Beer foam longevity depends strongly on the degree of fermentation—single, double, or triple—with higher degrees associated with longer-lasting heads. Foams form when air is whipped into a liquid containing surfactants, such as fats or proteins in edible foams, which strengthen the liquid films between bubbles. Individual bubbles are initially spherical to minimize surface area, allowing tight packing into foams. Over time gravity drains liquid, thinning film walls and causing bubbles to coarsen as smaller bubbles are absorbed by larger ones. Increasing bubble packing leads to jamming, greater internal pressure, and solid-like rigidity. Multiple factors influence overall foam stability.
For many beer lovers, a nice thick head of foam is one of life's pure pleasures, and the longer that foam lasts, the better the beer-drinking experience. A team of Swiss researchers spent seven years studying why some beer foams last longer than others and found that the degree of fermentation-i.e., whether a given beer has been singly, doubly, or triply fermented-is crucial, according to a new paper published in the journal Physics of Fluids.
Individual bubbles typically form a sphere because that's the shape with the minimum surface area for any volume and hence is the most energy-efficient. One reason for the minimizing principle when it comes to a bubble's shape is that many bubbles can then tightly pack together to form a foam. But bubbles "coarsen" over time, the result of gravity pulling down on the liquid and thinning out the walls. Eventually, they start to look more like soccer balls (polyhedrons).
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