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The relevance of pH to SO₂ use in wine processing.

• The pH of grape juice and wine influences many aspects of wine production.
  
  The aspect we are concerned with here is its influence on the equilibrium between the different forms of free sulphur dioxide (FSO₂).
  
  
• A decrease in pH of juice or wine will increase the most effective form of FSO₂ (free sulphur dioxide), that is molecular SO₂. (see table or equation for calculations)
  
  
• It must be noted that because of the logarithmic function of pH, an increase of one pH unit equals a ten fold increase in the hydrogen ion concentration.
  
  
• While a decrease in pH only increases the molecular SO₂ by a small amount, it has a great significance in increasing its antimicrobial activity and its role in reducing oxidation.
  
  
• Juice and wine pH generally range from 2.9 - 4.0.
  
  
  White wine average pH is 3.3.
  Red wine average pH is 3.4
  
  
• Red wine's pH is higher due to the extraction of potassium ions (K⁺) from the grape skins, during fermentation on skins.
  
  The extra K⁺ (pottasium ions) force the hydrogen ions (H⁺) back onto the acidic compounds that released them, to maintain charge balance.
  
  This will decrease the free [H⁺] (hydrogen ion concentration) and hence increase the pH (see pH Scale).
  
  
• The conversion of malic acid to the weaker lactic acid during malolactic fermentation (MLF) will also result in an increase of wine pH.