Wine Chemistry and Biochemistry

8D Yeasts and Wine Flavour 315

The term ‘flavour’ will be used in its broadest sense to incorporate aroma, taste,

and mouth-feel. Yeast taxonomy is according to Kurtzman and Fell (1998), with the

exception of the former speciesSaccharomyces uvarum, for which species status

is still unresolved (Nguyen and Gaillardin 2005), and will therefore be recorded

asSaccharomycesspeciesbayanus/uvarumYeast gene and enzyme notations are

according to theSaccharomyces GenomeDatabase (http://yeastgenome.org).

8D.2 Overview of Yeast Interactions with Grape Compounds

In standard winemaking conditions, fermentation can only commence when sug-

ars and other nutrients necessary for yeast growth are released from the berry by

crushing. The yeasts associated with the grapes and processing equipment, often

referred to as indigenous, ‘native’ or ‘wild’ yeasts, can initiate fermentation or,

more commonly, a starter culture of a selected yeast is added. Sulfur dioxide is

commonly added as an antioxidant and antimicrobial compound, which reduces the

populations of indigenous yeasts and bacteria. Crushing also releases volatile and
non-volatile compounds associated with the berry pulp into the must, whereas mac-

eration, often with added exogenous pectinolytic enzymes, is required to facilitate

release of flavour and precursor flavour compounds associated with berry skin, and

to a lesser extent seeds. Must compounds can be considered to fall into three broad

functional groups, ‘nutrients’, ‘flavour precursors’ and ‘non-precursor flavour-active

compounds’ (Fig 8D.1). The action of yeasts on many of these compounds results in

the complex chemistry of wine, which forms the basis of ‘appearance’, ‘fermenta-

tion bouquet’, ‘varietal character’ and ‘mouth-feel’ of wine. Known key compounds

are summarised in Table 8D.1.

Fermentation Bouquet

Esters, Higher alcohols, Acids

Carbonyls

Polysaccharides

Higher alcohols, Acids, Carbonyls

Volatile sulfur compounds

Varietal compounds

Monoterpenes, Norisoprenoids,

Aliphatics, Benzene-derivatives

Long-chain polyfunctional thiols

Transformation products

Flavour-active

Compounds

Carboxylic acids, Transformation

products

Phenolic adducts and polymers

Grape compound Metabolism Metabolite/product

Sugar metabolism (Glycolysis/TCA pathway) &

Lipid metabolism

Sugar metabolism

Nitrogen metabolism

Sulfur metabolism

Hydrolysis

Non-hydrolytic cleavage

Reduction, Esterification, Decarboxylation

Catabolic/anabolic pathways

Biotransformations

Metabolism/Biotransformations

TCA pathway

(Yeast metabolites)

Nutrients

Hexoses

Amino acids, ammonium, peptides

Sulfate, (sulfite)

Flavour precursors

Glycosides

Cysteinylated conjugates

Non-conjugated secondary

metabolites

Non-precursor flavour-

active compounds

Carboxylic acids

Phenolic compounds

Fig. 8D.1Interactions between yeasts and grape compounds