Chapter 18

Sulfur Dioxide and Winemaking

Sulfur Dioxide Calculations

To adjust a wine to a given molecular sulfur dioxide level, first determine the wine's pH and free sulfur dioxide, then consult Table 18.1 Amount of Free SO2 Given Molecular SO2 Level. This will indicate the amount of free sulfur dioxide necessary to yield a given 0.8 ppm or 0.5 ppm molecular sulfur dioxide. As previously mentioned, a certain percentage of the added sulfur dioxide will bind with wine constituents (such as acetaldehyde, sugars, phenolic compounds, anthocyanins, and others). Since only a portion of the sulfur dioxide that is added will be free sulfur dioxide, it is necessary to estimate what percentage will become free sulfur dioxide (Section 18.3). Always follow-up with analysis of the wine to determine whether the added sulfur dioxide was sufficient.

Potassium Metabisulfite

The use of solid salts that release sulfur dioxide is practically compatible with all the phases of winemaking, and it is certainly preferable in wine sulfitation, since it does not involve the addition of water. Further, the dosage is precise, given that the salts are easily weighable and more stable over time compared to aqueous solutions, when stored in closed containers and in dry environments.

Added to Harvest Bins

Sulfur dioxide can be added to harvesting bins if there is a risk of spoilage due to high temperatures, high pH, diseased or damaged fruit, long transport distances to the winery, or long wait times until the fruit can be processed at the winery. High temperatures and pH increase proliferation of microbes after infection and increase the rate of oxidation.

Added to Juice/Wine

By using the following equation, you can determine the amount (weight) of potassium metabisulfite to add to the juice/wine depending on the specific needs. Potassium metabisulfite is dissolved in warm water before being used.

Liquid Stock Solution

Medium and small wineries generally prefer to use liquid stock solutions. Solutions of 5 to 10 percent are commonly recommended and convenient for making several sulfur dioxide additions to various wines at one time. Stock solutions should be prepared immediately prior to use, as they are not stable for long periods of time. To make a 10 percent sulfur dioxide stock solution add 174 grams of KMBS to a 1-liter volumetric container (100 g SO2 / 0.576 = 174 g KMBS).

Gaseous Form

Compressed gas has the advantages of being economical and avoiding the addition of potassium into the juice or wine.

Timing of Sulfur Dioxide Additions

When adding sulfur dioxide to must or wine it is important to consider the stage of winemaking that it is in, such as fermentation, maturation, or pre-bottling.

Harvest

When fruit is harvested there is likely to be berry breakage, especially if mechanically harvested, which can lead to quicker browning, wild fermentations, and spoilage.

Pre-Fermentation

A sulfur dioxide addition is typically done immediately after crushing or just following pressing of white grapes and before yeast inoculation. Levels used depended on the variety and maturity as well as fruit integrity.

Fermentation

During fermentation carbon dioxide is formed, which protects the wine against oxidation. Sulfur dioxide should not be added during fermentation, as it may kill the yeasts.

Post-Fermentation

Wines, having completed their primary or secondary (malolactic) fermentation are very susceptible to oxidation and bacterial spoilage. Therefore, a second timely addition is needed to keep them sound.

Maturation

During maturation sulfur dioxide levels will continue to decline throughout the winemaking process - topping, racking, etc.

Bottling

Finally, sulfur dioxide levels are usually adjusted immediately before the wine is packaged to ensure that a sufficient amount is present to protect against oxidation and microbial activity for its foreseeable shelf life.

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