8D Yeasts and Wine Flavour 325
over use of SO 2 , stimulates glycerol production (Bataillon et al. 1996). Due to
its essential role as cofactor in the conversion of acetaldehyde to ethanol by alco-
hol dehy drogenase (Adhp), NADH oxidation depends on Gpdp activity, thereby
increasing glycerol formation (Radler and Sch ̈utz 1982). SO 2 addition to must
can also stimulate glycerol production by forming a hydroxysulfonate adduct with
acetaldehyde, again limiting NADH oxidation by Adhp, although the increase in
glycerol is relatively small at the rates of SO 2 addition in winemaking (Rankine and
Bridson 1971). Higher fermentation temperature, 25◦C compared to 15◦C, and
higher pH, 3.8 compared to 3.3, can increase glycerol production to a small extent
(Rankine and Bridson 1971).
8D.3.3 Non-volatile Organic Acids
8D.3.3.1 Significance
Acidity and pH constitute fundamental importance to the sensory perception of
wine, essentially defining its structure and balance. Excessive acidity increases the
perception of sourness whereas low acidity decreases flavour harmony. The percep-
tion of sourness and astringency of organic acids depends on concentration, pH and
anion species (Sowalsky and Noble 1998). Wine acidity is largely determined by
the major grape acidsL(+)-tartaric andL(–)-malic, with smaller contributions from
citric andD(–)-lactic acids. Grape acids can range from<5 g/L for over-ripened
grapes and exceed 10 g/L in early season grapes from cool regions intended for
sparkling wine, with 6–8 g/L usually being preferred for dry wines. Wine also
contains various non-volatileacids of microbial origin succinic, keto acids pyruvic
and -ketoglutaric,L(+)-lactic and lesser amounts of other non-volatile acids, which
contribute to acidity (Boulton et al. 1998; Fowles 1992). Volatile fatty acids of either
grape or microbial origin, which can further contribute to acidity and aroma, are
discussed in Sect. 8D.4.3
8D.3.3.2 Metabolism and Modulating Factors
Organic acid metabolism serves several important functions, as precursors for
biosynthetic pathways and in maintenance of redox balance. Of the major grape
acids, tartaric acid is not metabolised bySaccharomyces cerevisiaewhereasL-malic
acid can be partially degraded (3–45%) by most strains, but this pathway is restricted
by diffusion of the non-dissociated acid into the cell and active transport of malate
out of the cell (Salmon 1987). Several wine strains ofSaccharomyces cerevisiae
have a variable ability to produceL-malic acid, which can cause difficulties in pre-
dicting and controlling titratable acidity in new wines (Holgate 1997). Some strains
ofSaccharomyces bayanus/uvarumproduce malic acid, though this properties tends
to be strain dependent (Antonelli et al. 1999; Giudici et al. 1995).L-Malate is formed