Chapter 18

Sulfur Dioxide Use in Winemaking

(book excerpts)

Sulfur dioxide is the most common additive in winemaking. Sulfur dioxide plays two important roles. First, it acts as an antioxidant, safeguarding the wine’s fruit integrity and protecting it against browning. Second, it is an anti-microbial agent, and as such is used to help curtail the growth of undesirable fault producing yeasts and bacteria. Sulfur dioxide exists in three forms: bisulfite (HSO3-), molecular SO2, and sulfite (SO32-). The bisulfite and molecular forms make up what is termed as “free sulfur dioxide.” Because the sulfite form is quite negligible (less than 1 percent), the percentage of free sulfur dioxide in the sulfite form is not often calculated. The majority (over 90 percent) of free sulfur dioxide is composed of the bisulfite form, which acts as an antioxidant in wine. A small portion of the free sulfur dioxide also exists as the molecular form, which acts as an antimicrobial agent. The amount of sulfur dioxide in the free form, and the effectiveness of the free sulfur dioxide, depends on the pH of the wine. The higher the pH, the less sulfur dioxide will be in the useful free form AND the less effective this free sulfur dioxide will be. There are a number of techniques that are used to measure the concentration of sulfur dioxide in wine. Automated systems are available and offer considerable benefits to laboratories that routinely analyze relatively large numbers of samples. Sulfur dioxide may be applied in powder form, as an aqueous solution, or as a gas. Otherwise, sulfur can be applied by the combustion of sulfur wicks or discs. The maximum quantities allowed in winemaking are established by specific laws in force in each country.

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