How does acetaldehyde impurity arise in fermentation

There are several tools that a winemaker can use to limit the production and impact of acetaldehyde in wine. Impact of acetaldehyde on wine Acetaldehyde is the main aldehyde present in wine. Acetaldehyde is the main binder of added sulfite molecules.

This binding produces a molecule that has low volatility and low odour, thus limiting the sensory effect of acetaldehyde. However this binding also reduces the efficacy of the added sulfite. Properties Acetaldehyde is a colourless liquid with the chemical formula C 2 H 4 O and the systematic name ethanal not to be confused with ethanol.

It is frequently used in manufacturing as an intermediate in the synthesis of other chemicals, including perfumes and dyes. In large doses it is not pleasant — and in some literature it even gets the blame for the onset of hangovers although technically speaking by the time a hangover kicks in, acetaldehyde has left the body. Carbonyl compounds aldehydes and ketones in wine such as acetaldehyde bind with bisulfite to form hydroxysulfonates, which render the acetaldehyde compound odourless.

Acetaldehyde is not the only compound in wine that binds to SO 2 — different sugars, acids and polyphenols also form bound complexes. In most cases though, acetaldehyde is the major compound associated with bound SO 2 see Table 1.

Table 1 : Proportion of sulfur dioxide typically bound to wine components 2. During fermentation, sugars are primarily converted to ethanol and carbon dioxide. Fermentation is not, however, a simple process and many intermediate steps occur.

Acetaldehyde is involved in one of these intermediate steps. Acetaldehyde is not only formed in the fermentative production of ethanol, it is also present in the pathway of production of acetic acid from ethanol by bacteria. Acetic acid bacteria AAB usually produce significant levels of acetic acid under conditions of high oxygen and low alcohol, however in wine environments with higher alcohol and low oxygen, there is a tendency for AAB to favour production of acetaldehyde.

Whilst yeast typically reduces acetaldehyde to ethanol, under certain conditions of high oxygen, yeast can also convert ethanol back to acetaldehyde via an oxidative pathway. This is typical of yeasts appearing on the surface of ullaged tanks or barrels. In some instances the activity of these yeasts is encouraged and the oxidative processes utilised in the production of certain styles of wine such as flor sherry.

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