The tendency to foam and the stability of the head of a beer is often seen as an indicator of successful fermentation. A very detailed research paper on the stability of such foam has recently been published by the American Institute of Physics in their Journal, ‘Physics of Fluids.’ The authors compared the stability of the head produced by six beers: four Belgian: Bosteels Tripel Karmeliet (8.4%), Westmalle Triple (9.5%), Westmalle Dubble (7%) and Westmalle Extra (4.8%) and two Swiss Lagers: Chopfab (4.7%) and Feldschlösschen (4.8%).
The conclusion is that Triple fermented beers have the most stable foams, while the froth rim created by single fermentation beers, including lagers, are more likely to collapse.
Like any other foam, a beer-head is made of small air bubbles, separated by thin films of liquid. Under the pull of gravity and the pressure exerted by surrounding bubbles, over time, the bubbles burst and the foam collapses. The rate at which this occurs depends on the form of a barley-derived protein: Liquid Transfer Protein 1 (LTP1).
In single fermentation beers, LPT1 proteins have a globular form and arrange themselves densely as small, spherical particles on the surface of the bubbles. “It’s not a very stable foam,” said Prof Jan Vermant, a chemical engineer at ETH Zurich, who led the study.
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.
During the third fermentation, the LPT1 proteins become broken down into fragments. These protein fragments stabilise foams in many everyday applications such as detergents. Some of the triple fermented beers had foams that were stable for 15 minutes.
As Vermont says: “We now know the mechanism exactly and are able to help the brewery improve the foam of their beers.”
Abstract:
A recent academic study indicates that Triple fermented beers have the most stable foams, while the froth rim created by single fermentation beers, including lagers, are more likely to collapse.
Photos: Westmalle, taken by Dave Pickersgill
Link:
https://pubs.aip.org/aip/pof/article/37/8/082139/3360405/The-hidden-subtlety-of-beer-foam-stability-A The hidden subtlety of beer foam stability: A blueprint for advanced foam formulations | Physics of Fluids | AIP Publishing