Three Forms of SO2 in Solution

What do the different forms of SO₂ really do? Why does this matter? The activity of SO₂ in grape juice and wine depends on the chemical form it is in. Undersanding the rold of each form can lead to better management decisions. Following is a summary of  the activities of each form.

Molecular SO₂

Molecular sulfur (SO₂) is prized because of its antimicrobial activities. As the only form of sulfur dioxide that is not charged, molecular sulfur can penetrate the cell membranes of microbes and cause cellular damage and death. Once inside the cells of yeast and bacteria, which themselves have a pH around 6.5, molecular sulfur converts to the bisulfite form and denatures proteins, disrupts cell membranes, and ends cell functioning (1–3). Many Saccharomyces cerevisciae have a special cellular pump to rid the cell of sulfide (4), which means they are less affected by SO₂ than other organisms. Molecular SO₂ also has antioxidant properties as it binds to hydrogen peroxide, a main component of oxidative cascades in wine (3). This form is volatile, and in high enough concentration, can cause negative sensory aromas in the headspace of wine. It is also the one way that free SO₂ is lost during aging, as it diffuses into the headspace of barrels and is dissipated (3). The ability of SO₂ to volatilize is used to measure the concentration of SO₂ during the aeration oxidation test, when strong acid is used to convert all free SO₂ in solution to the molecular form, which is bubbled to a catchment vessel and titrated with base (3).

Bisulfite 

The bisulfite form of sulfur dioxide (HSO₃^-) dominates at wine pH. Bisulfite is a potent inhibitor of enzymes such as tyrosinase (aka polyphenoloxidase) that causes enzymatic browning in juice and wine, though it is somewhat less effective against laccase, the oxidative enzyme produced by Botrytis (2). The activity of bisulfite is limited by the fact that it binds many constituents in the wine including acetaldehyde, anthocyanins, and sugars (1–3). Once bound, it no longer acts as an antioxidant. Bisulfite binding to acetaldehyde forms a compound without sensory impact, improving the nutty or bruised apple aroma that accumulates during malolactic fermentation (2,3). Binding of bisulfide to anthocyanins in red and Rose wines can cause color bleaching, as the bound form is colorless. This binding is reversible, so anthocyanins may be released if sulfite is bound up and the equilibrium shifts, however, binding also blocks the polymerization reactions with tannins that would instead stabilize anthocyanins for longer term aging (1,2). Bisulfite also binds sugar. This is the primary fate of most of the sulfur dioxide added to grape must (1,2). New oak barrels have many un-bound sugars that get bound up by bisulfite, a primary reason new barrels tend to require higher SO2 additions than older barrels.

Sulfite 

At the pH of wine, the sulfite ion (SO₃^-2) is nearly non-existent. This form of sulfur dioxide binds directly with oxygen in solution in a slow reaction, so it does have antioxidant properties. However, this reaction is very slow so only has an effect during long bottle aging, where it can be important in phenolic maturation (3).

References

(1)Ribereau-Gayon, P.; Dubourdieu, D.; Doneche, B.; Lonvaud, A. Handbook of Enology Volume 1: The Microbiology of Wine and Vinifications, 2nd ed.; John Wiley & Sons: West Sussex, England, 2006.

(2)Zoecklein, D. B. Sulfur Dioxide (SO2). Enology Notes Downloads, 16.

(3)Boulton, R.; Singleton, V. L.; Bisson, L. F.; Kunkee, R. E. Principles and Practices in Winemaking; Chapman and Hall, Inc: New York, 1996.

(4) Bisson, L. F. Geographic Origin and Diversity of Wine Strains of Saccharomyces. American Journal of Enology and Viticulture 2012, 63 (2), 165–176.