Wine Chemistry and Biochemistry

8D Yeasts and Wine Flavour 341

anthocyanins

pyranoanthocyanins,

ethyl-bridged

anthocyanin

conjugates

acetaldehyde

pyruvate

glucose

CO 2

ethanol

NAD+

acetate

acetyl-CoA

fatty acids

NAD(P)H

ATP

acetaldehyde

hydroxysulfonate

HSO 3 –

acetoin

2,3-butanediol

NADP+

Acetyl-CoA

Thiamine-PP

diacetyl

2-methyl-

1-propanal

3-methyl-

1-butanal

α-aceto-

lactate

higher aldehydes

CO 2 CO 2

NAD+

CO 2

2-methyl-

1-propanol

3-methyl-

1-butanol

NADP+

Acs1p NAD+

Ald3p

Ald4p

Ald5p

Ald6p

Adh1p

Pdc1p

Pdc5p

Pdc6p

Bdh1p

CO 2

Ilv2p

Fig. 8D.7Aldehyde metabolism, showing key role of acetaldehyde in formation of branched-chain
and higher aldehydes, and diacetyl and acetoin
Acetaldehyde is formed from pyruvate catalysed by pyruvate decarboxylases (Pdc1p,5p,6p),
with most being reduced to ethanol by alcohol dehydrogenase (Adh1p). Higher aldehydes, exclud-
ing hexanal, are formed during fatty acid biosynthesis from acetaldehyde, which is first oxidised
to acetate by aldehyde dehydrogenases (Ald3–Ald6p) and activated with ATP by acetyl-CoA syn-
thase (Acs1p). Branched-chain aldehydes, and diacetyl and acetoin are formed by decarboxylation
reactions from -acetolactate, which is derived by condensation of pyruvate and acetaldehyde
thiamine-PP complex, catalysed by acetolactate synthase (Ilv2p). As fermentation completes, free
aldehydes are largely reduced by dehydrogenase reactions to corresponding alcohols. Extracellular
acetaldehyde can react with bisulfite ion to form acetaldehyde-hydroxysulfonate, and can react
with anthocyanins to form stable pigments, pyranoanthocyanins and ethyl-bridged anthocyanins
conjugates with proanthocyanidins (tannins)

the higher aldehydes [branched-chain aldehydes (2-methyl 1-propanal, 3-methyl 1-

butanal), and the C 1 ,C 3 ,C 5 ,C 6 and C 7 saturated aldehydes] (Frivik and Ebeler 2003).

The saturated higher aldehydes, usually produced in trace amounts, are derived

from the biosynthesis of fatty acids fromacetyl-CoA, which is derived from acetic

acid(Fig8D.7).TheC 6 aldehydes, hexanal,cis-3-hexenal andtrans-2-hexenal, are

derived by enzymatic oxidation reactions of grape C 18 unsaturated fatty acids during

must processing (Rib ́ereau-Gayon et al. 2000a).

Acetaldehyde plays an important role during red wine fermentation in which

it can react directly with anthocyaninspigments to form vinyl adducts (pyroan-

thocyanins) (Fulcrand et al. 1998), and can participate in the condensation of

proanthocyanidins (tannins) and anthocyanins to form ethyl-bridge linked dimers