(IrishTimes) - Medical science argues that SO2 does not cause most people any problems, but a small number will have a severe allergic reaction

Look at the back label on any bottle of wine and you may be reassured by the lack of any added ingredients. Yet in a fascinating talk at last year’s Ballymaloe LitFest, wine writer Alice Feiring pointed out that there are no less than 70 permitted additives or treatments for wine. The only one that legally must be mentioned is sulphur dioxide, with the statement “contains sulphites”.

Sulphur dioxide (SO2) prevents oxidation in a wine and wards off harmful bacteria. All wines contain small amounts; it is a natural byproduct of fermentation. Most winemakers add further amounts either when crushing grapes or prior to bottling. White wines, which are more prone to oxidation, typically have more than red wines, and sweet wines, which can start refermenting, have the highest levels of all.

Medical science argues that SO2 does not cause most people any problems, but a small number will have a severe allergic reaction, hence the warning. Yet dried fruits typically contain far greater levels of sulphur and none of the bags in my local health-food shop carry any warning. Some argue it causes headaches but there is no evidence for this, although excess alcohol certainly does.

It is impossible to know if a wine is low in sulphur or not. Almost every wine will have natural levels above 10ppm and must carry the warning, including organic wines – SO2 is an organic compound and therefore permitted. So-called natural wines should have lower levels, but you cannot be sure. It is difficult for winemakers to measure levels of SO2 accurately, as some of it binds with other compounds and is harmless. It is only “free” sulphur that causes reactions.

Proponents of low sulphur wines argue that it dulls the natural terroir and flavour of a wine. Opponents say it prevents off-flavours and infection. In my experience it is young fruity red wines with good levels of acidity that work best with low sulphur winemaking.

Should we be concerned about sulphur? Probably not. Levels are far lower than in the recent past. Of greater interest are the levels of pesticides and fungicides used in producing grapes.