How long does a bottle of wine keep after you have opened it?

Oxygen is the cause. As soon as you open the bottle, the wine comes into contact with air and the color, aroma and flavor begin to change.

Oxygen reacts with the phenols and the alcohol in wine. The fresh fruit notes flatten out, white wine gets darker in color and red wine turns towards brick red. The sulfur dioxide in the wine absorbs part of those reactions, but is gradually used up.

The emptier the bottle, the more air is in it. In a bottle, more than 60 percent of the oxygen is in the headspace above the wine.

Research on oxygen in packaged wine shows that most of the oxygen is not dissolved in the wine but in the air above it. The more you pour out, the larger that headspace becomes. Pour the rest into a smaller, well-sealing bottle and you remove most of that air.

Cooling slows down oxidation. At a lower temperature wine consumes oxygen noticeably more slowly, and that applies to red wine as well.

In laboratory tests, wine at 35 degrees consumes oxygen much faster than at 16 degrees. What is consumed faster is also oxidised faster. So simply put an opened red wine in the fridge and take it out half an hour before pouring.

Count on one to two days. A bottle of champagne contains about 9 grams of dissolved carbon dioxide, good for roughly 5 litres of gas, and that escapes as soon as the bottle is open.

The carbon dioxide keeps sparkling wine fresh and carries the aroma upwards. Once the pressure is off the bottle, the gas keeps escaping, even with a good stopper on it.

Fortified wine contains 15 to 22 percent alcohol, and many port and sherry styles are deliberately exposed to oxygen during ageing.

Styles such as tawny port, oloroso and madeira mature in contact with oxygen; what has already oxidised then changes only slowly. Fino and manzanilla are the exception: they mature under a layer of yeast and remain fragile, so treat them like a white wine.

Close the bottle immediately, stand it upright in the fridge and pour the rest into a smaller bottle once less than half is left.

There are no fixed standards here. The time periods you read everywhere are rules of thumb, not legal or scientifically established limits. A vacuum pump or an inert gas system slows oxidation further.