Wine Chemistry and Biochemistry

334 M. Ugliano and P.A. Henschke

R

|

C = O

|

COOH

R

|

H – C – NH 2

|

COOH

R

|

H – C = O

R

|

H –C –OH

|

H

NH 3 CO 2 NADH NAD+

Amino acid α-Keto acid Aldehyde H+ ‘Fusel

Alcohol’

Glycolysis

Ehrlich pathway

Amino acid sufficiency

Biosynthesis pathway

Amino acid deficiency

Deamination Decarboxylation Reduction

Transport

Amino

acid

Oxidation

NAD+

NAD(P)H

H+

Sugars

Gap1p Bap2p,

Tat1p, Tat2p,

Mup1p Mup3p

Bat1p, Bat2p,

Aro8p, Aro9p

Pdc1p, Pdc5p,

Pdc6p, Aro10p Adh1-6p, Sfa1p

Acid

Ald1-6p

Acid

Pdr12p

R

|

C = O

|

OH

Fig. 8D.5Formation of higher alcohols from sugar and amino acids by the Ehrlich pathway
A deficiency of amino acids during growth activates their synthesis from -ketoacids, derived
from sugars via glycolysis. If insufficient nitrogen is available for transamination reactions, surplus
-ketoacids are excreted as higher alcohols (Biosynthetic pathway). During amino acid sufficiency,
transamination of amino acids can produce a surplus of -ketoacids, some of which are decarboxy-
lated and reduced to alcohols (Ehrlich pathway)

et al. 2008) (Fig 8D.5). Pyruvate decarboxylases (Pdc1p, Pdc5p, Pdc6p, Aro10p)

convert -keto acids to their corresponding aldehydes, which are then reduced to

alcohols by alcohol dehydrogenases (Adh1p-Adh6p, Sfa1p). Catalytic oxidation

of aldehydes by aldehyde dehydrogenases (Ald1p-Ald6p) forms the corresponding

acids, which are then removed from the cell by the weak organic acid permease

Pdr12p (Hazelwood et al. 2008). Over-expression of theBAT1gene in a commercial

wine yeast (VIN13) increased 3-methybutanol, its acetate ester, 2-methylpropanol

and 2-methylpropanoic acid, whereas over-expression of theBAT2gene increased 2-

methylpropanol, 2-methylpropanoic acid and propanoic acid. Deletion of theBAT2

gene decreased these compounds. Altered expression of these transaminase genes

might provide a means for modulating the aroma of wine (Lilly et al. 2006ab).

The production ofn-propanol is directly related to initial nitrogen and yeast

growth, and appears not to be influenced by the structurally-related amino acids,

threonine and -aminobutyric acid (Rapp and Versini 1996). A negative relation-

ship has been observed betweenn-propanol production and H 2 S formation (Giudici

et al. 1993) but impact on wine quality was not reported. Methionol production

is related to methionine concentration,and is therefore limited by the generally

low methionine in grape must. Hexanol is believed to be reductively formed by

yeast from hexanal, which in turn is formed from linoleic (C18:2) acid during must

processing (Rib ́ereau-Gayon et al. 2000a).