Sulphur in the winery
Sulphur dioxide (SO2) is the most widely used and controversial additive in winemaking. Its main functions are to inhibit or kill unwanted yeasts and bacteria, and to protect wine from oxidation.
Oxidation is the reaction of wine with oxygen. It can alter its colour and odour (tending to make wines darker and dryer) and is often dismissed as a fault.
Excessive oxidation does ruin wine. But controlled oxidisation can add complexity, and is crucial to certain styles (some sherries, for example, and the vins de voile of the Jura). It is also an important part of the ageing process.
This is why most wine authorities will tell you that it is impossible to make a wine which ages well without using sulphur dioxide.
The SO2 drastically inhibits the process of oxidation. Whether what you have is a wine which ages well, or merely one which ages is slowly, is a moot point.
There are four points at which sulphur dioxide is commonly used in conventional winemaking, although the winemaker may choose to make further additions if he is feeling nervous.
Applied in the form of metabisulfite to inhibit the action of wild yeasts and prevent oxidation. It means the grapes don't have to be rushed to the winery.
To prevent fermentation from beginning with wild yeasts before cultured yeasts can be added. Cultured yeasts are bred to be more resistant to SO2.
At any point during fermentation, but most commonly at the end to stop or prevent malolactic fermentation. A natural winemaker has to wait for the malo to finish naturally.
To prevent oxidation (or any other microbial action) in the bottled wine. In sweet wines there is the danger that fermentation will restart.
A natural winemaker would only ever use sulphur dioxide at bottling, only in white wines, and only in very small quantities. Many natural winemakers use none at all.