The role of SO2 in wine
In order to understand the role of added SO2 in the formation of volatile sulphur compounds, it is important to understand why it is added to wine in the first place. SO2 is added to wine for its antiseptic, antioxidant, and antioxidasic properties. It can be sprayed on the grapes, added to the must, used to arrest the primary (alcoholic) fermentation, used to arrest the malo-lactic fermentation, and added at bottling. SO2 exists in wine in more than one form. In winemaking, reference is often made to the “total” SO2 being a combination of “free” SO2 and “bound” SO2. The “bound” SO2 is the fraction that has formed other sulphur containing molecular compounds and can thus no longer play an antiseptic/antioxidant role. The “free” SO2 is the useful fraction, but even this does not exist purely as molecular SO2. In solution, SO2 exists as a combination of hydrogen sulphite (HSO3–), sulphite (SO32-), and SO2, like this:
The balance between these three forms depends on pH. At wine’s pH, the dominant form is HSO3–, although SO2 and SO32- will also be present. As the pH goes lower, the proportion of SO2 goes up, and that of HSO3– goes down; as the pH goes higher, the proportion of HSO3– once again goes down, and the proportion of SO32- goes up. Hence, it is a generally known dictum in winemaking that the more acidic the wine, the more SO2 is available to do its job, and the less of it that needs to be added.
It should be noted that SO2 in wine is not only useful but generally also harmless. The amount of total SO2 permitted in wine is regulated by law, and as such, will only be present in quantities which are safe for human consumption (of course this will depend on the laws of the region or country of production and adherence to those laws). In the EU, for instance, the basic legal limit for dry wines is 200 mg per litre for white wine and 150 mg per litre for red wine (red wine needs less sulphur dioxide because contains more polyphenolic compounds, which act as natural anti-oxidants). That said, SO2 can cause an allergic reaction in some people, especially if they are prone to asthma or other allergies, and it is for this reason that most bottles now carry the phrase “contains sulphites”. Also interesting to note is the fact that even organic winemaking permits the use of sulphur dioxide, and it is rare to see a wine made entirely without SO2.
The use of SO2 goes hand in hand with modern day anaerobic winemaking, in which great care is taken to avoid oxidation at all stages of the winemaking process. Wines made in this way tend to be incredibly clean, fresh, and fruit-driven, in line with current preferences of the general public. The irony is that wines made in this way are also at a much higher risk of developing sulphur taint. Some experts think the levels of added SO2 should come down to as little as 5 mg per litre, especially for wines bottled under screwcap (check back soon for an article on Screwcaps and Sulphur Taint).
- Understanding Wine Technology, The Science of Wine Explained, D. Bird, DBQA Publishing 2010.
- R. B. Boulton, personal communication, September 2015.