For the end-consumer, sulfur dioxide (SO2) in wine has been perceived as an evil chemical compound. In fact, the total absence of sulfur dioxide in wine is not even possible, as the yeasts produce small quantities of SO2 during fermentation, mostly no more than 10 mg/l.
But why do winemakers add more SO2 to wine? Why is it needed? Let’s have a look at the advantages first:
First of all, SO2 has antiseptic properties. It blocks the activities of bacteria and yeasts and prevents microbial spoilage in wine. Bacteria are more vulnerable to SO2 compared to yeasts. Antiseptic attribute is specifically important during the storage of wine as SO2 stops the activities of yeasts, acetic acid bacteria that might turn the wine into vinegar or Brettanomyces, which contaminates the wine over a certain threshold.
The second important characteristic of SO2 is its being an antioxidant. In order to understand how it works, we need to know what happens when SO2 dissolves in wine:
H2O (water) + SO2 (molecular SO2) <=> H+ + SO3– (bisulfite) <=> 2H+ + SO3= (sulfite)
(Little chemistry here: The double arrows indicate that the reaction is in equilibrium. The reaction between the different types of sulfite is going both ways at a steady state so the concentration of these compounds is stable. This does not mean sulfite and bisulfite have equal amounts. This depends on the pH of wine)
Let’s come back to the antioxidant effect of SO2! SO2 does not directly react with O2. (SO3= can bind directly to O2 but there is almost no SO3= present in wine due to its pH range) During oxidation, O2 reacts with flavonoids (such as catechin or epicatechin) and this results in reactive agents such as quinones. SO2 plays an important role in reducing quinones formed during oxidation and reverts them back to their nonreactive phenol form. Furthermore, SO2 reacts with hydrogen peroxide (oxygen reduced form) and inhibits aldehydes formation.
Another advantage of SO2 is its antioxidasic feature. It simply inhibits the oxidation enzymes (tyrosinase, laccase) and ensures that they are destructed in time. This is why SO2 is crucial especially before fermentation. It also stops oxidasic casse, a fault caused by an enzyme that makes wine cloud when exposed to air and changes the color of wine. There is one drawback though, at doses high enough to stop oxidasic casse SO2 also blocks malolactic fermentation. One other advantage of SO2 is that it can partially correct the wine after oxidation. It simply binds with acetaldehyde (oxidation product) and creates a tasteless bisulfite addition.
So if SO2 has so many advantages, why is it treated like a villain?
Well, here is the other side of the coin. First of all, there are some health concerns about SO2, especially about allergic reactions. These reactions may occur at very low ingested levels especially with asthmatics. Therefore, it is required to mention that the wine contains sulfites when the concentration exceeds 10 mg/l. In Europe, there are upper limits for the use of SO2. For red wines (with less than 4 g/l of sugar) the limit is 150 mg/l and for whites and roses (with less than 4 g/l of sugar) it’s 200 mg/l. The reason of the difference is the higher percentage of phenolic protects the red wine from oxidation so the required amount is less compared to white wines. For sweeter wines the limits are higher as more SO2 is needed. Within the legal limits, it is also important to mention that in lower pH (which means higher acidity) less SO2 is needed because there are more “free SO2” in wine to be active. (pH and SO2 relation is another big topic which can be an essay itself) (Also check the difference between free and bound SO2: Sulfur Dioxide)
Many people believe that headaches occur due to SO2 but there is no scientific proof. Recent studies show that the headaches are caused by biogenic amines such as histamine and research on this topic still continues.
As another disadvantage, high doses of SO2 also neutralize aroma and even more amounts produce faulty aromas like wet wool. It is also known that sulfite binds to anthocyanins and therefore, the wines start losing color. The effect is more obvious in light reds or roses.
Using SO2 cleverly and minimizing the amounts can avoid the disadvantages of sulfur dioxide. I believe that people are interested in wines with “no added sulfites” due to the rising trend of natural wines. As the winemakers of Frey Vineyards claim, it might be possible to make wine without sulfites with healthy grapes and great care in the winery with temperature control. I like the idea of drinking “natural” wine, but to be honest, my taste buds do not enjoy oxidation. There are indeed some natural wines that are very well protected with great complexity. However, considering the supply chain for different wines of the world, it doesn’t seem very likely to keep the wine under optiomal conditions at all times. In many occasions wine travels long way to reach its final destination. I personally do not see SO2 as the biggest enemy. And if I were a winemaker, I’d rather use minimum amounts of sulfur dioxide – only enough to keep the wine safe and focus on growing organic grapes and many other sustainable projects to protect the environment.
The Science of Wine, Jamie Goode
The Handbook of Enology, Professor Pascal Ribéreau-Gayon, Denis Dubourdieu, Bernard Donèche, Aline Lonvaud