10 F. Z a m o r a
Krebs cycle and the constituents of respiratory chains are repressed (Gancedo 1992;
Polakis et al. 1965; Barnett and Entian 2005). Therefore, under wine fermenta-
tion conditions,Saccharomyces cerevisiaecan only ferment sugars.Saccharomyces
cerevisiaecan only use respiration when the sugar concentration is really low and
when oxygen is present in the medium. These conditions are used for the industrial
production of selected dry yeast.
1.6 Alcoholic Fermentation
As was quoted above, when fermenting grape juice,Saccharomyces cerevisiae
mainly directs the pyruvate to produce of ethanol in order to regenerate the NAD+
consumed by glycolysis. This process, called alcoholic fermentation, is shown in
Fig. 1.4.
P l a s m a M e m b r a n eDihydoxyacetone
phosphate Glyceraldehyde-3-phosphateAlcoholic
FermentationHexose
transporterD-GlucoseD-Fructose1,3-diphosphoglycerateAlcohol
dehydrogenaseGlyceraldehyde
3-phosphate
dehydrogenaseEthanalEthanol
NAD+NADH + H+PyruvateCO 2Pyruvate
decarboxylase2 ATP2 ADP +2 Pi4 ATP 4 ADP +4 PiP l a s m a M e m b r a n eGrape juiceDihydoxyacetone
phosphateGlyceraldehyde-3-phosphateAlcoholic
FermentationD-GlucoseD-FructoseHexose
transporterHexose
transporterD-GlucoseD-FructoseD-GlucoseD-Fructose1,3-diphosphoglycerateAlcohol
dehydrogenaseGlyceraldehyde
3-phosphate
dehydrogenaseEthanalEthanol
NAD+NADH + H+EthanolPyruvateCO 2Pyruvate
CO decarboxylase
22 ATP2 ADP +2 Pi4 ATP 4 ADP +4 PiFig. 1.4Alcoholic fermentation