remarkably well while a mild one often will not.

This is also why a head is worth protecting rather than skimming off out of habit. The film of gas and protein sitting on top of a beer is a barrier between the liquid and the air, and it slows the carbon dioxide escaping from the glass and the oxygen coming in. Drink straight from the bottle and you lose the head, the beer goes flat faster, and the aromatics that were in the head space are gone with it. The most striking version of this is an imperial stout, where a head left in place for a few minutes measurably changes how much of the roast and chocolate aroma you can still smell when you come back to the glass.

What The Glass Is Doing

Glass is not a passive container, because a clean glass has a charged surface that attracts the protein film and a dirty one has an oily surface that repels it, and that single difference accounts for most of the bad pours people complain about. Detergent, fat from a fingerprint, and a trace of dishwater all leave a film that stops the bubble wall from adhering, so the gas escapes without building structure. Rinse the glass in cold water without detergent after washing it and the problem disappears. Beyond cleanliness, the shape of the glass controls the rate at which the foam survives, since a narrow taper at the bottom gathers the bubbles up into a tight column where they press against each other and hold, while a very wide bowl spreads the same liquid over more surface and gives the head nowhere to sit.

Factor	What it changes	Which direction you want
Glass washed with detergent	Blocks protein adhesion on the inner wall	Rinse without detergent
Fatty or oily glass	Same effect, worse and harder to see	Never pour into one
Bubbles made in a clean glass	Adhere, forming a stable film	What you want
Bubbles made in a dirty glass	Bounce off, merge, and escape as air	Why the head never forms
Nitrogen or added gas	Very small bubbles, creamy and dense	Creamy but heavy mouthfeel
Carbohydrate or malt addition	Raises dissolved solids, thickens the liquid	Improves hold and body

How To Pour One That Behaves

The pour that people call correct in a taproom is really a two stage move, and each stage is doing a different job. Tilt the glass to about forty five degrees so the beer runs down a clean wall rather than onto the bottom, and start low, letting the liquid build a shallow pool in the base while the first bubbles are still large. Then raise the glass towards vertical and pour the remainder down the middle in one steady stream, which drives the coarse bubbles out through the foam and leaves the finer ones behind. The result is a head that is dense, tight to the glass and a centimetre or two thick rather than a large, loose cap of half-inch bubbles that collapses before you have picked the glass up.

The other half of technique is temperature, both of the beer and of the glass. A warm glass gives you a thin head because the gas comes off faster through the warm wall, so a glass taken out of a warm cupboard in summer will underperform the same beer poured into a cold one. A cold glass taken straight from a fridge also has condensation on the inside, and water on the wall is not a neutral surface, so wiping the inside of the rim is worth doing even on a cold day. Leave the glass out for a moment so the wall warms toward the beer rather than freezing it, and you will find the head holds noticeably longer than it did straight from the fridge.

What The Beer Itself Brings

Two things in the liquid decide how much structure there is to build with, and neither is adjustable by the person drinking. The first is the concentration of dissolved high molecular weight protein, which comes mainly from a grist with a decent share of malted wheat or a light kilned malt, and it is why a hefeweizen, an English bitter or a nitrogen-charged stout will hold a head far longer than a stripped pilsner from the same brewery. The second is dissolved carbohydrate, since sugars and dextrins thicken the liquid and slow the coalescence of small bubbles into large ones. A brewery that wants a stable head therefore has three levers, which are the malt bill, the addition of a hydrolysable starch or sugar syrup, and the choice of gas.

The choice of gas is the one that produces the biggest visible difference and the one that changes the beer rather than the foam alone. Carbon dioxide forms bubbles quickly, and they rise in a lively, aggressive column that looks like a sparkling wine. Nitrogen dissolves far more slowly and forms much smaller bubbles, so a nitrogen charged stout pours with a dense, smooth, almost solid-looking head that sits like a cap and barely moves, and the drinking experience is creamy and dense rather than sharp. Nitrogen is not a substitute for carbonation in every case, because it does not carry the same crisp bite on the tongue, and a brewery that wants both uses a blend. If you have never tasted the same stout on one and then the other, the difference is the most immediately obvious thing about head retention that you can find in a glass.

