320 M. Ugliano and P.A. Henschke
Esterification reactions involving alcohols are also favoured by yeast but the rea-
son why this reaction occurs is not clear. It could involve detoxification or to produce
attractant molecules to assist yeast dispersal by insects. Acetate esters, such as ethyl
acetate are formed from ethanol and acetyl-CoA by alcohol acyltransferase (Fujii
et al. 1994). An example of a grape derived compound is the esterification of 3-MH
to the more aroma active 3-MHA (Swiegers et al. 2006).
The occurrence of many of these reactions depends on the presence of appro-
priate precursor in the grape must and together with the concentration of precursor
and the ability of the yeast to (i) produce an appropriate extracellular enzyme for
non-transportable precursors, (ii) take upthe precursor, and (iii) transform the pre-
cursor; different combinations of yeast and grape variety can lead to small or large
differences in the aroma and flavour profile of the wine.
8D.2.3 Non-precursor Flavour Compounds
Some grape-derived compounds, which are released from the berry pulp during
crushing and from the skin, are not metabolised by microorganisms during fermen-
tation, and therefore they accumulatein wine in a chemically unmodified form.
Examples are certain carboxylic acids, phenolics and aroma volatiles.
L-Tartaric acid, the major acid ofVitis viniferagrapes, together withL-malic acid
and smaller amounts of citric acid form the acidic structure of wine and are responsi-
ble for balance and mouth-feel (Jackson 2000); tartaric acid is stable to degradation
by fermentation microorganisms whereas malic acid can be partially metabolised
by yeast and malic and citric acids are metabolised by lactic acid bacteria during the
malolactic fermentation, thereby reducing wine titratable acidity and increasing pH
(Swiegers et al. 2005).
Grapes are rich in phenolic compounds that are responsible for colour, astrin-
gency and bitterness in wine. The latter two flavour attributes are contributed by
a large group of compounds derived from flavan-3-ols, which are represented by
monomers, oligomers and polymers (condensed tannins or proanthocyanidins). A
smaller group of phenolics are the flavonols, of which quercetin is best known for
its antioxidant and beneficial pharmacological properties of wine. These groups
of compounds are largely unaffected by yeast metabolism; however, a proportion
become bound to the cell wall of yeast. The pigmented polyphenols, anthocyanins,
which give the red/purple colour of wine, and the flavan-3-ols, monomeric pre-
cursors of tannins, react with yeast metabolites to form stable pigments. A small
proportion of pigmented phenolics bind to the yeast cell wall. Phenolic acids are, on
the other hand, metabolised intracellularly and their products secreted (Chatonnet
et al. 1993; Kennedy et al. 2006). Metabolism of phenolics is discussed in more
detail in Sect. 8D.5.3
Methoxypyrazines represent an important group of flavour compounds, which
contribute the vegetative, herbaceous, capsicum-like aromas of certain grape vari-
eties, especially Cabernet Sauvignon and Sauvignon Blanc (Lacey et al. 1991