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Volatile Acid Measurement - Introduction

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• The measurement of volatile acidity (VA) is important at all stages of wine production.
  
  
• VA (volatile acidity) is removed and collected by steam distillation
  
  
• The predominant volatile acid is acetic acid.
  
  
• VA (volatile acidity) analysis, measures a number of volatile acids with acetic acid predominating and some that should not be included in the measurement and need to be removed or allowed for.
  
  Three of these acids are lactic acid, sulphurous acid and carbonic acid.
  
  
  • Lactic acid, although only slightly volatile, can distill across and interfere if present in significant quantities.
    
    Lactic acid is produced from malic acid during MLF.
    
    High quantities will be produced if the grapes were grown in a cool climate with a high initial malic acid concentration and a complete MLF is conducted.
    
    
  • Sulphur dioxide (SO₂) additions to juice and wine can distill across and exist as sulphurous acid ().
    
    SO₂ can be converted to sulphuric acid (H₂SO₄), a non-volatile acid, by small additions of hydrogen peroxide (H₂O₂) to the sample being tested.
    
    
  • Carbon dioxide (CO₂), if distilled across can exists in solution as carbonic acid.
    
    
• VA is measured by removing and collecting the volatile acids from the sample by steam distillation, using a Markham still, or a modified Markham still.
  
  The collected sample is titrated with a sodium hydroxide (NaOH ) solution to determine the acid concentration.
  
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