The Part You Control, Which Is Not The Pour

Fat is the enemy of a good head, and this is the part that surprises people, because the effect is immediate and it is nothing to do with the beer. The lipids in butter, oil, cheese and rich meat coat the inside of the glass in a film that behaves exactly like detergent, and a glass that has held a slice of pizza has no chance of holding a foam. It also coats the surface of the liquid itself, which stops the bubble film forming, so the head collapses faster and the beer arrives in the mouth with less aromatic surface to smell. Waiting a few minutes for the glass to be rinsed by the beer, or drinking from a wider glass where your mouth does not touch the rim, both work around it.

• Rinse every glass after washing it, with cold water and nothing else, since a single trace of detergent is the most common reason a head will not form.
• Never drink from a glass that has held fatty food, and if you have, rinse it before pouring into it rather than assuming the pour will save the head.
• Pour down the side of a tilted glass and then raise it, which gives a tight, dense head rather than a loose cap of large bubbles.
• Let the glass reach the temperature of the beer, and wipe the inside of the rim where condensation collects.
• Choose a shape that tapers, since a narrow base gathers the bubbles into a column and gives the foam something to hold onto.
• Do not skim the head off, because it is the barrier that keeps the carbonation and the aromatics in the glass.

Read The Head Before You Drink

Once you know what the foam is made of, it becomes a useful diagnostic rather than a matter of taste, and you can read a beer before you have swallowed any of it. A dense, tight, off white head that stays put suggests a beer with body, with protein from the malt bill and with enough residual carbonation, and it usually correlates with a beer that will also have a good mouthfeel. A large, coarse, fast-vanishing foam that leaves a ring on the glass usually means a thin, highly carbonated beer, which is not a fault but does mean the aromatics are escaping quickly. A head that forms and then flattens to a thin lace across the bottom within a minute is telling you the liquid is thin and the glass is dirty, and that is a pair of facts you can act on the next time you are ordering.

None of this is decoration. The head is the part of the beer that is still gas, protein and aroma rather than liquid, and it is the reason a glass of well poured stout on a cold day in a Victoria pub in December tastes like more than the same beer poured badly in a warm room in July. Our piece on bottle conditioning covers where the gas in that head came from in the first place, and if you want the vocabulary for what you are smelling once the foam has settled, the tasting glossary will get you through it.

The Maillard Reaction: Where Beer Colour and Roast Flavour Come From

The Maillard reaction is the chemical process that turns raw malt into the brown and black colours and the roasted, nutty, sometimes coffee-like flavours that dominate dark beer, and it happens whenever amino acids and sugars are heated together. Kilning malt is the first and largest application of it in brewing, since a green malt is pale and grassy while a heavily kilned malt is brown and tastes of toast, and the same reaction runs a second time in your boil and again in the kettle or the brewing vessel, adding more colour and more of the bready, biscuity character underneath. It is also the reason colour and roast are two separate things in beer, because the reaction produces a whole family of compounds at once, and the ones that give a stout its tarry, coffee and cocoa depth form at different temperatures than the ones that give a brown ale its toffee sweetness.

What The Reaction Actually Is

A reducing sugar and an amino acid meet under heat, and instead of staying separate they begin a long chain of rearrangements that ends in a large brown polymer with a nitrogen group hanging off it. That polymer is what food scientists and brewers call a melanoidin, and the first, lighter members of the family are the ones that give bread crust and roasted malt their colour. The reaction is not a single step but a cascade, so it never finishes and it never resolves into one compound, which is why two malts kilned to the same colour by two different kilns can taste noticeably different. Duration, temperature, moisture and the ratio of protein to sugar all move the result, and a kilned malt held longer at a lower temperature goes further down the same path than one flashed quickly at a higher one.

Where It Happens In The Brew House

There are three places in a brewery where the reaction is doing work, and each one adds a different layer rather than repeating the same note. The first and largest is the kiln, where green malt is dried and then held at temperatures high enough to drive off water and to push the reaction forward, and the colour of that malt is set there and can never be added back later. The second is the mash, where the malts sit in hot water and enzymes are at work, and while the mash is not hot enough to roast anything, the extracted melanoidins and the caramelised sugars formed in the kiln dissolve into the wort, so the wort arriving in the kettle is already coloured before anything else happens. The third is the boil, where wort held near its boiling point drives more of the reaction forward and produces the Maillard compounds that read as bread crust, toffee, and dried fruit rather than as plain roast.

Specialty malts are simply the kiln pushed further along that path, and their names describe how far. A kilned amber malt is taken just past the pale stage into light biscuit and caramel territory, a crystal malt is taken much further and has a strong sweet toffee note that survives into the finished glass, and a roasted barley or chocolate malt is taken far enough that the reaction has produced something close to coffee and dark cocoa. The further a malt is pushed, the less of its own grain character remains, so the very darkest malts contribute colour and roast rather than a malt flavour, and a well built dark beer uses them as a background against a base of pale malt that still supplies the bread and biscuit underneath.

