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Spoilage yeasts

Spoilage yeasts include -

• Film or surface yeasts, which form in the presence of oxygen and low sulphur dioxide (SO₂) levels.
  
  They typically grow on the surface of wines that have large head spaces containing air (ie oxygen).
  
  This technique is used to encouraged the desired growth of film yeasts or the flor (flower) in the production of flor sherry.
  
  Surface yeasts convert alcohol to acetaldehyde and can also form acetic acid (the ingredient of vinegar).
  
  
  
• The yeast strain, Brettanomyces, can form "mousy", as in mouse cage, mouse urine, band aid and other off flavour compounds in wine.
  
  Often introduced to wine through infected cooperage.
  
  Brettanomyces is quite sensitive to SO₂ (sulphur dioxide) and its presence would indicate poor hygiene.
  
  
  
• Naturally occurring yeasts are often termed wild yeasts, with the majority being weak fermenters, in that that they have a low alcohol tolerance.
  
  However, these yeasts can dominate the early stages of fermentation , especially in the absence of adequate levels of SO₂, resulting in the production of off flavours or dominating flavours.