Beer School: How to get a head in beer
Foam. What is it, how did it get there, and why do we love it?
Sunday 29 November 2020
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Foam is unique to beer. Lots of drinks have bubbles, of course, but in no other drinks do those bubbles naturally form into foam. Think about Champagne, soda, or sparkling cider, and the bubbles appear when you pour them, but they disappear almost immediately. The foam on a well-made and well-served beer can last until after you’ve finished drinking it. So what is the white stuff that sits on top of your beer?
Beer foam is made of lots of small bubbles of gas and a little bit of beer. In the majority of beers the gas is carbon dioxide (CO2) and it’s dissolved under pressure into the beer, sometimes naturally within the maturation tank, bottle or cask, and sometimes force-carbonated like a soda. That gas is saturated in the liquid and sealed inside the container, waiting to be released when the beer is opened.
When you open a beer (or soda or Champagne) you change the pressure inside the container and the liquid can no longer hold onto all of the saturated CO2, so the gas starts to escape; you won’t see foam in the headspace of a full sealed bottle, but open it and you’ll hear the pssst of gas coming out and immediately see some bubbles, but you won’t yet see foam.
If you left an open bottle or can of beer alone then the CO2 would gradually leak out and it’ll become flat, but when we open a beer it’s usually because we’re going to pour it out, and it’s the beer hitting the bottom of a glass which enables foam to form.
The disruption of beer being poured knocks the CO2 out of suspension and the gas escapes the liquid, but it doesn’t simply float away like balloons released from a large net. Inside the glass are tiny nucleation points, which could be any slight imperfection on the smooth surface or from etched nucleation patterns in the base of the glass. The gas collects in those points and grows into bubbles which are released into the beer. Bubbles continue to form in those nucleation sites while you drink, and the higher the carbonation, the more bubbles you’ll get.
The bubbles head up through the beer until they reach the surface. What makes beer special is that the bubbles don’t pop at the top and instead they form into foam and grip to the glass, and that’s because some of the natural ingredients in beer are ‘foam positive agents.’
Some of the natural ingredients in beer are ‘foam positive agents'
Lipid Transfer Protein 1 (LTP1) is a kind of protein naturally found in barley and it’s hydrophobic, meaning it doesn’t like water. The proteins cling onto and wrap around the rising CO2 bubbles as if they’re life jackets, then at the top of the glass the LTP1-protected bubbles don’t pop and instead they group together.
A lone CO2 bubble is spherical, but when many bubbles combine they shift shapes and become polyhedral, forming into a tight and stable foam structure. There is no LTP1 in Champagne, soda, or sparkling cider, which is why the bubbles burst and don’t become foam.
Beer has other foam positive agents, too, and where LTP1 builds the foam, another natural barley protein called Protein Z stabilises the foam and helps keep it there. Iso-alpha acids in hops (they give beer its bitterness) can grip onto proteins and help further stabilise the foam, while also making it stick as lacing down the glass. Other ingredients and processes can also aid foam formation: wheat can increase foam as it has a high protein content; roasted barley stabilises foam, whereas crystal malt de-stabilises it; keeping beer cold will hold onto the carbonation better (that’s why warm beer usually tastes flat); and high alcohol beers inhibit foam stability (around 5% ABV is ideal for lasting foam).
But all bubbles will eventually pop.
As bubbles rise to the surface, a small amount of beer gets trapped with the gas and that helps to create the smooth texture of the foam, but that beer will fall through the foam via gravity, and bubbles burst on the way. If there aren’t new bubbles rising to replace the popped ones, then the foam diminishes. Small bubbles can also diffuse into bigger bubbles, giving less structure and less foam overall, before the big bubbles also pop.
There are some beers, however, with long-lasting foams, the most iconic being Irish stout with its smooth, creamy head, and that’s achieved with the use of nitrogen alongside a small amount of natural CO2.
