An opened bottle changes continuously, first improving and then declining. The turning point varies enormously between wines, and it is governed by a small number of measurable factors.
Oxygen does two opposite things
Initial exposure releases volatile compounds that were trapped under pressure and allows aromatics to express themselves.
It also begins oxidation, converting alcohol toward acetaldehyde and eventually toward acetic acid, which is the same process that produces vinegar.
Both start immediately. The wine improves while the first effect dominates and declines once the second overtakes it.
Everything about decanting, aerating and resealing is an attempt to manage the balance between those two processes.
Why some wines need air and others do not
Young tannic reds benefit most, because oxygen begins softening the tannins and dissipating reductive aromas that mask the fruit.
Delicate older wines have already done that work in bottle, and their remaining aromatics are fragile enough to disappear within an hour.
Light whites and sparkling wines generally have little to gain, since their appeal rests on freshness and carbonation that air removes.
The rule of thumb is that structure benefits from air and delicacy suffers from it.
Sulphur dioxide is the wine's own defence
Sulphur dioxide added at bottling binds oxygen and inhibits the microbes that convert alcohol to acid.
It is consumed as it works, so a wine's protection depletes steadily once the bottle is open.
Wines made with very low sulphur additions therefore have a shorter open life, which is one of the practical consequences of that style.
The same wines often also show more dramatic changes in the glass, which some drinkers value and others find unpredictable.
Surface area matters more than volume
Oxidation happens at the boundary between wine and air, so the exposed surface governs the rate.
A half-empty standard bottle standing upright exposes a small circle of wine to a large volume of air. Standing it is better than laying it down.
Transferring the remainder to a smaller bottle filled to the neck reduces both the surface and the available oxygen, and is the most effective simple measure.
Vacuum stoppers reduce the amount of oxygen present but also pull out some aromatics, which is a trade rather than a solution.
Cold slows everything down
Chemical reaction rates fall with temperature, so refrigerating an opened bottle slows oxidation substantially.
This applies to red wine as well, despite the instinct to leave reds at room temperature. A cold red can be warmed before serving.
Refrigeration also slows the microbial activity that produces volatile acidity, which is the more damaging change in a wine left several days.
The combination of a small filled bottle and a cold fridge extends usable life from a day to several for most wines.
Inert gas systems address the problem directly
Spraying a heavier-than-air inert gas over the surface displaces oxygen and forms a temporary barrier.
The barrier is disturbed each time the bottle is moved or reopened, so the technique works best with bottles left undisturbed.
Commercial preservation systems dispense wine under gas pressure without ever admitting air, which is what makes by-the-glass programmes of expensive wine viable.
The equipment cost is why such systems appear in restaurants with serious lists rather than in general use.
Fortified wines follow different rules
Higher alcohol slows microbial spoilage, and oxidatively aged styles have already been exposed to air deliberately during production.
Those styles change slowly after opening and remain drinkable for weeks rather than days.
Fortified wines aged without oxygen behave more like table wine, declining within days once the bottle is opened.
The distinction is not the alcohol level but whether oxidation is part of the intended character, which determines whether further exposure is damaging.
How to tell the wine has turned
The first sign is a loss of aromatic lift rather than an off smell. The wine goes quiet before it goes wrong.
Next comes a flattening of fruit and a slight bruised-apple note, which indicates acetaldehyde formation.
Sharp vinegar aromas and a nail-varnish note come later and mean the microbial stage is well advanced.
Wine at the quiet stage is still perfectly usable for cooking, which is where most of an over-aged open bottle can reasonably end up rather than in a glass.