1. Sulfite Chemistry in Wine
Case Study for Chemical Equilibrium
Sirromet Wines Pty Ltd
Mount Cotton Rd
Mount Cotton Queensland, Australia 4165
Courtesy of Jessica Ferguson
Assistant Winemaker & Site Chemist
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2. What is sulphur dioxide?
Sulfur dioxide is a pungent gas used as preservative in winemaking
May be added as a liquefied gas or as a sulfite salt
Various species of sulphites exist in equilibrium in wine
Very effective preservative as it acts in several different ways:

3. (i) Antimicrobial properties
Inhibits development of micro-organisms
Generally more effective on bacteria than yeast
At low concentrations, protection may be transitory
High concentrations may destroy a percentage of microbe population
Effect is highly pH dependent

4. (ii) Antioxidant Properties
Binds with dissolved oxygen in the presence of catalysers:
SO2 + ½O2 SO3
This reaction is slow – protects from some chemical oxidation but not microbial or enzymatic oxidations
May appear to be antioxidant as it can bleach or hide oxidised compounds (but can’t reverse these reactions)

5. (iii)Antioxidasic properties
SO2 inhibits the functioning of oxidation enzymes
Can also destroy some of these enzymes over time
Used in must prior to beginning fermentation for this reason
Particularly prevents browning in white juice

6. (iv) Binding properties
The bisulphite ion readily binds to various compounds in wine
Most significant is acetaldehyde, and the complex formed is very stable
Other binding compounds include sugars, pigments, phenolics and some spoilage compounds caused by bacteria
Of any addition of SO2 to wine, some will be ‘bound SO2’ and some remain as ‘free SO2’

7. Free SO2 vs. Bound SO2
Unstable binding is somewhat reversible under certain conditions (equilibrium will shift if free SO2 is consumed)
Binding with acetaldehyde is permanent – stable complex
However this is a positive as excess acetaldehyde will give bruised apple and sherry characteristics to wine

8. Chemical Equilibrium of Free SO2
H2O + SO2 ⇌ HSO3- + H+ ⇌ SO3= + 2H+
 decrease  pH  increase 
Equilibrium is pH driven as shown

9. Ka Values for SO2 in wine

10. Molecular SO2 Minimum safe levels
critical level for effective protection is around 0.8mg/L molecular SO2
Levels of molecular SO2 directly affected by pH
the free SO2 requirement to obtain 0.8ppm molecular SO2 will vary with pH
As seen on this table, lower wine pH is desirable as free SO2 is far more efficient at lower pH

13. Red wine bleaching by SO2
Anthocyanins are the compounds responsible for colour in red wines
Free anthocyanins are relatively unstable and will react with SO2 to form a colourless complex
Young red wines are particularly susceptible
In older red wines, anthocyanins have formed condensation products with tannins, which are less reactive with SO2

14. SO2 use at bottling At bottling the dissolved oxygen level is critical
The bottling process itself has the potential to increase dissolved oxygen
Dissolved oxygen at bottling can cause browning and flavour oxidation in the bottle
4mg of FSO2 required to eliminate every 1mg O2
Sulfite (SO3=) ion will scavenge dissolved oxygen but the reaction is very slow due to the low concentration of SO3= at wine pH
Other forms of sulfite do not react directly with O2 but only with its oxidation products

15. Winemaking Implications
Winemakers need to be extremely vigilant with maintaining safe levels of SO2
All wines need to be tested frequently
Emerging problems with some spoilage yeast that can tolerate very high molecular sulphur dioxide levels
Need to maintain high level of cellar hygiene and good winemaking practice

16. SO2 - Health Concerns
By law, wine must be labelled as containing sulphites
No wine is sulphite free – some SO2 is produced by fermentation (10-50ppm)
Many people blame sulphites for various ill-effects caused by wine such as headaches, flushing or hives
Many studies have failed to show that SO2 in wine causes headaches – no link could be found
Headaches in wine drinkers may be caused by biogenic amines (such as histamine) – research continues
One study shows that red wines that have gone through malo-lactic fermentation may contain higher levels of these amine compounds

17. SO2 – Health Concerns (cont)
Headaches are most commonly caused by overindulgence! The side effects of alcohol metabolism can be very rapid (within an hour of drinking) and is underestimated
White wines, both sweet and dry, contain higher sulphite concentrations than red wine
Around 0.1% of the population do have a serious allergy to sulphites and cannot drink most wines
Some asthma sufferers are sensitive to high levels of sulfites – can trigger an attack

18. References
Zoecklin, Fugelsang, Gump & Nury, Production Wine Analysis, Van Nostrand Reinhold, ©1990
Ribéreau-Gayon, Glories, Maujean & Dubourdieu, Handbook of Enology Vol 2, Wiley ©2000
Rotter, B: Sulphur Dioxide article
Iland, Ewart, Sitters, Markides & Bruer, Techinques for Chemical Analysis and Quality Monitoring during Winemaking, 2000