This is caused by yeasts such as Saccharomyces or Candida, which grow as a film on the surface of the tank if the surface is allowed contact with air. These oxidise ethanol (alcohol) to produce acetaldehyde (ethanal), producing a ‘sherry-like’ aroma and combining the free sulphur dioxide, thus allowing for further oxidation and spoilage.
In order to prevent this development, tanks and barrels must be kept fully topped up or regularly blanketed with inert gas. Free sulphur dioxide levels should also be kept at around 25 mg/l and good winery hygiene is essential.