Aldehydes:
Since the formation of an SO2- aldehyde adjunct is one way cells become tolerant to sulfite, excessive sulfite use may lead to release of high concentrations of acetaldehyde that would not then be reconsumed by the yeast. But in general when acetaldehyde is found in wine it comes from the chemical re-oxidation of ethanol during aging and oxygen exposure of the wine. Acetaldehyde levels in wine range from 1 – 160 mg/L. It has a putative threshold of 100 mg/L. It is described most often as sherry-like in wines, but has bruised apple and nutty notes as well. The higher aldehydes derived from amino acids can have strong nutty and rancid nutty notes at high concentrations and are made under the same chemical conditions conducive to acetaldehyde formation. At lower concentrations they may confer notes of coffee, chocolate or stone fruits.