Why does old, opened beer taste flat and stale?
Show answer & explanation
Answer: CO2 escapes and oxygen oxidizes it
CO2 escapes and oxygen oxidizes it ✓ — Correct! Two things happen: the dissolved CO2 escapes (so it goes flat), and oxygen slowly oxidizes the beer into stale, wet-cardboard flavors. Warmth speeds both up, which is why beer keeps best cold, sealed, and not for too long.
The alcohol turns into water — Wrong. Alcohol doesn't turn into water. The staleness comes from losing CO2 and from oxidation, not from the alcohol changing.
The hops dissolve away — Wrong. The hops don't dissolve away. Flatness is escaping CO2, and the off-flavors come from oxygen reacting with the beer.