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? (Continued)
• Crystals of Rochelle salt () and cream of tartar () form in wine because of its alcohol concentration. This affects the solubility of the salt, so that a concentration which is soluble in the starting grape juice is no longer soluble in the resulting wine. This crystallisation is a slow process, and because solubility decreases at lower temperatures, crystal formation can continue after the wine is bottled.
In wine vinegar, bacteria convert the alcohol to acetic acid. The latter’s effect on the salts’ solubilities will not be the same, and the introduction of hydrogen ions further complicates the matter.
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Microbes are probable contributors to the sediment. Even when the production process leaves substantial amounts of the bacteria used to oxidise ethanol to acetic acid in the vinegar, it will remain clear while it is in a full and unopened bottle. Upon opening it and exposing the contents to oxygen, however, the bacteria will quickly proliferate and the vinegar will become cloudy. This cloudiness is called “mothery” from the prescientific name for the bacterial slime, ““, which acknowledges its biological role.
“Exposing the contents to oxygen means the bacteria will proliferate and the vinegar becomes cloudy”
Modern production techniques ensure that when the vinegar leaves the producer, it is pasteurised, but the organisms are ubiquitous and exposure to airborne cells rapidly leads to a harmless but unsightly infection dominated by organisms that can further oxidise the acetic acid to carbon dioxide and water. This can happen in all vinegar types, including both red and white wine vinegars.
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 observed deposit.
Brian Wood, Lenzie, Dunbartonshire, UK