The Chemistry
Oxygen is both medicine and poison. A small dose of oxygen (the kind a wine gets in a decanter for 30 minutes) softens tannins and unlocks aromas. A large dose (the kind from sitting open for days) browns the wine, flattens fruit, and eventually turns it to vinegar.
Two reactions are happening in an open bottle: oxidation — oxygen reacts with wine compounds, gradually creating brown color and tired flavors, the same chemistry that browns a cut apple; and acetic acid formation — bacteria slowly convert alcohol to vinegar, which is what eventually makes very old open wine smell sharp and sour.
Cold slows everything. Light damages aromatics. Surface area exposed to air is the single biggest factor. A bottle with one glass left has 4× more air per drop of wine than a full bottle — and dies 4× faster.