Malt or step	How far the reaction has gone	What it contributes
Pale malt, kilned lightly	Early, pale golden to light amber	Bread crust, biscuit, a neutral base colour
Amber malt	Moderate, amber to copper	Toffee, caramel, a round mid-tone colour
Caramel or crystal malt	Advanced, deep amber to brown	Sweet toffee, dark fruit, real body
Roasted barley	Far advanced, brown to black	Coffee, toast, dark grain, no fermentable sugar
Chocolate or black malt	Nearly complete, black	Cocoa, char, bitterness, colour only
The boil, on any wort	A further increment in all of them	Bread crust, biscuit, dried fruit, stability

Colour And Roast Are Not The Same Thing

This is where most people are caught, and it matters when you are reading a label or trying to brew at home. A beer can be black and taste almost nothing of roast, which is what happens when a light kilned or a highly modified pale malt is dissolved in a large volume and the colour is being carried by melanoidins formed in the kiln rather than by anything roasted. It can also be a deep chestnut brown and taste strongly of coffee and cocoa, which is what a small quantity of a heavily roasted malt will do. Colour is a measure of how much polymer has formed, while roast is a measure of which members of the family that polymer is made of, and the higher compounds that smell of coffee and char need a good deal more heat than the ones that only darken. Two beers can therefore match in colour and be nothing alike in the glass, and if you want a roasted character you have to ask about the roasted malts, not about the colour.

What It Does For Flavour And For Keeping

The same reaction that makes the colour is doing several useful jobs at once in the finished beer. The brown polymers are unreactive with oxygen compared with the colourless compounds that would otherwise oxidise, so a well kilned malt contributes to the stability of a beer and a bottle that has sat in a warm room for a summer will taste more like its younger self than an unmalted equivalent would. The high molecular weight products also carry a lot of flavour, and much of the apparent body and the drying, mouth-coating quality in a stout comes from dissolved melanoidin rather than from alcohol or from residual sugar. Alongside that, the reaction creates hundreds of volatile aroma compounds, and the ones that survive fermentation are the reason a beer built on caramel and roasted malt still reads as bread and toast rather than as sweetness alone.

Fermentation changes all of that somewhat, since yeast consumes the simplest sugars and leaves the heavier molecules alone, which is how the kilned character survives into the bottle. What it does not do is re-create the reaction, so nothing that was not present in the wort can appear in the finished beer. This is the reason a roasty port or a coffee stout that tastes of neither has almost always had its roast character stripped out somewhere upstream, either by a light kilned substitute malt, by a blander pale base that dilutes the roasted portion, or by simply being under-attenuated and thin.

Using It When You Brew At Home

If you want more colour and more of the kilned kind of flavour, the base malt and the specialty malt are two separate controls, and working them one at a time is how you find out which one you were actually short of. Start by swapping your base two-row for an amber or a Vienna and taste before adding anything dark, since that changes body and colour together without pushing the reaction as far. Then add a caramel or crystal malt for sweetness and body. Only once those are right should you introduce a roasted barley or a chocolate malt, and when you do, add it in small steps, because a roasted malt has no fermentable sugar of its own and it takes a percentage point or two to change the character while a larger addition will simply make the beer taste of ash.

• Treat colour as the thing that happens first and roast as the thing that happens last, since a heavy kiln can make a black malt that adds almost no roast flavour to the finished beer.
• Keep a light kilned pale or two-row malt as the majority of the grist in a dark beer, because that is where the bread, biscuit and body come from, and the roasted malts belong in the minority.
• Add caramel and crystal malts for sweetness and mouthfeel rather than for roast, since the reaction at that stage produces toffee and dark fruit rather than coffee.
• Watch the boil as the place where character can go wrong, because a long vigorous boil on a wort already heavy with melanoidin pushes it toward a cooked, gravy-like note that no one wants.
• Do not expect a boozy, strong beer to taste roasted on its own, since the roasted character comes from the kilned malts and alcohol adds warmth and nothing of the sort.

