The year 1487 marked a turning point in the history of good winemaking. In that year, a Prussian royal decree officially permitted the use of the wine additive sulfur dioxide (or SO2) for the first time. Not that it hadn't been used before then. To help preserve their wines during transport, Dutch and English wine traders regularly burnt sulfur candles inside barrels before filling them. It was a trick that they learned from the Romans who conducted the same practice over a millennia before.

Today, the use of the additive sulfur dioxide (SO2) is an almost universally accepted winemaking practice. Sulfur dioxide is best known to most as the food additive 220 or 202. It is a substance that has a simple chemical structure; two oxygen atoms bound to a single sulfur atom. Although it is naturally produced in small amounts by wine yeast during alcoholic fermentation, most of the SO2 found in wines has been put there by the winemaker. It is added at most stages of the white winemaking process, from crushing through to bottling. It is used less liberally during red winemaking, but with an almost mandatory addition being made following the completion of the malolactic fermentation of these wines. SO2 is added in the form of a powder, or is directly fed into the wine as a gas from a dosing gun.

Many would be aware that sulfur dioxide can cause severe allergic reactions in some people. So why do winemakers use it? Put simply, it is very difficult to make wines that have an aging potential beyond a few months if sulfur dioxide is not used during winemaking. A big statement but true.

Sulfur dioxide plays two important roles. Firstly, it is an anti-microbial agent, and as such is used to help curtail the growth of undesirable fault producing yeasts and bacteria. Secondly, it acts as an antioxidant, safeguarding the wine's fruit integrity and protecting it against browning. Despite its chemical simplicity, SO2 can take on a few different forms in a wine. One form is called 'molecular SO2'. When in this form, it is around 500 times more effective in killing wine microbes than when in any of the other forms that it can take (Sneyd et al. 1992). Luckily for us, the desirable yeasts that undertake wine fermentation are more resistant to SO2 than most of the spoilage yeasts. So having some SO2 around helps give the desirable bugs a leg-up in their competitive dog eat dog world in which they co-exist.

When dissolved in wine, SO2 can also exist in what is called an ionised form. This form has the greatest antioxidant effect. In reality, SO2 isn't that keen to react directly with oxygen, but it does readily combine with other oxidising agents that are formed in the presence of oxygen. In conjunction with molecular SO2 which plays a further role of 'knocking out' the naturally occurring enzymes in grapes that cause wines to brown (called polyphenoloxidases), SO2 is a powerful force in keeping wines fresher longer.

One less known but important property of SO2, is its ability to bind with acetaldehyde. This compound has an unpleasant smell of bruised apple or rank sherry, and is produced when wines undergo some oxidation. When the SO2 and acetaldehyde molecules bind to each other, the resultant substance is odourless. The SO2 effectively strips the wine of its oxidative character.

Well if it's so good, why not add bucket loads of the stuff? As with most things, it does have its downsides. When present in excess, it can cause a healthy ferment to stop before the yeast has converted all the grape sugar to alcohol. Furthermore, as the bacteria responsible for malolactic fermentation are particularly sensitive to SO2, excessive amounts will almost certainly block its onset. This lack of resistance to SO2 by malolactic bacteria can be very handy when a winemaker is attempting to make a wine that does not benefit from malo, such as a crisp fruity Riesling or a Rose´. However, malo is a defining component of the style of most full bodied dry white, and nearly all red wines. Therefore, having excessive SO2 present at the time when malo would normally occur is particularly undesirable.

When SO2 is greatly in excess, it can also produce a pungent aroma in white wines, considered by most to be a fault. The aroma is best described as that of a match that has just been struck. Many people have trouble smelling the sulfurous aroma of SO2, but instead perceive it as a irritation of the membranes of the nose. High SO2 can also render the palate of the wine harsh, metallic and frequently bitter.

Sulfur dioxide's ability to bind to many other substances can present a few problems in red wines. SO2 readily binds to anthocyanins, the compounds that make red wine red. When bound, the anthocyanins are effectively bleached, which results in lighter coloured and less commercially desirable wines. This is more of an issue with very young red wines as they contain higher amounts of these simple anthocyanins. In older wines, most of the anthocyanins are chemically connected to tannin molecules, something which renders them far more resistant to bleaching by SO2. In any case, the bleaching is not a terminal problem for wines in the medium to long term. The SO2 level of all wines naturally declines over time, and as this occurs, the colour returns. Incidentally, the ability of SO2 to bind to these and many substances in red wine is the reason why it is close to impossible to smell the struck match character of SO2 in these wines. Many people talk about 'sulfur' character in reds. What they are probably referring to is the presence of the pongy smelling Hydrogen sulphide (H2S), a wine fault that is unrelated to the level of sulfur dioxide in the wine.

One final point. The trend to the use in ROTE (Stelvin™) capsules appears to have recently resulted in a higher incidence of the struck match SO2 character in young white wines. This is because the closure is very effective in both keeping the oxygen out, and the SO2 in, the bottle (Godden et al. 2002). But don't be too critical of the wine if it is very young. The addition of SO2 to the wine at bottling was made to protect it from oxidation in the longer run. So copping a burst of sulfurous 'Beelzebub's bottom breath' when the wine is 3 months old, is perhaps a small price to pay for longer lived fresher wines.

References

Godden, P.W., Francis, I.L., Field, J.B.F., Gishen, M., Coulter, A.D., Valente, P.J., Hoj, P.B. and Robinson, E.M.C. (2002) An evaluation of the technical performance of wine bottle closures. Proceedings of the 11th Wine Industry Technical Conference. Eds. R. Blair, P. Williams and P. Hoj (AWITC: Glen Osmond, South Australia) pp. 44-52.

Sneyd, T.N., Leske, P.J. and Dunsford, P.A. (1993) How much Sulfur? Proceedings of the 8th Wine Industry Technical Conference. Eds. C. Stockley, R. Johnstone, P. Leske and T. Lee (Winetitles: Adelaide, South Australia) pp. 161-166.