You know those beer commercials where glasses are overflowing with foam and precious suds falls down the sides, part of the beer basically wasted on a tabletop? It's cringeworthy for me, but it's not all wrong: Some foam is, in fact, important when pouring a beer. (An overflowing glass, however, is sad. Don't do that.)
Foam, or head, is a layer of bubbles formed from a beer’s natural carbonation. Proteins and other chemicals like polypeptides (chains of amino acids) cause head to form on a beer when poured into a glass. The amount of head has to do not only with how a beer is poured, but also how the beer was brewed. The density of the foam and so-called "head retention" is directly related to what type of malts were used during the brewing process. Oats and wheat cause a fluffy, long-lasting head, while barley generally results in a less frothy brew.
Too much head is obviously not a desirable trait in beer—a customer at a bar may even view a super-foamy pour as a rip-off—but some will help deliver aromas to your nose and can add an extra layer of texture when sipping beer. It's also good to know that some beer will naturally have very little foam or carbonation, especially aged beers, boozy ales, and some sours, and this is not necessarily a defect. It's just one more little fact to know when ordering or pouring what you've got at home.
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