20 F. Z a m o r a
1.11 Other Subproducts of Alcoholic Fermentation
As mentioned previously, fermentation produces mainly ethanol and glycerol but
also several other substances that contribute to their complexity (Lambrechts and
Pretorius 2000). Figure 1.9 shows schematically the metabolic origin of some of the
most important subproducts of alcoholic fermentation.
1.Diacetyl, acetoin and 2,3-butanediol: These compounds are produced by con-
densing of pyruvate with ethanal. This reaction produces acetolactate which
is later decarboxylated. Diacetyl is produced if the decarboxylation is oxida-
tive, whereas acetoin is produced if the decarboxylation is not oxidative. Ace-
toin can also be formed by directly reducing diacetyl. Finally, acetoin can be
reduced to form 2,3-butanediol. This last reaction is reversible (Rib ́ereau-Gayon
et al. 2000c). Acetoin and especially diacetyl give off a buttery smell that mayHexose
transporterEthanal PyruvateAcetolactateDiacetyleAcetoine2,3 -ButanediolSugarsNAD+NADH
+ H+Acetyl -coAAcetic
acidHScoAAmino acidH+PermeaseKeto acidsFatty
acyl-coAFatty
acidsHigher
alcoholsEstersNAD+NADH
+ H+EthanolEsters Partially
Operative
Krebs
CycleSuccinic
acidGrape juiceHexose
transporterHexose
transporterEthanal Ethanal PyruvateEthanolAcetolactateDiacetyleAcetoine2,3 -ButanediolSugars SugarsNADNADH
+ H
NAD+NADH
+ H+DiacetyleAcetoine2,3-ButanediolAcetyl-coAAcetic
acidHScoAAcetic
acidAmino
acidH+Amino acidH+PermeasePermeaseKeto acidsFatty
acyl-coAFatty acids FattyacidsHigher
alcoholsEstersNAD+NADH
+ H+Higher
alcoholsEstersEthanolEsters Esters Partially
Operative
Krebs
CyclePartially
Operative
Krebs
CycleSuccinic
acidSuccinic
acidOther acidsFig. 1.9Biosynthesis of the other subproducts