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  articipation 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. The acetaldehyde concentration will also generally increase with higher temperatures. At wine pH (3 to 4), SO2 consists mainly of bisulphite (HSO3-) and small amounts of molecular (SO2) and sulphite ion (SO32-). The bisulphite can form complexes with carbonyl compounds, predominately acetaldehyde, and this binding of acetaldehyde to bisulphite limits its sensory contribution to wine. The addition of SO2 to ‘inhibit’ acetaldehyde production may therefore reduce the perceived aldehyde character, but is most likely only masking the aroma contribution of the acetaldehyde, instead of actually inhibiting its production. Three types of aldehydes (acetaldehyde, methyl-propanal and methyl-butanal) are formed early in the fermentation and most are affected by the presence of SO2, increasing as SO2 levels increase. In contrast, the aldehyde types that are formed towards the end of the fermentation (formaldehyde, propanal, pentanal, hexanal, and heptanal) are not sensitive to the level of SO2. On average, red wines contain 30 mg/L acetaldehyde, white wine 80 mg/L and sherry 300 mg/L. At low levels, acetaldehyde can contribute pleasant fruity aromas to a wine. However, at higher levels the aroma is considered a defect. The threshold in wine ranges between 100 – 125 mg/L, while the high levels in sherry are considered a unique feature of this wine style.

As a wine fault:
At too high concentrations, acetaldehyde is considered a wine fault, commonly described as distinctive, straw-like, somewhat acrid and sherry-like. Too high concentrations of acetaldehyde can be prevented by minimising the exposure of finished wine to air. In the cellar, it is also advised to maintain 25 parts per million (ppm) free SO2, while further keeping fermentation and storage containers topped up. Because of their higher phenolic content, red wines are less susceptible to acetaldehyde production than whites. In sherry Fino is a sherry style produced with the ‘help’ of a natural phenomenon called flor. This is a strain of Saccharomyces yeast that appears as a grey-white film floating on a wine’s surface. It is believed to occur naturally in the microflora of the Palomino grapes used for sherry in Jerez in Spain.

The effect of flor on sherry is to absorb remaining traces of sugar, diminish glycerine and volatile acids, and greatly increase esters and aldehydes. The typical aroma and flavour of fino sherry is mostly related to acetaldehyde and its derivatives, such as acetate, and because SO2 binds acetaldehyde, it will impair typical flor character; no free SO2 should therefore be added to flor sherry. Aldehydes, especially acetaldehyde, are generally considered a wine fault caused by oxidation, although trace amounts could be seen as adding complexity to wine. Acetaldehyde is, however, an important
compound in the production of fino sherry, giving rise to the common acetaldehyde description of a wine having ‘sherry-like’ flavour characteristics.

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