8D Yeasts and Wine Flavour 353
considered to lower wine quality. Ethyl phenols indicate growth ofDekkera brux-
ellensis, a wine spoilage yeast (Chatonnet et al. 1992b; Su ́arez et al. 2007). In red
wines, colour is based on anthocyanin pigments and derivatives. Monomeric pig-
ments are the major determinant of colour in young red wines but decline as the wine
ages through chemical modifications and condensation reactions with flavan-3-ols
and proanthocyanidins. These reactions form more stable pigments, pyranoantho-
cyanins and polymeric pigments. Red wines also contain up to 10-fold higher con-
centrations of flavan-3-ol monomers (catechins) than white wines that provide bit-
terness and probably contribute astringency. Red wine astringency is, however, pri-
marily determined by the types and content of proanthocyanidins, generally referred
to as tannins in the wine literature (Kennedy et al. 2006).
8D.5.3.2 Metabolism
Grapes contain several hydroxycinnamic acids,p-coumaric, caffeic, ferulic and
sinapic acids, which exist as free acids and esterified with tartaric acid.Saccha-
romycesspecies can take up free acids to produce the corresponding vinyl phe-
nol catalysed by hydroxycinnamate decarboxylase (phenylacrylic acid decarboxy-
lase; Pad1p) (Fig 8D.11) (Chatonnet et al. 1992b; Chatonnet et al. 1993; Edlin
et al. 1995). Vinyl phenols areunstable and highly reactive.Dekkera bruxellensis
is one of few wine microorganisms that can further reduce vinyl phenols to highly
stable ethyl phenols in wine. Vinyl phenolscan also react with anthocyanins to form
vinyl derivatives, a reaction that is favoured by fermentation yeast having hydrox-
ycinnamate decarboxylase activity (Morata et al. 2006).
Various yeast metabolic interactions with grape phenolics, involving several
vinyl and carbonyl metabolites, can affect red wine colour and astringency
(Fig 8D.11). Acetaldehyde, pyruvic acid, -ketoglutaric acid and vinyl phenols
can react with anthocyanins to form pyranoanthocyanins, such as Vitisins (Morata
et al. 2006). These pigments, which are more stable than anthocyanins to pH and
SO 2 bleaching effects, have different spectral properties to anthocyanins and con-
tribute to the less purple and more red orange colour of aging wines (Hayasaka
et al. 2007). Acetaldehyde can also form ethyl-linked conjugates of anthocyanins
with flavanol monomers, oligomers and polymers (tannins), which also stabilise
wine colour. Yeast polysaccharides, in addition to grape polysaccharides, inter-
act with anthocyanins to improve colour stability (Escot et al. 2001). Several
yeast mediated reactions can also negatively impact on wine colour development,
such as by binding anthocyanins to the cell wall (2–6% of total anthocyanins)
(Morata et al. 2003) or by producing anthocyanin- - D-glucosidase or possibly exo-
glucanase that releases the corresponding, but unstable, anthocyanidin (Manzanares
et al. 2000; Gil et al.2005).
These various reactions involving yeast metabolites and non-pigmented phe-
nolics are also believed to lead to changes that affect the astringency of tannins
(Eglinton et al. 2005). Sensory evaluation of wines made with twoSaccharomyces
yeast which differed in acetaldehyde production led to differences in mouth-feel
attributes associated with tannins, namely grainy, silky, velvet, drying and pucker.