Brilliant article over at The Economist on how bubbles form in Stout beer (how can I get that job??). As anyone knows who has enjoyed a Guinness from the pressurized can, it takes a special widget containing nitrogen to make all those awesome creamy bubbles instead of the regular CO2 used in ales and lagers.
Now it turns out that you may be able to do the same thing using a coffee filter instead. Read the whole article here.
Conventional wisdom used to hold that once the pressure inside a bottle or can has been released by opening it, the bubbles in a fizzy drink, whether alcoholic or not, are seeded by pockets of air trapped in scratches and imperfections on the surface of the glass being drunk from. Over the past decade, however, scientific scrutiny has revealed that most bubbles actually form on fibres of cellulose that have either fallen into the glass from the air or been left behind when it was dried with a towel. These fibres, which are generally hollow, trap a small amount of air in their interiors.
To see what is going on in stouts, Dr Lee and his team wrote down the equations governing the physics of the dissolved gases and fibres. They found that molecules of nitrogen and carbon dioxide are able to diffuse from the liquid through the walls of the fibres into the air pockets trapped inside them, causing those pockets of air to expand. If a pocket reaches the end of a fibre, it breaks off as a bubble.
The problem, as far as stouts are concerned, is that the low concentration of dissolved nitrogen means the process works at only a 15th of the rate seen in ales and lagers. But Dr Lee has an answer to that: more cellulose. He and his team spiked their beer with extra fibres from a cut-up coffee filter and watched the bubbles form under a microscope.