Which factor affects the free sulfur dioxide [SO2] in must?

• A. pH
• B. Temperature
• C. Alcohol content
• D. Yeast strain

Answer: (A)

The key idea is how acidity controls the balance between molecular SO2 and its inactive forms. Dissolved SO2 becomes sulfurous acid (H2SO3) in water, which can dissociate to bisulfite and sulfite forms. The amount of molecular SO2—the active form that provides antioxidant and antimicrobial protection—depends on how much of H2SO3 remains undissociated. At lower pH (more acidic must), more SO2 stays as molecular SO2; at higher pH, more converts to bisulfite and sulfite, so the fraction available as the active form drops. Because pH directly shifts this equilibrium, it most strongly determines the level of free, active SO2 in must. Temperature, alcohol content, and yeast strain don’t shift this acid-base balance as directly, so their effect on the active free SO2 is much less.