8D Yeasts and Wine Flavour 327
determined but environmental conditions modulateL-malate metabolism; produc-
tion is favoured by low nitrogen availability and low pH whereas low sugar concen-
tration can promote malate consumption (Radler 1993; Salmon et al. 1987).
Succinic acid, which has an ‘unusual salty, bitter taste’, is the major organic
acid produced by yeast metabolism, (Coulter et al. 2004; Radler 1993).Saccha-
romyces cerevisiaestrains produce succinic acid in highly variable amounts of up to
2 g/L whereasSaccharomyces bayanus/uvarumproduce slightly greater amounts
(Antonelli et al. 1999; Eglinton et al. 2000; Giudici et al. 1995; Radler 1993).
Succinate is primarily formed via the reductive arm of the tricarboxylic pathway
(Fig 8D.3). Oxalacetate is first formed from pyruvate by a carboxylation reac-
tion (Pyc1,2p), oxalacetate is reduced toL-malate, and hydrated to form fumarate,
from which succinate is formed in a reductive step catalysed by fumarate reduc-
tase (Frdsp) (Enomoto et al. 2002). Succinate formation from pyruvate provides an
alternative pathway for the oxidation of reduced dinucleotides formed by anabolic
reactions required for cell growth under anaerobic conditions (Salmon et al. 1987;
Camarasa et al. 2003). A variety of fermentation conditions affect succinic acid
accumulation during fermentation, including fermentation temperature and must
clarity and composition, including sugar concentration, assimilable nitrogen, biotin,
pH, acidity and SO 2 (Coulter et al. 2004). Abnormal amounts of succinic acid result
from high levels ofγ-amino butyric acid, which can form in grape must under
certain conditions (Bach et al. 2004). Succinate can also form by the oxidative
decarboxylation ofα-ketoglutarate/glutamate catalysed by the oxo-glutarate dehy-
drogenase (OGDH) complex (Camarasa et al. 2003).
Several important keto acids are formed in relatively small amounts, notably
pyruvic acid andα-ketoglutaric acid, which have implications for wine chemical
and microbiological stability but also play a role in formation of stable wine pig-
ments. Keto acids can bind SO 2 , thereby lowering its efficacy as an antioxidant
and antimicrobial (Rankine 1968). Ketoacids can react with anthocyanins to form
pyranoanthocyanins, which are more stable to oxidative degradation than antho-
cyanin pigments (Asenstorfer et al. 2003; Bakker and Timberlake 1997; Benab-
deljalil et al. 2000). Keto acids can form from sugar catabolism and from their
respective amino acids, alanine and glutamic acid, respectively, by the Ehrlich path-
way. Strains vary in keto acid formation which is further modulated by assimilable
nitrogen composition of must (Radler 1993).
8D.4 Yeast Volatile Aroma Compounds
The anaerobic fermentation of sugars bySaccharomyceswine yeasts generates
a variety of volatile metabolites that contribute to the sensory profile of wine.
The important compounds include esters, higher alcohols, volatile fatty acids, car-
bonyls, and volatile sulfur compounds. The accumulation of these compounds
in wine depends on the strain of yeast,must composition (chemical, physical
and nutrient composition) and fermentation conditions. In addition, a variety of