Match Your Malt To The Style

Once you know that one reaction produces the whole range from pale biscuit to black cocoa, the malt list of any dark beer becomes readable rather than a wall of unfamiliar names. An English brown ale is amber and crystal malts carrying the colour with very little roast, a porter leans on a moderate amount of roasted barley for a dry coffee note, and an Irish dry stout is built from a pale base with enough black malt to give colour and cocoa while staying clean enough to drink early in the evening. If the beer in front of you is not doing one of those things, the problem is nearly always in the grist rather than in the brewery, and you can read the answer for yourself on a bag of malt. Our explanation of what malt does in brewing goes further on the other things the grain contributes, and our look at dry stout shows what a low-roast dark beer actually tastes like next to the roasted version.

Carbonation Levels Explained: What PSI Actually Does to Your Nose

Pounds per square inch is a measure of pressure inside a sealed vessel, and in beer it decides how much dissolved carbon dioxide is waiting to escape when you open the bottle or pull the tap. Higher pressure holds more gas in solution, so the same beer at 20 PSI tastes sharper and more insistent in the nose than the identical beer at 10 PSI, and the difference shows up in aroma intensity, in the bite on the tongue, and in how long the bubbles keep climbing the side of the glass. Low pressure gives you a soft, gently sparkling beer with a delicate nose, which is why lagers and ales built for long pours sit where they do, and it is why a beer that tastes flat in a warm glass can taste entirely alive when it is cold and poured hard.

What PSI Is Actually Measuring

PSI counts how hard the gas is pressing on the inside of the container, and beer holds its gas there by pressure alone. When the seal breaks, the pressure differential does two things at once. It pushes the gas out of solution all at once, which is the fizz you hear and the foam you get, and it lets the volatile aroma compounds that were trapped in the liquid escape into the air above the glass, which is the part people call the nose. Without that release of pressure the aromas simply stay dissolved in the beer, and you taste them rather than smell them. This is why a beer tastes duller in a glass you drink from slowly than in a fresh one, since the head space fills with carbon dioxide and the aroma compounds have less room to move.

Where The Typical Ranges Sit

The numbers used in cellars and tap rooms are ordinary engineering figures rather than marketing claims, and they vary by style in a way that tracks bitterness and body closely. Force carbonation is the modern default, where beer is chilled and pushed with carbon dioxide in a bright steel tank until it holds the target pressure, then it is filled and sealed at that same figure. A lager poured in a bar is commonly held somewhere around 12 to 16 PSI, a pale ale usually sits a little higher, and a double or imperial IPA can be pushed as far as 25 to 30 PSI without anything going wrong. The low end belongs to older methods and to deliberate styles, since cask ale in a UK pub is served at what amounts to roughly one atmosphere of carbon dioxide, and deliberately under-carbonated beer is a feature of several traditional German and Belgian ales.

Beer and setting	Rough pressure	What you notice
Cask ale, naturally conditioned	Near atmospheric	Very soft, no sharp bite, faint persistent sparkle
Bottle conditioning, typical	About 12 to 16 PSI	Even sparkle, a head that settles slowly
Force carbonated lager in a tap	About 12 to 16 PSI	Clean and crisp, moderate aroma
Force carbonated pale ale	About 16 to 20 PSI	Brighter hop aroma, more tongue bite
Double or imperial IPA	About 25 to 30 PSI	Very high carbonation, sometimes masking aroma

Reading a pressure off a bottle tells you almost nothing useful. Breweries do not print a PSI figure on a label, and a packaged bottle that was force carbonated and then settled in the warehouse is not carrying anything like its tank pressure by the time you hold it. What the package does tell you is how the gas got in. A crown cap on a 341 mL bottle is the common North American format for something force carbonated and packed cold, a cork and cage signals a bottle conditioned beer that finished in the glass or bottle, and a plastic or screw cap in a larger format is usually a draught-style filling, since screw caps seal poorly against carbon dioxide over time.

The Nose Connection Nobody Expects

Carbon dioxide is a scentless gas, so the gas itself carries no aroma, and the whole nose effect comes from what the pressure change lets escape. Aroma compounds in beer are mostly dissolved in the liquid, and a higher pressure keeps them there along with the gas. The common shorthand that carbonation carries hop aroma to your nose is directionally right but mechanically sloppy, since the aroma rides out on the gas that comes off when you break the seal, and it comes off most vigorously in the first few seconds, which is why the first sniff from a freshly poured glass is the loudest one you will get. Overcarbonation works against aroma as much as for it. At very high pressure the gas leaves so quickly that it pins the foam to the glass and drives the aroma compounds away from the surface faster than you can smell them, so an aggressively carbonated double IPA can taste thinner and harsher than a gentler pour of the same beer at half the pressure.

