8D Yeasts and Wine Flavour 323
(Nieuwoudt et al. 2002). The sensory impact of glycerol is not entirely clear. Effects
on perceivable viscosity and body depend on the wine and glycerol concentrations
(Gawel et al. 2007b; Noble and Bursick 1984). Having a sensory threshold of about
5.2 g/L for sweetness, glycerol can be expected to contribute this attribute in dry
wines, although this effect is probably not consistent, as is the impact on acidity,
and aroma and flavour intensity (Nieuwoudt 2004; Gawel et al. 2007b). Although
2,3-butanediol has a mildly bitter sweet taste, it probably has limited impact on wine
flavour due to its relatively high odour threshold (Jackson 2000).
8D.3.2.2 Metabolism
Glycerol production provides a major route for the reoxidation of NADH gen-
erated from cellular anabolic reactionsunder anaerobic growth conditions. It is
also a compatible intracellular solute generated to offset osmotic stress. Glycerol
is derived from the glycolytic intermediate, dihydroxyacetonephosphate, by reduc-
tion to glycerol-3-phosphate followed by dephosphorylation to glycerol. The first
step is carried out by two NADH-dependent glycerol-phosphate dehydrogenase
isoenzymes (Gpd1,2p), encoded byGPD1andGPD2genes (Albertyn et al. 1994;
Eriksson et al. 1995) (Fig 8D.2). This stepis rate limiting for glycerol production
since over-expression of either gene increases glycerol production. Intracellular
glycerol concentration is also regulated by the glycerol permease Fps1p, which
controls glycerol release from the cell.GPD1is expressed in response to hyper-
osmotic conditions in order to protect the cell from dehydration whereasGPD2is
expressed under anaerobic growth conditions as a key step in the reoxidation of
NADH generated by anabolic metabolism (amino acid biosynthesis) and as the first
step in the biosynthesis of triacylglycerols and glycerophospholipids for membrane
growth. Several studies suggest that manyredox reactions are involved in restoring
the high NAD+:NADH balance generated from non-growth related glycerol produc-
tion, with the main NAD+sink being the oxidation of acetaldehyde to acetic acid
by NAD(P)+-dependent aldehyde dehydrogenases (Eglinton et al. 2002; Remize
et al. 1999). Other significant NADH reoxidation steps with flavour implications
include the production of 2,3-butanediol andL-malic and succinic acids, as sum-
marised in (Fig 8D.2).
8D.3.2.3 Modulating Factors
Wine yeast vary widely in glycerol production in dry wines, ranging from 4.2 g/L
to 10.4 g/L (Radler and Sch ̈utz 1982; Rankine and Bridson 1971). Compared to
mesophilicSaccharomyces cerevisiaestrains, thermotolerant strains and those of
the cryotolerant yeastSaccharomyces bayanus/uvarumproduce substantially more
glycerol under similar conditions (Antonelliet al. 1999; Giudici et al. 1995; Rainieri
et al. 1998). Some non-Saccharomycesyeasts, particularlyCandida stellataand
Saccharomycodes ludwigii, produce comparatively high amounts of glycerol (Ciani