This issue, Louise Crane gets herself in a lather over the surprisingly complex chemistry that goes into creating the perfect head
Tuesday 14 April 2020
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Whether it’s a creamy, brilliant-white meringue or a sticky, lacy topping, the head of a beer is a vital component of the classic pint. Trapped within this moustache-making foam is an array of aromas that hint at the flavour beneath. But how come beer has a head when other, equally bubbly drinks don’t?
Inhale a freshly poured, or pulled, beer and you’ll be met with smells that tease at the taste to come, from fruity to nutty, citrusy to earthy and everything in between. These smells are the result of volatile compounds evaporating from the surface of the beer, a surface that often, initially, is covered in a thick foamy head made up of hundreds of thousands of bubbles filled with the carbon dioxide resulting from fermentation (and sometimes nitrogen, but more of that later). The bubbles in champagne are made from the same ‘waste’ product, but these escape the surface straight away.
Bubble scientist (yes that’s an actual job, and an important one at that) Dr Helen Czerski explains: “A newly-formed bubble will continue to grow while it rises, as gas from the surrounding drink moves into it. Beer is full of surfactants – molecules that sit on the bubble surface – which slow down gas flow into the bubble (leaving you with smaller bubbles) and also stabilise it upon reaching the drink’s surface, causing a long-lasting head. Sparkling drinks have far fewer surfactant molecules, so these bubbles grow to be bigger and also burst more quickly, leaving no head behind.”
DID YOU KNOW?
The proteins from grains are called albumins, and are very similar to the albumins found in blood plasma and eggs (though note that the white of an egg is the albumen, with an e). The albumins are formed in the shape of a heart. Aww.
So, here’s the science bit. We hope you are concentrating. These surfactants are a combination of molecules that come from grains and hops. Malted barley and wheat releases proteins in the mashing process, and also polypeptides, another, smaller type of molecule made up of two or more amino acids (amino acids are the building blocks of proteins).
Proteins are big, complex molecules, with distinct areas that react to substances differently. There are ‘water loving’ parts, known as hydrophilic, and ‘water hating’ parts, called hydrophobic. So, when a protein is in water and encounters a carbon dioxide gas bubble, the hydrophobic portion sees an opportunity to get away from its hated water, and is drawn towards the wall of the gas bubble. There it meets other proteins doing exactly the same thing, and they all link up with each other.
Dr Czerski again: “The hydrophilic part of a molecule is very “sticky” when it’s surrounded by water, and the hydrophobic part is “sticky” when it’s surrounded by something that isn’t water. So these molecules tend to stay put when one end is in water and the other end is stuck to something else.” In this way, proteins create a stable skin around the gas bubble.
These grain proteins aren’t the only thing in beer that would rather be cuddling up to a gas bubble. Recall the first taste of beer foam on your lips – isn’t it much more bitter than the sip that follows? That’s because of the iso-alpha acids. These are bitter-tasting compounds that are present courtesy of hops, and they are also hydrophobic. If there’s a sticky bubble wall forming, they will rush to join it, jostling amongst the bigger proteins to link up with those aforementioned polypeptides, which are also hydrophobic, and so doing exactly the same thing. The iso-alpha acids and the polypeptides cross link, and the bubble’s skin becomes even stronger, which leads to a foam that is much more rigid, stable and clingy.
Foam has a profound trigeminal sensation – an effect of taste that is perceived physically, like “fullness”. Since the head is so important for smell, taste and visual appeal, some brewers pay special attention to creating the perfect head. Wheat has more protein, and more stabilising power, than barley, so this might be added to the mash bill if a more persistent head is in order. Oat and rye beers also have a good head for similar reasons.
Proteins are big, complex molecules, with distinct areas that react to substances differently.
The gas bubbles themselves can be manipulated by the very common, mass market approach of pumping carbon dioxide through kegs at the bar. For beers with a super-creamy head – Guinness, perhaps – you need fine, small bubbles. Smaller bubbles require higher pressure; more pressure than you can get from just carbon dioxide (unless you want a ridiculous Mr Whippy-style foam top to your pint). Nitrogen gas provides the extra pressure at a ratio of around 70:30, and the resulting mix of gas and stout is forced through small holes in a ‘restrictor’ in the pub’s tap, causing it to ‘surge’, and then settle into a wonderful, well-defined head. Nitrogenation is normally only applied to malty beers like porters or stouts, because they retain their aromas and flavours better than hoppy beers during the process. You’ll probably have noticed that a beer with a tall but fine head will be a little flatter than a carbonated brew.
WHAT IS LACING?
The proteins which form a coating around every foam bubble interact with other compounds that also happen to rise to the top of your glass. This interaction causes them to become denser, undergoing a textural transition, and they begin to stick to the sides of the glass when left alone for a bit. This is why a beer consumed slowly will accumulate much more lacing than its guzzled counterpart.
Some beers only form a dusting of bubbles on the top when poured, others have a luxurious topping, inches thick. You'll notice this in particular with hefeweizens, farmhouse ales, and other wheat beers; pints that need the head sliced off, which, while looking really cool, is mainly about keeping the carpet clean. On the other hand, barley wines and imperial stouts have the barest of toppings. The level of alcohol is a big factor here; strong beers don’t have a head because high alcohol kills foam, and a session beer at around 3% won’t have a lasting head either.
Acidity appears to be important too, but scientists can’t agree why, or how. The biggest culprits for destroying beer head are lipids, which make up fats and oils. Good brewing practice should ensure that very low levels of lipids survive into the final beer, but just the oil on your lips will interfere with the protein-laden bubble skin, breaking up the bonds like a bouncer at a rowdy nightclub. That’s why some people dip a chip into their beer – it’s to break up the head.
One of the most important factors in bubble formation is the ‘site of nucleation’, which might sound like a chapter from Donald Trump’s defence strategy, but is actually where bubbles are born. This could be a crack in the vessel, or something manufactured like the etchings seen on the bottom of modern pint glasses. These markings help to maintain the head by allowing the gas to be released more easily.
But just as all good things come to those who wait, all good things must also come to an end. After several minutes of settling, where rising bubbles join the head, gravity has an overwhelming effect of dragging down the beer trapped between the bubbles. Their walls start to touch and then merge or coalesce to produce larger, more unstable bubbles and the foam becomes coarser, a mix of different-sized bubbles.
This leads to the final physical process involved in foam collapse, disproportionation. The smaller bubbles join with their bigger cousins until just a few big bubbles are left. The colder the beer and the higher the pressure above the foam, the longer this takes, which is a good explanation for why German beer steins with handles and lids came about - keeping warm hands away from cold beers, and keeping the pressure on.
As you watch the bubbles merge together, until just one remains, worry not. Unless you’re gulping, or are on a very short third, you’ll have plenty of beer from beneath the head to savour, and a delicate lacing of bubbles down the side of your glass to leave.
DID YOU KNOW?
A High Court Judgement found that foam should be regarded as an integral feature of beer but that the customer can, if they wish, ask for a full measure of beer.
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