Wine Chemistry and Biochemistry

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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

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