Wine Chemistry and Biochemistry

8D Yeasts and Wine Flavour 337

Cytosolic aldehyde dehydrogenases

Ald2p, Ald3p, Ald6p

Mitochondrial aldehyde dehydrogenases

Ald4p, Ald5p

Acetaldehyde Aceticacid

NAD(P)+ NAD(P)H

Acetic acid

Acetyl CoA

Tricarboxylic acid pathway

Lipid biosynthesis

Acs1p

Fps1p

Fig. 8D.6Acetic acid metabolism inSaccharomyces cerevisiae
Acetaldehyde is oxidised to acetic acid by co-reduction of NAD(P)+catalysed by cytosolic and
mitochondrial aldehyde dehydrogenases (Ald2-Ald6p). Acetic acid is activated with coenzyme A
for lipid biosynthetic reactions, including fatty acids. Surplus acetic acid generated from redox
balancing reactions (growth and stress induced) is secreted into the fermentation medium. The
different aldehyde dehydrogenase genes are expressed according to their physiological role, with
ALD6being the major isogene under dry wine fermentation conditions

are the predominant final products, which are incorporated into phospholipids, the

backbone of cell membranes. Growth limiting factors, which inhibit acetyl-CoA

carboxylase, a key enzyme in regulating fatty acid synthesis, causes early release of

the fatty acid from the fatty acid synthase complex. This results in the generation of

short and medium chain fatty acids (Wakil et al. 1983; Bardi et al. 1999).

Branched chain fatty acids, such as 2-methylpropanoic, and 2-methylbutanoic

and 3-methylbutanoic acids, are not products of the fatty acid synthetic pathway.

They are instead derived from oxidation of the aldehydes formed from -keto acids

during amino acid metabolism (Fig 8D.5). The mechanism of regulation is not
known.

8D.4.3.3 Modulating Factors

Yeast strain, and nutrient status of themust and fermentation conditions, many of

which affect growth or induce physiological stress, modulate the accumulation of

acetic and other fatty acids in wine. Reported factors include must sugar concen-

tration, nutrient balance, inoculum level, fermentation temperature, pH and aeration

(Delfini and Costa 1993; Henschke and Jiranek 1993; Shimazu and Watanabe 1981).

The effects of osmotic stress, as induced by sugar concentration, on acetic acid

production are discussed in Sect. 8D.3.2.

Strains ofSaccharomyces cerevisiaevary widely in acetic acid production (rang-

ing from 37 mg/L to 999 mg/L; mean 300 mg/L) such that strain selection has been

the most important factor for controlling the acetic acid content of wines during

fermentation (Giudici and Zambonelli 1992; Heard 1999; Paraggio and Fiore 2004;

Radler 1993; Reynolds et al. 2001; Shimazu and Watanabe 1981). Strains selected

for winemaking show minimal variation (Reynolds et al. 2001) although strain

interaction with musts can be considerable (Paraggio and Fiore 2004). Cryotoler-

antSaccharomyces bayanus/uvarumstrains are characterised by producing very