Part of the benefit of writing this blog is that I'm always looking for the next topic, which in turn makes me curious and questioning about everyday things I see. So after sixty-nine-plus years on planet Earth, my brain finally got around to asking this simple question: Why is beer foam white?
Whether it's a light, yellow lager like in the image below, or my favorite, a black Guinness stout, the foamy head is white. And when the head slowly settles, it turns back into the beer's color.
In truth, beer foam isn’t exactly white, but because it's so much lighter than the beer below it, it appears to the eye much more white than it is. That effect is called “White’s Illusion”, named after Michael and Tony White, not the foam's color. More on that in a future blog. Beer foam appears lighter because its structure is almost impenetrable to light, and the reflected light is scattered in all directions, making it appear white. The same effect as the top of a cresting wave will be white as light is scattered by millions of tiny water droplets.
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