Sulfur Dioxide Management in Winemaking: Dosing and Legal Limits

Correction of Free SO2

Determining the free SO2 content requires knowing the fraction of SO2 that can combine with certain wine components. This combination is referenced in index tables.

Dose of SO2 and Timing of Sulfite Addition

Initial Sulfiting

The first sulfite addition occurs immediately after pressing or squeezing the juice. This step is crucial to:

• Prevent fermentation under inadequate conditions.
• Aid in selecting appropriate yeasts.
• Prevent enzymatic browning and oxidation.

This initial sulfiting is of low intensity, typically around 45 mg/L total SO2.

Post-Fermentation Adjustment

After fermentation, the SO2 level must be restored, usually after the wine has been decanted, unless malolactic fermentation is desired. The target dose is 20 mg/L of free SO2.

SO2 Loss During Storage

During wine storage, SO2 can be lost due to several factors:

1. A portion transforms into sulfuric acid (H2SO4).
2. Temperature variations in the cellar can cause free SO2 to transition into a gaseous state.
3. Air ingress into inadequately sealed storage areas.
4. Excess movement during racking (trasiego) affects the free SO2 content.

Required Free SO2 Conservation Doses

The free SO2 content that must be maintained in the wine varies:

• Red Wines: 10–20 mg/L
• Dry White Wines: 18–25 mg/L

Lower levels (below 10 mg/L) may suffice for protection in some cases. For expedition (bottling/shipping), the recommended doses are:

• Red Wines: 20–30 mg/L
• Dry White Wines: 30–35 mg/L

Legal Limits of Total SO2

Legal maximum limits for total SO2 are:

• White Wines: 210 mg/L (with lower values in specific DDOOs).
• Red Wines: 160 mg/L.

SO2 Adjunct Products

Due to the disadvantages associated with SO2, alternatives that could replace it are sought. An ideal replacement should:

• Possess the positive effects of SO2.
• Be less toxic.
• Be organoleptically neutral.
• Be easily metered and practical.
• Have a reasonable cost.

Currently, no single product meets all these characteristics. However, substances that reinforce SO2's action have been found:

1. Ascorbic Acid

Naturally present in grapes, adding ascorbic acid to wine consumes dissolved O2 and transforms into dehydroascorbic acid. It acts as a reductant to prevent oxidation and protects against minor aeration during filling. The typical dose is 4–8 g/hL, but it should not exceed 150 mg/L to avoid imparting 'green' fruity notes.

2. Sorbic Acid

Sorbic acid acts against yeasts, preventing the formation of 'flowers' (film yeasts) in wine. However, it does not act against lactic acid bacteria or acetic acid bacteria. It can impart a geranium-like smell, and the recommended dose is 200 mg/L.