William Lee

Stout Beers


Stout beer

One of my research interests is the science of stout beers.

Stout beers have fascinating physical properties. Chemically the only difference between stout beers and other beer is that the stout beers contain dissolved nitrogen as well as carbon dioxide.

This apparently small chemical change confers a host of interesting properties on the beer, of interest both to beer connoisseurs and to physics aficionados. Below, some of these properties are discussed, along with the physics behind them

Smooth less bitter taste

In carbonated beers the dissolved carbon dioxide forms carbonic acid in solution, which has a bitter taste. Dissolved nitrogen is unreactive and so does not add an acid taste to the beer.

Creamy long lasting head

The "creamy mouthfeel" of the heads of stout beers is due to the small size of the bubbles found in stout beers. These small bubbles are due to the very low solubility of nitrogen in the beer. There is very little gas dissolved in the beer, which makes it difficult for the bubbles to grow.

The long lifetime of the heads on stout beers has the same ultimate cause: the low solubility of nitrogen. The head of a beer decays by a process called disproportionation: large bubbles grow larger while small bubbles grow smaller. But for this to happen the gas in the bubble must go into solution in the bubble wall while travelling from one bubble to another. The higher the solubility of the gas the more quickly this can happen. So the head of carbonated beers has a much shorter lifetime than stout beers.

Sinking bubbles

Bubbles in stout beers sink because there is a circulatory current in the glass and because the small size of bubbles makes them more susceptible to drag forces so are pulled down by the current faster than they rise due to their buoyancy. For the large bubbles of carbonated beers the opposite is true: the upwards velocity imparted to them by their buoyancy is much greater than the downwards motion from drag forces. For my own research in this area see Sinking bubbles in stout beers.

Foaming technology

A disadvantage of adding nitrogen to beers is that it makes it difficult for them to foam. A carbonated beer will foam (apparently) spontaneously. This is not true for stout beer which need special technology such as the widgets found in canned stout beers to promote foaming. For my own research into alternatives to the current widgets see Bubble nucleation in stout beers.

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