Acetaldehyde is a volatile compound formed when ethanol is oxidised by bacteria and yeasts, especially acetic acid bacteria and film yeasts, and can be formed either as a by-product of fermentation or due to spoilage of finished wine. It is a by-product of the oxidation of ethanol to acetic acid. Acetaldehyde can also be formed from the oxidation of phenolic compounds, as hydrogen peroxide, a product of phenolic oxidation, will also oxidise ethanol to acetaldehyde.
At low levels, acetaldehyde can contribute pleasant fruity aromas to a wine. At high levels the aroma is considered a fault and is reminiscent of rotten-apples. Acetaldehyde intoxication is also implicated in hangovers. The sensory threshold for acetaldehyde is 100-125 mg/L. Above this level, acetaldehyde leads to a reduction of varietal character, and begins to impart a sherry-like aroma to the wine. Aromas associated with acetaldehyde include green apples, nuts, straw, grass, sherry and rotten apples.
- An overview of formation and roles of acetaldehyde in winemaking with emphasis on microbiological implications, S. Q. Liu and G. J. Pilone, International Journal of Food Science and Technology Vol. 35, 2000.