A flat pint of beer with no head is a common gripe among pub-goers. And while the bar staff\u2019s pint-pulling technique is often assumed to be the cause, scientists have discovered that the stability of beer foam is also highly dependent on the chemical makeup of the brew.<\/p>\n
Triple fermented beers have the most stable foams, the study found, while the froth created by single fermentation beers, including lagers, are inherently more likely to collapse before you have time to take the first sip.<\/p>\n
\u201cWe now know the mechanism exactly and are able to help the brewery improve the foam of their beers,\u201d said Prof Jan Vermant, a chemical engineer at ETH Zurich, who led the study.<\/p>\n
The research began as a \u201ctypical Friday afternoon project\u201d, Vermant said. \u201cWe decided to study beer and found this really rich physics going on.\u201d<\/p>\n
Like any other foam, beer foam is made of many small bubbles of air, separated from each other by thin films of liquid. Under the pull of gravity and the pressure exerted by surrounding bubbles, the films of liquid slowly thin out, the bubbles pop and the foam collapses. But the rate at which this process occurs was found to vary depending on the form of a barley-derived protein, called Liquid Transfer Protein 1 (LTP1).<\/p>\n
\u201cThe idea was to directly study what happens in the thin film that separates two neighbouring bubbles,\u201d said Dr Emmanouil Chatzigiannakis, an assistant professor at Eindhoven University of Technology and first author of the study.<\/p>\n
Turning to a collection of scientific imaging techniques, the team was able to determine how these thin films could hold together to make a stable foam.<\/p>\n
\u201cWe can directly visualise what\u2019s happening when two bubbles come into close proximity,\u201d Chatzigiannakis said. \u201cWe can directly see the bubble\u2019s protein aggregates, their interface and their structure.\u201d<\/p>\n
In single fermentation beers, the LPT1 proteins have a globular form and arrange themselves densely as small, spherical particles on the surface of the bubbles. \u201cIt\u2019s not a very stable foam,\u201d Vermant said.<\/p>\n
During the second fermentation, the proteins become slightly unravelled and form a net-like structure that acts as a stretchy elastic skin on the surface of bubbles. This makes the liquid more viscous and the bubbles more stable.<\/p>\n
During the third fermentation, the LPT1 proteins become broken down into fragments that have a water-repellent (hydrophobic) end and a \u201cwater-loving\u201d (hydrophilic) end. In these beers, a phenomenon called the Marangoni effect comes into play, which drives liquid flow from protein-rich (thicker) areas of the bubble to protein-depleted (thinner) areas, delaying the bursting of bubbles. A similar effect is seen with soap bubbles, where swirling can often be seen on the surface for the same reason.<\/p>\n
\u201cThese protein fragments function like surfactants, which stabilise foams in many everyday applications such as detergents,\u201d Vermant said. Some of the triple fermented beers had foams that were stable for 15 minutes.<\/p>\n
Vermant, who is Belgian, said the findings could help brewers increase or decrease the amount of foam as desired. There may be differing views, though, on whether a foamy pint is desirable or represents poor value for money. \u201cFoam isn\u2019t that important everywhere beer is served \u2013 it\u2019s basically a cultural thing,\u201d he said.<\/p>\n