Article by Elona Hesseling of Wineland Magazine

Aldehydes are chemical compounds formed by the oxidation of alcohols in wine. While these compounds are responsible for the oxidation characteristics in healthy wine, they can contribute positively to the overall aroma in trace concentrations, especially in certain wine styles such as sherry.

Al-de-hydes derive their name from the fact that they are alcohols that have been de-hydrogenated. These by-products of fermentation contribute to a wine’s flavour. They also affect colour, by reacting with sulphites and preventing bleaching, and by participating in the binding of anthocyanins to tannins, thus stabilising colour. Finally, due to the participation in tannin polymerisation reactions, aldehydes also have an impact on a wine’s texture. An aldehyde is an organic compound containing a formyl group. This functional group consists of a carbonyl center (a carbon double bonded to oxygen) bound to hydrogen and an R group, which is any generic alkyl or side chain. The different aldehydes are named after the amount of carbons in their chains. From one to nine carbons, these are known as formaldehyde, acetaldehyde, propanal, butanal, pentanal, hexanal, heptanal, octanal and nonanal. Acetaldehyde (CH3CHO) is the most important and familiar sensory aldehyde in wine and constitutes about 90% of the total aldehyde content. It is described as a colourless, volatile water-soluble compound found naturally in grapes and wine in trace amounts, and produced both by fermentation and oxidation. This compound has the unmistakable smell of cut apples, sherry and nuts. Acetaldehyde can be formed by yeast and acetic acid bacteria. Acetic acid bacteria form acetaldehyde by oxidising ethanol. The amount formed by yeast varies with species, but is considered to be a leakage product of the alcoholic fermentation. Additionally, film yeasts (important in sherry production) oxidise ethanol to form acetaldehyde. Acetaldehyde can also be formed as a result of the oxidation of phenolic compounds. Hydrogen peroxide, a product of phenolic oxidation, oxidises ethanol to acetaldehyde. Several factors can affect the production of aldehydes, namely yeast strain, temperature, pH, oxygen level, sulphur dioxide (SO2) level and nutrient availability. Of these, SO2 is particularly important because it affects the enzyme that converts acetaldehyde into ethanol (aldehyde dehydrogenase). SO2 also binds directly with acetaldehyde, preventing its transformation to alcohol. Oxygen and SO2 impacts the amount of acetaldehyde formed by yeasts. Wine fermented in the presence of SO2 generally has considerably higher amounts of acetaldehyde. This is related to SO2 resistance of certain yeasts …