Wine Chemistry and Biochemistry

324 M. Ugliano and P.A. Henschke

dihydroxy-

acetone

phosphate

glucose

glyceraldehyde

3-phosphate

glycerol

3-phosphate

glycerol

pyruvate acetaldehyde

ethanol

Gpd1p

Gpd2p

glycerol

NAD+

NADH

NAD+

acetic

acid

NAD(P)H

biomass

NADH

Fps1p

Adh1p

Aldp

Pdcp

acetoin 2,3-butanediol

NAD+

Bdh1p

L-malate

NAD+

oxalacetate

NAD+

Mdhp Frdsp

succinate

CO 2

CO 2

CO 2

succinic

acid

acetate

Fig. 8D.2Production of glycerol and associated redox reactions with flavour implications
Ethanol production is essentially redox neutral; however metabolism associated with biomass
production generates nett NADH, which is oxidised largely by glycerol production. Other impor-
tant NADH oxidising reactions with flavour implications are the production of 2,3-butanediol,
L-malic acid and succinic acid. When glycerol production is stimulated by non-growth associated
reactions (i.e. osmotic stress) NAD+reduction occurs by other reactions including the oxidation
of acetaldehyde to acetic acid

and Maccarelli 1998). Strain variability is largely due to differences in the regulation

of glyceraldehyde-3-phosphate dehydrogenase activity.

Glycerol production is modulated by fermentation conditions, especially those

that affect growth or physiological stress. Higher maturity grapes, having higher

sugar concentration, increases glycerol production by hyperosmotic stress (Pigeau

and Inglis 2005; Rankine and Bridson 1971). Nutrient availability appears to affect

glycerol mainly through redox maintenance mechanisms. Unlike aerobic growth

which use O 2 as the terminal electron acceptor,anaerobic growth depends on glyc-

erol production to restore NAD+:NADH balance (Verduyn et al. 1990b). Nitro-

gen availability generally stimulates biomass formation and hence NADH pro-

duction, although the effects are strain dependent (Albers et al. 1998; Radler and

Sch ̈utz 1982; Vilanova et al. 2007). Growth on inorganic nitrogen (ammonium

salts) compared to mixtures of amino acid also generates NADH due to amino acid

biosynthetic reactions, which stimulates glycerol production (Albers et al. 1996).

Thiamin deficiency, which can be inducedby significant growth of wild yeast or