8D Yeasts and Wine Flavour 339
Volatile fatty acids are by-products in the formation of long-chain fatty acids,
which are required for cell membrane phospholipid biosynthesis. The biosynthesis
of volatile fatty acids is generally controlled by the same factors that control the
formation of ethyl fatty acid esters, that is, oxygen, ergosterol and various insoluble
solids (grape solids, clarification solids, yeast hulls) tends to suppress production
whereas sugar concentration and clarification are stimulatory (Bardi et al. 1999;
Delfini et al. 1992, 1993; Edwards et al. 1990; Houtman et al. 1980).
8D.4.4 Carbonyl Compounds
8D.4.4.1 Significance
Yeasts produce various carbonyl compounds from sugar metabolism, including
aldehydes, ketones and keto acids; the latter are discussed in Sect. 8D.3.3. Acetalde-
hyde is quantitatively the most important saturated aldehyde. It has a sensory thresh-
old of 100 mg/L wine, and ranges in concentration from 10 mg/L to 75 mg/L
in dry wines to which it contributes a ‘bruised apple’ and ‘nutty’ character when
present at sensorially detectable concentrations (Schreier1979). In addition, higher
saturated aldehydes (C 3 –C 9 ) contribute herbaceous, grassy, green, fatty, fruity and
pungent flavours (Ebeler and Spaulding 1998). Hexanal,cis-3-hexenal andtrans-2-
hexenal contribute a green character evident in grape must. From a sensory point of
view, high concentrations of acetaldehyde are generally associated with oxidation
off-flavors (aldehydic) in dry wines, although this compound plays a key role in
the aroma of particular wines made by oxidative processes, such as Spanish sher-
ries, Frenchvin jaune, Sicilian Marsala and Sardinian Vernaccia (Jackson 2000;
Schreier 1979). Evident aldehydic character in young wines can be suppressed by
treatment with sulfite, which binds to acetaldehyde to form the non-volatile adduct
acetaldehyde hydroxysulfonate. Unintended exposure of wines to oxygen (air), such
as wine stored under ullage or aeration during wine transfer operations, increases
acetaldehyde content, which leads to higher concentrations of bound sulfite. This
occurrence is both undesirable from a consumer perspective and total SO 2 content of
wine is subject to wine regulatory requirements. Furthermore, bound SO 2 provides
limited protective antioxidant and antimicrobial activity.
The most significant ketoneproduced by yeast is diacetyl(2,3-butanedione), a
vicinal diketone, although malolactic fermentation is a more important source, when
it is used in wine production. Having a sensory threshold of 0.2–2.9 mg/L, accord-
ing to the type of wine, it is characterised by a ‘nutty’, ‘toasty’ or ‘buttery’ aroma
depending on concentration (Martineau et al. 1995). Dry white wines tend to con-
tain lower concentrations (0.1–2.3 mg/L) than red wines (0–7.5 mg/L) (Bartowsky
et al. 2002; Martineau et al. 1995). Acetoin, which produces a buttery flavour, is
formed by partial reduction of diacetyl, and is itself reduced to 2,3-butanediol.
Acetoin is usually present at concentrations (<80 mg/L) much lower than its sensory
threshold of 150 mg/L (Romano and Suzzi 1996).
Lactones are another group of carbonyl compounds formed by yeast during
alcoholic fermentation. Concentrations are expecially high during oxidative mat-