Wine Chemistry and Biochemistry

352 M. Ugliano and P.A. Henschke

Carbon-sulfur

lyase

3-(Hexan-1-ol)- 3-(Hexan-1-ol)- 3-Mercapto- 3-Mercapto-

L-cysteine L-cysteine hexanol hexyl acetate

Alcohol

acetyltransferase

General amino

acid permease

Fig. 8D.10Enzymatic cleavage of S-cysteinyl conjugate and subsequent esterification

lyase is one of the enzymes potentially involved in this clevage process, as overex-

pression of this gene resulted in enhanced release of 4-MMP and 3-MH (Swiegers

et al. 2007).

Formation of 3-MHA occurs thorough a more complex mechanism, that involves

first liberation of 3-MH from the cysteinyl-conjugate precursor, followed by yeast-

driven esterification with acetic acid (Fig 8D.10). The formation of 3-MHA from

3-MH occurs through the same pathway leading to the formation of acetate esters,

since over expression of the alcohol acetyltransferase geneATF1increased for-

mation and overexpression of esterase geneIAH1decreased formation (Swiegers

et al. 2006). 3-MH can also form chemically by reaction between H 2 S produced by

the yeast and carbonyl compounds present in the must, such as 2-hexenal. This
pathway only accounts for 10% of the 3-MH typically formed in fermentation

(Schneider et al. 2006).

Because the ability of different strains ofS. cerevisiaeto liberate long-chain poly-

functional thiols from their precursors is genetically determined, selection of yeast

strain is a powerful tool for controlling the release of 4-MMP and 3-MH during

fermentation (Dubourdieu et al. 2006; Murat et al. 2001; Swiegers et al. 2008c).

Similarly, the ability to form 3-MHA from 3-MH depends on genetic characteristics

of individual strains. Strain characterisation studies have indicated that some yeast

strain exhibit higher ability to hydrolyseS-cysteinyl-conjugates, while other strains

are characterised by increased ester synthetic activity (Swiegers et al. 2008a). Based

on these findings, the use of mixed cultures containing two or more yeasts, one with

high cysteine-lyase activity and the other with high acetate production has been pro-

posed as a tool to modulate the composition of the pool of long-chain polyfunctional

thiols formed during fermentation.

8D.5.3 Anthocyanins and Tannins

8D.5.3.1 Significance

Grape phenolics compounds are important to wine colour, flavour, astringency and

bitterness, with red wines generally containing 1200–1800 mggallic acid equiv-

alents/L of total phenolics, six- to ninefold more than present in white wines

(Kennedy et al. 2006). Hydroxycinnamic acids (non-flavonoid phenolics) are major

phenolic compounds of white wines and are responsible for their colour. Other non-

flavonoid phenolics contribute flavour, such as vanillin, vinyl phenols and gallic

acid. Vinyl and ethyl phenols, which can be present to variable extents, elicit pheno-

lics, medical, ‘Bandaid’, barnyard and spicy characters in wine, which are generally