Levels of acetaldehyde, the major volatile carbonyl compound found in wine, vary between 10 and 75 mg/l. Wines exposed to air or to aerobic film-forming yeast will develop high levels of acetaldehyde as the oxidation of ethanol, through chemical or metabolic means, will produce it. It is characteristic of the aroma of oxidized or ‘madeirized’ wines like Madeira (Section 2.10.5) and also of flor wines like fino Sherry (Section 2.10.3), and can be detected sensorially at concentrations as low as 0.5 mg/l in aqueous ethanol (Francis and Newton, 2005), although the detection threshold would be significantly higher in wine (see Chapter 4.7 for discussion of sensory analysis). Acetaldehyde is frequently confused with ethyl acetate, which is a component of volatile acidity. The latter has the aroma of pear drops at lower concentrations, and is sharp and vinegary at higher concentrations.