Sometimes I open a room-temperature bottle of red wine and put in a valved spout. On operating the valve the next day, at the same temperature, the noise I hear suggests that the air in the bottle was at a lower pressure than that outside. Why should this be?
鈥� When you open a bottle of wine, you allow oxygen to flood in. If you then leave the bottle overnight, this oxygen will react with numerous substances in the wine. This will incorporate the oxygen (which, like all gases, takes up lots of space) into the much denser liquid, thereby lowering the pressure of the gases in the bottle.
One example of this is when alcohol in wine oxidises to form vinegar. This eventually causes wine to spoil, but, even if the process is slow, it uses up some oxygen gas.
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The phenolic compounds that give wine its colour also react with oxygen to produce hydrogen peroxide. This goes on to react with gaseous, as well as dissolved, sulphur dioxide in the bottle. So, phenols also cause a drop in pressure by removing both oxygen and sulphur dioxide from the air inside the sealed bottle.
Emily Fox, Fortrose, Highland, UK
鈥� There are many processes that might cause this effect. Some involve the bitter tannins in wine, which are phenols and can form longer, chain-like polymers when oxygen is present. A wine rich in tannins can be opened in advance of drinking to allow this oxidation process to soften the flavour.
The colourants of red wine also oxidise and can eventually precipitate as a polymeric sludge or sediment at the bottom of the bottle. Even sulphites, added to wine to inhibit yeasts and bacteria, oxidise to sulphate at an appreciable rate in air. All of these processes deplete oxygen in the air pocket over the wine, thereby depressurising it to some extent.
Peter Urben, Kenilworth, Warwickshire, UK
鈥� Some of the oxygen in the air would be consumed by bacteria that turn ethanol into acetic acid. However, only a small amount of alcohol would be used up and very little acetic acid would be made.
If you had, say, a quarter of a litre of air in the bottle, that represents only about 64 milligrams of oxygen, which if it were all consumed would produce only 120 milligrams of acetic acid, using up only 92 milligrams of ethanol. The half-litre of wine left in the bottle originally had about 50 grams of ethanol, so you would be unlikely to detect much change in flavour.
鈥淎 wine rich in tannins can be opened in advance of drinking to allow oxidation to soften the flavour鈥�
Eric Kvaalen, Les Essarts-le-Roi, France
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