Brewers dissolve nitrogen into these beers before packaging them. When the beer is poured on tap it flows through a ‘restrictor plate,’ which is a disk with tiny holes in it like a shower head, and that breaks the nitrogen out of solution. Nitrogen has much smaller bubbles than CO2 and the bubbles are less buoyant, so when the beer hits the bottom of the glass the nitrogen slowly escapes and creates the characteristic cascade of tiny bubbles on its way to becoming a thick foam. In a canned nitro beer there’ll be a small plastic ball – a widget – infused with nitrogen which does a similar job to the restrictor plate and knocks the bubbles out of the beer.
When we drink nitro beer it looks and feels creamy, but the actual beer isn’t creamy at all. The combination of low carbonation and the smooth foam creates a textural impression of creaminess; drink the beer minus the foam and it’ll taste quite thin and flat.
For regular draft beer, it’ll be fully carbonated in the brewery and filled into pressurised kegs. In the pub cellar, the kegs are connected to a cannister of CO2 and when the tap is opened to pour a beer, the gas pushes the beer out, where if a pint of beer leaves the keg then it’s replaced with the equivalent volume of gas in the headspace, which maintains an equilibrium of pressure (in nitro beers, the kegs are connected to ‘mixed gas’ of usually 70/30 N2/CO2 and the same theory applies). The CO2 breaks out in the line between the keg and the glass, then those bubbles form into foam (‘fobbing’ or foaming beer is often down to issues in the cellar, so perhaps the pub hasn’t balanced the gas pressure, or the lines might be dirty which creates nucleation points, and therefore too many bubbles, between keg and glass). As they’re pressurised, a tapped keg can hold its carbonation for a couple of weeks. Cask ale, by contrast, will last no more than a couple of days once it’s been opened.
Brewers fill casks with beer which has a very low level of natural CO2 and then seal that cask shut. A small secondary fermentation takes places inside the cask, producing some natural CO2, and because the cask is sealed the gas can’t escape, so it’s dissolved into the beer. When the cask is tapped, the pressure changes and CO2 starts to escape (plus oxygen starts to come in), which is why casks need to be served quicky before they become flat. If the beer has CO2 in it then those bubbles will become foam when it pours; if there’s no CO2, then it’ll mean no foam. In the north of England, cask ales are often served through a ‘sparkler’, a small nozzle similar to nitro’s restrictor plate which fits onto the cask tap and forces CO2 to be released from the beer, resulting in smaller bubbles which slowly settle into a tight, persistent, creamy foam.
All of the theory of bubbles becoming foam relies on one thing: the glass.
As we’ve seen, nucleation points allow bubbles to be released from the base of the glass up into the foam, but there can be impurities elsewhere in the glass which don’t release the bubbles, and therefore they don’t form as foam and the beer tastes flat.
If you’ve ever had a beer with bubbles clinging to the inside of the glass then that’s a sign the glass is dirty. This ‘dirt’ could just be residual soap or detergent, or from glasses being washed in dirty water or alongside milky teacups or greasy plates.
Foam lacing is the best evidence of a clean glass and a good beer
Dirty glasses are the biggest inhibitor to beer foam, while foam lacing is the best evidence of a clean glass and a good beer. Next time you’re in the pub, look at the empty glasses, not the full ones. If an empty glass has lacing all the way down it then you know that the beer has been well brewed and it’s been properly poured into a clean glass. Also, the lacing will tell a story about the drinker: lots of lines close together shows they’re slowly sipping, whereas big gaps mean big gulps.
There is a stigma to foam in Britain. A feeling that too much means we’re being shorted a sip of beer and coming with sarcastic shouts for a Flake, and you can have too much of it, but all beer should have foam because it shows us that a beer is well-made and well-served. And more than anything else, it makes beer look beautiful and appetising. Just picture the perfect beer in your mind and it’ll almost definitely have a thick whip of white foam on top. Foam is unique to beer, and beer isn’t beer without its foam.
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