Being given a pint without a nice thick foamy head is a common cause of frustration for pub-goers.
But according to a new study, it might not be due to bar staff botching the pour.
Scientists in Switzerland and the Netherlands have found that the stability of beer foam is due to a protein present in the liquid.
And this protein tends to be more abundant in ‘triple-fermented’ beers, which are especially popular in Belgium.
This is why foams on Belgian beers last so long, especially compared with ales and lagers commonly found in British pubs, which lose foam in seconds.
Beer foam is considered important because it makes beer up to twice as aromatic, improves mouthfeel, and makes a beer look more inviting.
‘We now know the mechanism exactly and are able to help the brewery improve the foam of their beers,’ said study author Jan Vermant, a Belgian scientist and beer fan at ETH Zurich.
‘There are many areas where the knowledge we have gained from beer is proving useful.’
Scientists in Switzerland and the Netherlands have found that the stability of beer foam is influenced by a protein present in the liquid (file photo)
These images of beer bubbles, superimposed onto a glass of beer, are from two ‘triple-fermented’ beers (left) and two lagers (right)
According to the team at ETH Zurich and Eindhoven University of Technology, individual beer bubbles are separated by thin films of liquid, which gradually become thinner after the beer has been poured, due to pressure from surrounding bubbles.
When the liquid films get thin enough, the bubbles burst and the foam collapses.
But the rate at which this occurs is closely related to the fermentation process, which converts sugars to CO2 and alcohol.
Sometimes, brewers repeat fermentation to make double- or triple-fermented beers, generally known in Belgium as dubbel and tripel, which tend to be stronger and more complex than once-fermented beers (‘singel’).
Using microscopic imaging techniques, the scientists studied six commercial beers, including Westmalle Tripel, a Belgian tripel with an alcohol content of 9.5 per cent.
Also included was Westmalle’s dubbel and singel, plus Tripel Karmeliet (another Belgian tripel with an ABV of 8.4 per cent), as well as Swiss lagers Feldschlösschen and Chopfab.
The team found that triple fermented beers have the most stable foam, followed by double-fermented beers and finally the single-fermented lagers.
Generally, beer foam collapses due to LTP1, a kind of protein that comes from grain used to make beer, such as barley and wheat, they explain.
Sometimes, brewers repeat the fermentation process – creating double-fermented or triple-fermented beers, traditionally known in Belgium as dubbel and tripel
One a microscopic level, a thin film separates two neighbouring bubbles in a beer foam. This image shows a very thin film of beer between two bubbles. The different colors correspond to different film thicknesses, similar to a topographic map with contour lines
In single-fermented beers, like the lagers analysed, LTP1 proteins are small, spherical particles that arrange themselves densely on the surface of the bubbles.
‘The little ‘spheres’ touch each other but do not stick,’ Professor Vermant told the Daily Mail.
‘So they do provide some resistance, the interface becomes more thicker, more viscous (compared to pure water), but the interface cannot yet carry a lot of stress.’
In double-fermented beers, the proteins in the beer come together to form a two-dimensional structure, giving the thin films an elastic quality that keeps them intact longer.
Meanwhile, in triple-fermented beers, LPT1 proteins are broken down into fragments with a water-repellent and a ‘water-loving’ end.
These fragments reduce the tensions between the bubbles and stabilize the bubbles to the maximum possible extent, the experts found.
‘These protein fragments function like surfactants, which stabilize foams in many everyday applications such as detergents,’ said Professor Vermant.
Because British pubs usually focus on serving ales and lagers that are only fermented once – which may be why they commonly lose their foam so quickly.
In triple-fermented beers, there’s less surface tension between the bubbles, stabilising them to the maximum possible extent compared with lagers
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The new findings, published in the journal Physics of Fluids, could help brewers create beer with more long-lasting foam.
Brewers could put their beers through more fermentation steps or manipulate the nature of LPT1 proteins to provide a stronger barrier between bubbles.
‘[We] indicate that this protein [LPT1] is the molecule to look at,’ added Professor Vermant.
However, the academic admits that foam isn’t that important everywhere beer is served as it’s ‘basically a cultural thing’.
‘I’m Belgian, and in my home country a foam froth is seen as a sign of quality,’ he told the Daily Mail.
‘This may not be an opinion everyone shares.’
Study co-author Emmanouil Chatzigiannakis at Eindhoven University of Technology said foam ‘usually is associated with consumer satisfaction’.
‘It really depends on cultural things, but it also modifies texture and mouthfeel and helps retain the aroma of beer,’ he said.