Sulfur Dioxide In Wine Has two properties: –Antimicrobial : Active form is Molecular SO 2 –Antioxidant: Molecular form of SO 2 is the reactive form at wine pHs –Both forms are dependent on the pH of the Wine copyright © 2008 Stuart Robinson

The Effect Of pH On The Disassociation Of Sulfur Dioxide In Wine Antimicrobial : Active form is Molecular SO 2 copyright © 2008 Stuart Robinson

SO 2 aq HSO 3 - SO 3 -- Free SO2 Bound SO2 MolecularBisulfite ionSulfite ion Acetaldehye, keto acids, glucose, quinones, monomeric anthocyanins of red wine making them colorless Hydroxy-sulfonates Chemical Species Of Sulfur Dioxide copyright © 2008 Stuart Robinson

Molecular SO 2 is the Antimicrobial Active form pH Dependant Need ppm (mg/L) molecular SO 2 for microbial effectiveness pH 3 –6% of SO 2 is in molecular form Need 14.8 mg/L pH 4 –0.6% of SO 2 is in molecular form Need mg / L = copyright © 2008 Stuart Robinson

Figure 1 copyright © 2008 Stuart Robinson

Questions, use Figure 1 At pH 3.1 how much sulfur dioxide do you need to add to obtain a molecular level of milligrams per litter? At pH 3.8 how much sulfur dioxide do you need to add to obtain a molecular level of milligrams per litter? copyright © 2008 Stuart Robinson

Figure Two copyright © 2008 Stuart Robinson

Questions, use Figure 2 At pH 3.1; what percent of the sulfur dioxide that you added, to obtain a molecular level of milligrams per litter, is in the molecular form? What percent is in the bisulfite form? At pH 3.8; what percent of the sulfur dioxide that you added, to obtain a molecular level of milligrams per litter, is in the molecular form? What percent is in the bisulfite form? copyright © 2008 Stuart Robinson

Antioxidant: Molecular form of SO 2 is the reactive form at wine pHs SO 2 aq HSO 3 - SO 3 -- MolecularBisulfite ionSulfite ion copyright © 2008 Stuart Robinson

Reaction at Wine pHs SO 2 aq HSO 3 - SO 3 -- MolecularBisulfite ionSulfite ion 4 -- SO 4 -- O2O2 H2O2H2O2 reaction not at Wine pH SO 2 aq HSO 3 - Molecular Bisulfite ion Depends On wine pH copyright © 2008 Stuart Robinson

Molecular form of SO 2 is a Quenching compound Ascorbic Acid + O 2  H 2 O 2 Vicinal-Dihydroxy Phenol + O 2  H 2 O 2 –Caffeic (or caftaric ) acid Andrew L. Waterhouse and V. Felipe Laurie Oxidation of Wine Phenolics: A Critical Evaluation and Hypotheses Am. J. Enol. Vitic., Sep 2006; 57: SO 2 aq 4 -- SO 4 -- copyright © 2008 Stuart Robinson

H2O2H2O2 O2O2 +Vicinal-Dihydroxy Phenol, Ascorbic acid Ethanol + Acetaldehye + SO 2 Reduces Free SO 2 shows that oxygen reacts with phenolics and ascorbic acid (1) to produce hydrogen peroxide, in turn hydrogen peroxide reacts with ethanol to produce acetaldehyde. Once acetaldehyde is formed it binds reversibly with sulfur dioxide thus reducing the free sulfur dioxide concentration in the wine solution. copyright © 2008 Stuart Robinson

The big picture then is to understand that these reactions are going on all the time in the bottle -acetaldehyde is always being formed at a slow rate and that there's a differential going on between acetaldehyde binding sulfur dioxide and the dissipation of sulfur dioxide in the bottle, so once there is no more free sulfur dioxide then we begin to perceive the acetaldehyde present. (Roger B.Boulton personal communication, 2005) Another way of stating this is that the free sulfur dioxide in solution depending on the pH of the wine disassociates into two forms, molecular and the bisulfite form, the bisulfite form of sulfur dioxide binds acetaldehyde, this in turn masks our perception of acetaldehyde in the wine, then know that acetaldehyde is always being formed, and when there is no more free sulfur dioxide we then began to perceive the acetaldehyde in the wine. (1) However,John Danilewicz (personal communication, 2006) observe that one of the most important effects of SO2 in wine is to react with hydrogen peroxide, reducing the oxidation potential because of the presence of this oxidant. In a similar way, ascorbic acid can also recycle quinones back to phenols but it can also have the opposite effect depending on its concentration. Under oxygen- rich conditions, ascorbic acid will quickly oxidize, producing peroxide in the same fashion as catechols (Bradshaw et al.2001). ( 1) Andrew L. Waterhouse and V. Felipe Laurie Oxidation of Wine Phenolics: A Critical Evaluation and Hypotheses Am. J. Enol. Vitic., Sep 2006; 57: copyright © 2008 Stuart Robinson

References Boulton, R.B., V.L. Singleton, L.F. Bisson, and R.E. Kunkee. Principles and Practices of Winemaking. Aspen, New York (1996). Andrew L. Waterhouse and V. Felipe Laurie Oxidation of Wine Phenolics: A Critical Evaluation and Hypotheses Am. J. Enol. Vitic., Sep 2006; 57: 306 – 313 Boulton, R.B. Chemical Stability Of Wine. Lecture University of California at Davis, Ven 126, 2005.