'Wine contains bacteria that will convert it to vinegar if they have the oxygen they need to begin the reaction. A well-made and full cask contains no oxygen, or only a very little. Then, too, a cold cellar slows the reaction, which is why the German and northern French vineyards were the first places to age wines in cask. A high degree of alcohol, found in the sweet wines of the Mediterranean, also inhibits the vinegar bacteria.
But it was the rediscovery (the Romans used it) of sulphur dioxide, in medieval Germany, that allowed wine to be properly stabilized. Sulphur, when burned in a cask prior to filling with wine, kills bacteria and prevents oxidation of the wine. By the early 18th century this was common practice in Bordeaux. This century has seen its use worldwide. It is sometimes over-used: many white wines, from Spain and France especially, gained a name some decades ago for heavy sulphur tastes.
Sulphur dioxide is used in solid, liquid or gas form at several stages. If a white wine is not to undergo malo-lactic fermentation, sulphur protects it and preserves the malic acid which contributes freshness. In red wine-making, sulphur is a general disinfectant for casks, grapes and wine.
Modern practice is to lessen and sometimes avoid the use of sulphur. Healthy grapes, and scrupulously clean winemaking equipment and premises, make it less necessary. Sulphur's use is also discouraged by mandatory labelling ("contains sul-fites") in the USA and Australia. A small fraction of the population is allergic to sulphur, but the labelling law alarms far more. There is at present no EC directive on such labelling.
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