Part 10 – Aldehyde and Acetaldehyde
==================================================
v Aldehyde
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.
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.