Carbonation, Glass Shape And Temperature

The pressure sets how much gas is available, and three other things decide how much of it you actually taste. Temperature comes first, because cold liquid holds carbon dioxide better and releases it more slowly, so a beer that seems flat at cellar temperature comes alive at four degrees. The glass matters next, since a narrow tulip or a stemmed pint concentrates the escaping gas into a tighter column and gives the foam something to cling to, while a wide shallow bowl spreads the same gas over a larger surface and loses more of it. Pour technique matters last and is worth naming, because pouring down the side of a tilted glass and then raising it produces a tight, dense head that seals the surface and slows the gas loss, which is exactly the opposite of dropping a stream straight into the middle of the glass.

There is a second half to the same story, which is what happens after the glass is empty of gas. Stretched proteins from the malt and from the yeast form a film around each bubble, and that film is what holds a head together and keeps bubbles from rising. A lower pressure means the bubbles grow larger as they expand on the way up, and a larger bubble has a thinner film, so a gently carbonated beer gives a coarse, loose, quickly vanishing foam. More pressure, and smaller bubbles with proportionally more surface area covered in protein, give a tighter, more durable head. That is a genuine practical reason to serve a strong beer at a high pressure, since the head it holds is protecting the aroma underneath it, and a beer with no head is a beer whose nose you are never going to smell properly.

Reading And Adjusting A Keg Yourself

Home brewers who force carbonate their own beer control this directly, and the routine is the same whether you are using a Cornelius ballard, a pin lock or a plastic bucket with a lid and a lockout. A pressure gauge on the tank and a regulator on the line are the whole system, and the sequence is short. Chill the beer to serving temperature, apply a starting pressure, let the tank sit sealed for a day or two so the gas can dissolve properly, then adjust the regulator down by a pound at a time and pull a glass to taste. The mistake almost everyone makes once is raising pressure and forgetting it, so keep a note of the figure and pull it back down before you pack for a gathering.

• Chill the beer first, because cold liquid absorbs carbon dioxide far better than warm liquid, and pressure applied to a warm tank takes noticeably longer to show up in the glass.
• Change the regulator pressure in small steps and taste after each one, since a two pound difference between neighbouring settings is a real difference in the mouth and not a rounding error.
• Stir or agitate the tank gently while a new pressure is soaking in, which lets the dissolved gas escape from the liquid before it reseals.
• Do not rely on the pressure gauge alone to tell you the beer is ready, because a tank can hold a steady pressure while the liquid underneath it has run out of gas to give.
• Expect a few days of settling after any change, and give a keg the same patience you would give a bottle, since the carbonation keeps working after the gauge stops moving.

What To Do About A Beer That Tastes Wrong

If a beer tastes aggressively fizzy, sharp and tiring, the pressure in the glass is too high for what you are drinking, and no amount of patient sipping will fix it, since the only gas you can remove is the gas you are holding. You can pour it into a larger glass and wait, since a bigger surface area and a longer pour releases the excess faster than anything else. You can drop the glass down a length of string and let it settle, and you can let it sit in a fridge for a while and drink it colder, since cold and a wider glass together will soften the bite noticeably. If a beer tastes completely flat with no bead on the glass and no bubbles on the tongue, the opposite is true and the liquid has lost its gas, which happens to anything opened and left for hours and cannot be reversed once the bubbles are gone. Freshness is the only real remedy, and that is worth keeping in mind when a shelf has been open for a while.

Pick The Right Level For The Style

A bottle conditioned beer and a force carbonated one are not better and worse versions of the same thing, they are different outcomes of the same pressure, and knowing which is which tells you what to expect in the glass. The gentle styles that people find calm, namely lagers, wheat beers and anything from a lager yeast strain, are usually built low, and the heavy hoppy and high alcohol styles are usually built high because they need the head to survive. If you take one thing away, let it be that the pressure figure is a choice rather than a natural law, and that the same beer at a different pressure is a different drink, so the next time a can or a tap seems too sharp, the answer is not that the beer is bad, it is that the gas is set a little too high for the glass you are holding.

A useful companion read is our piece on bottle conditioning explained, which covers where the gas comes from in a bottle rather than how much of it is waiting for you. If you are more interested in what the dissolved gas is carrying than in the pressure itself, the chemistry of hops sets out how hop compounds behave, and that is the part of the aroma that changes most when the pressure changes.