Part 10 - Aldehyde and Acetaldehyde

Part 10 – Aldehyde and Acetaldehyde

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v  Aldehyde

Aldehyde is an organic compound containing a  –CH=O  group. A few examples of aldehydes:

  

v  Acetaldehyde (or Ethanal)

The main aldehyde in wine is acetaldehyde, also called ethanal (CH₃-COH). Ethanal is an intermediary of alcholic fermentation obtained by decarboxylation of pyruvate (see Part 1, step 11 or image below). Ethanal is mainly reduced to ethanol, but small quantities of it may be released into the wine. A high concentration of ethanal gives off a characteristic aroma that contributes to the perception that the wine is oxidized. Normally alcoholic fermentation produces only small amounts of ethanal (average 70 mg/L) that remains below its detection threshold (125 mg/L).    

                          

  

v  Chemical oxidation

However, in the presence of oxygen, more ethanal can be formed by ethanol oxidation. For this reason, ethanal is also defined as ‘oxidized alcohol’. In the bottle, the limited oxygen oxidizes the ethanol in the wine into ethanal. This happens very slowly. It is a non-enzymatic oxidation because no enzyme is involved. It is also called a chemical oxidation. In the long run, if additional oxygen can get into the bottle (e.g. the cork dries out), ethanal will be oxidized into acetic acid, making the wine smell like vinegar and completely out of balance. This explains why an open wine can not be kept long.

                               

  

v  Biological oxidation

Some wines, such as Fino and Manzanilla from Jerez, or wine Jaune from Jura, give off a particular aroma which is characterized by a high concentration of ethanal. These wines are obtained by aging the wine under aerobic conditions and under a film of flor yeasts. The flor-film protects the wine from oxidation. Since the ‘fermentation yeasts’ have consumed all the sugars, the ‘flor yeasts’ have to use the ethanol, glycerol and acetic acid in the wine and the oxygen in the air to grow. Inside the flor yeasts, the ethanol is oxidized into ethanal, catalysed by enzyme. It is a “biological oxidation”. Some of the ethanal escapes from the flor yeasts into the wine. It gives a characteristic aroma to the wine (see below the general scheme of Peinado and Mauricio). That’s what makes biological oxidation beneficial to these wines. However, in bottle, without the protection of the flor, these wines are subject to chemical oxidation as well.

                                              

v Hate and love relationship between oxygen and wine

Oxygen is not always the enemy of wine. Especially to red wine, oxygen is a friend. The polymerization (= clumping) between anthocyanin molecules and tannin molecules and the polymerization of the tannin molecules themselves needs oxygen.

The color of red wine is determined by the anthocyanin and tannin molecules.The action of oxygen during the ripening promotes this polymerization process, which takes place at tank and especially in the slightly air-permeable wooden barrels. Polymerization of tannin molecules softens the astringent taste. That explains why young tannic red wine needs aeration after opening.

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P.S.

Buttery smell was once the trademark of California Chardonnay. Responsible voor this is the 'diacetyl' . Next post we'll take a look at how it is formed.