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Minimising The Use of Sulphur Dioxide (SO₂)

• Determination of sulphur dioxide (SO₂) concentrations will let you know -
  
  
  • the concentrations of the various forms of SO₂ (Sulphur Dioxide) and allow you to -
    
    
    • decide the urgency of SO₂ additions
      
    • determine whether SO₂ is being lost or consumed
    • decide on any remedial action required earlier, (e.g. fixing leaks or microbial activity), reducing SO₂ use.
      
      

• The amount of SO₂ required at the juice processing stage increases with -
  
  
  • increased poor condition of the grapes by -
    
    
    • rupture of grape berry skin do to -
      
      
      • disease
      • bird damage
      • swelling and splitting due to sudden water uptake of water
      • physical damage by -
        
        
        • pressure on lower grapes in tall harvest bins
        • rough and lengthy transport of grapes
          
        • handling and processing
    
    
  • increased temperature of grapes
    
    
  • increased time between harvest and processing
    
    
  • increased time between processing and the onset of fermentation
    
    
  • increased pH (low acidity) of grapes
    
    
  • increased grape particulate matter in juice
    
    
  • increased microorganism numbers due to
    
    
    • poor hygiene
      
    • housekeeping.
      
    • time of optimal conditions for microbial growth
      
      
  Therefore:
  
  Sound, intact, cool grapes grapes, processed, clarified (in the case of white wine production) and pH adjusted early in a well run winery will reduce the concentration of sulphur dioxide (SO₂) required.
  
  
  

• The amount of SO₂ required in wine increase with -
  
  
  • increased wine exposure to air and time of that exposure
    
  • increased storage temperatures
    
  • increased exposure of juice or wine to copper and iron surfaces other than stainless steel
    
  • high nutrient status of the wine for microbial growth
    
  • wines that have been handled protectively
    
  • white wine versus red wines
    
  • Delicate wines relying on fruit derived flavours for their appeal
    
  • lengthy bottle storage anticipated especially in white wines
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
    
  Rupture
  
  
  Grape juice is a nutrient rich medium for a host of spoilage microorganism and subject to oxidation on exposure to oxygen contained in the air around us. The grape berry skin is a protective barrier against microorganisms and contact with air. Once the grape berry skin is ruptured the exposed juice will begin to be oxidised and be attacked by microorganisms with undesirable consequences. The degree of spoilage increases with an increase in time and temperature. Sulphur dioxide is used to inhibit the growth and activity of microorganisms and the enzymes involved in juice oxidation. The smaller the colonies of microorganisms maintained at the early stages of wine processing the lower their activity at later stages.
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  Disease
  
  
  When grape berries are attacked by fungal diseases near or at maturity the berry skin looses its integrity. The disintegration of the skin results in the berry juice being exposed to oxygen and a host of microorganisms. The oxidation of juice and the by products formed during this uncontrolled activity of microorganisms such as wild yeast and acetic acid bacteria, will result in undesirable product formation such as acetic acid (vinegar) that will persist through to the wine.
  
  Increased sulphur dioxide (SO₂) concentrations will be required to subdue or inhibit the growth and activities of these microorganisms.
  
  When grape berries are attacked by the fungal disease, botrytis cinerea (sometimes termed noble rot when it is desired), a greater reliance on SO₂ is also required (see botrytis)
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  Temperature
  
  
  The rate of most chemical reactions and biochemical reactions, especially involving enzymes, during wine processing, storage and aging, increase with an increase in temperature. Juice and wine oxidation and microbial growth and activity are determined by biochemical and chemical reactions and will increase, up to a certain limit, with an increase in temperature. Therefore higher temperatures of grapes, juice and wine place a greater reliance on sulphur dioxide (SO₂) use.
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  Time
  
  
  The other variable that influences the degree of oxidation and microbial growth and activity is time. An increase in exposure time of grape juice or wine to oxygen, oxidative enzymes, microorganism, such as yeasts found on grape skins, without any preventative action, equals an increase in oxidation and microbial growth and activity. This will place a greater reliance on the use of sulphur dioxide (SO₂).
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  Hygiene
  
  
  Many winemakers fear what cant be seen.
  Microorganisms and bacteria, especially, fall into this category.
  However wine is a very hostile medium for bacterial growth.
  
  Sterility, the total exclusion of all microorganisms, is neither possible nor practicle in a winery environment.
  Rather aseptic conditions, where microorganism numbers are kept to a minimum are more important.
  
  No nutrients, equals no microbial growth
  
  Juice spillages and residues and dried up grape skins are nutrients for microbial growth and future contamination for the current vintage and even future vintages. Immediate water rinses removes most nutrient sources before they become difficult to remove, due to dehydration and sugar concentration.
  
  Where rough or porous surfaces are involved, such as oak barrels, a greater reliance on sulphur dioxide (SO₂) will be required.
  
  The larger the colonies of microorganisms, the greater the risk of their growth and activity, and hence the greater reliance on SO₂ to inhibit them will be required.