Free sulfur

Free sulfur meaning in wine

Free sulfur, usually written free SO₂, is the unbound portion of sulfur dioxide available to protect wine from oxidation and microbial spoilage. It exists in a pH-dependent equilibrium as molecular SO₂, bisulfite (HSO₃⁻), and a very small amount of sulfite (SO₃²⁻); at normal wine pH, bisulfite is the predominant form. Molecular SO₂ provides the principal antimicrobial effect, while the forms participate differently in protection against oxidation. Because pH controls how much free SO₂ is molecular, two wines with the same measured free SO₂ can have different levels of microbial protection. Winemakers measure free SO₂ in milligrams per liter—approximately parts per million—and set targets according to pH, wine style, oxygen exposure, and applicable limits. After an addition of potassium metabisulfite or sulfur dioxide, some SO₂ can bind to other wine compounds, so the free level must be retested rather than inferred from the dose alone.

Source: Australian Wine Research Institute overview of sulfur dioxide in wine.