Does wine really go off after opening?
Yes, though "goes off" overstates it for most wines. What happens is oxidation: exposure to air changes the flavour, first improving many wines and then progressively flattening them.
Oxygen reacts with compounds in wine. In small, controlled amounts this is beneficial, which is why decanting exists — a tight young red genuinely opens up over an hour or two as aromatics develop. Beyond that point the same reactions continue: fruit flavours fade, the wine tastes increasingly flat and nutty, and eventually acetic bacteria convert alcohol to acetic acid, at which point it tastes of vinegar.
How long you have depends mainly on the wine.
- Sparkling — 1 to 3 days with a proper stopper. It loses carbonation before it oxidises.
- Light whites and rosés — 3 to 5 days refrigerated.
- Full-bodied whites — 3 to 5 days.
- Light reds — around 3 days.
- Tannic, full-bodied reds — 3 to 5 days; tannins and higher alcohol act as antioxidants.
- Fortified wines such as port and sherry — weeks to months, because the added spirit inhibits spoilage.
Two pieces of practical advice that are more effective than most gadgets. Refrigerate everything, including reds; cold slows the reactions dramatically and a red can be brought back to temperature in twenty minutes. And reduce the air, by transferring a half-bottle into a smaller bottle filled to the neck, which removes far more oxygen than any pump extracts from a large bottle.
Oxidised wine is unpleasant, not unsafe. It is also perfectly usable for cooking, where oxidation matters far less.