Why does the bottle of red wine vinegar I buy from my supermarket start off without sediment and then acquire it after a couple of months?
鈥� The chemical explanations offered by previous answers may hold some truth. However, crystals of and ) form in wine because of its alcohol content. The salts鈥� solubility is lower in, say, 12 per cent alcohol than in the starting grape juice.
Crystallisation is a slow process and solubility decreases at lower temperatures, meaning crystal formation continues after the wine is finished and bottled. In wine vinegar the alcohol is converted to acetic acid. The salts鈥� solubilities in the acid will again be different, and the introduction of hydrogen ions from the cream of tartar further complicates the matter.
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Microbes are also probable contributors to the sediment. If substantial amounts of the bacteria used to oxidise ethanol to acetic acid remain in the finished vinegar, then once the bottle is opened and the contents exposed to oxygen, the bacteria quickly proliferate. This makes the vinegar cloudy 鈥� or 鈥渕othery鈥�, from the pre-scientific name for the bacterial slime that appears. This slime is known as 鈥渕other of vinegar鈥�, acknowledging its biological role.
Modern production ensures that new bottles leave the producer free of all bacteria, but the organisms are ubiquitous and exposing the vinegar to air will lead to the rapid development of a harmless but unsightly infection dominated by organisms that can further oxidise the acetic acid to carbon dioxide and water. This is true for all vinegar types, not just red wine.
鈥淓xposing the vinegar to air will lead to the rapid development of a harmless but unsightly infection鈥�
The idea that the pigments and tannins in red wine vinegar may become oxidised to insoluble forms has merit, but I think that the bacterial explanation accounts for much of the questioner鈥檚 observed deposit.
Brian Wood, Lenzie, Dunbartonshire, UK