Why does beer foam?

Answer: CO2 nucleates on proteins

Sugar ferments when poured — Wrong. Fermentation happened during brewing. Foam forms because dissolved CO2 escapes when poured, creating protein-stabilized bubbles.

Yeast cells create bubbles — Wrong. Yeast did produce CO2 during fermentation, but foam forms when pouring because dissolved CO2 escapes, stabilized by proteins.

CO2 nucleates on proteins ✓ — Correct! Beer contains dissolved CO2 (from fermentation). When poured, pressure drops and CO2 forms bubbles. Beer proteins (from barley/wheat) and hop compounds coat bubble surfaces, stabilizing foam. Without protein, bubbles pop quickly like soda. That's why beer head lasts—it's protein-stabilized foam!