The tendency to foam and the stability of the head on a beer is often seen as an indicator of successful fermentation and now new research has proved this to be true.

A research paper on the stability of a beer head has recently been published by the American Institute of Physics in its journal Physics of Fluids. The authors compared the stability of the head produced by four Belgian beers: Bosteels Tripel Karmeliet, 8.4 per cent ABV; Westmalle Triple, 9.5 per cent; Westmalle Dubble, 7 per cent; and Westmalle Extra, 4.8 per cent and two Swiss lagers: Chopfab, 4.7 per cent; and Feldschlösschen, 4.8 per cent.

The conclusion is that triple-fermented beers have the most stable foams, while the froth rim created by a single fermentation brew, including lagers, is more likely to collapse.

Like any other foam, a beer's 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.

Vermont added: “We now know the mechanism exactly and are able to help the brewery improve the foam of their beers.”

Photo: Dave Pickersgill