SO2, VA, pH: The Triad of Quality
You’ve worked hard all season to go from a quality fruit to a quality wine. Don’t let all that hard work be for nothing. Make sure you’ve got the right amount of molecular SO2 to prevent oxidation and microbial spoilage while wines are in storage.
The forms of SO2 in solution, and the activity of SO2 depends on pH.
In order to ensure sufficient SO2 to prevent spoilage, you must be at 0.825 molecular SO2, according to Y Margalit (Concepts in Wine Chemistry).
When first adding SO2 after fermentation is complete, the amount added will not always translate into a direct increase in Free SO2, as there are compounds in the wine that will bind some of the initial SO2 addition. It is recommended to check soon after adding and top up to ensure the wine is at the proper molecular SO2 level.
| Free SO2 required to achieve 0.825 molecular | |
| pH | Free SO2 (mg/L) |
| 3 | 14 |
| 3.1 | 17 |
| 3.2 | 22 |
| 3.3 | 27 |
| 3.4 | 34 |
| 3.5 | 42 |
| 3.6 | 53 |
| 3.7 | 66 |
| 3.8 | 83 |
| 3.9 | 105 |
Once the wine is at the target SO2 level it should remain stable unless these situations arise:
- Oxygen presence
- Aldehyde formation
- Microbial growth
- Additions (things like oak, tannin, bentonite and other products) can bind up some of the SO2. It is always a good idea to check SO2 again soon after adding anything to the wine.
Follow best practices and get your wines tested regularly.
After fermentation measure SO2, VA and pH.
Once the pH is known, monitor SO2 and VA every 2 weeks to a month to ensure your wine is being maintained in optimal condition.
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