THE ALCOHOL TEXTBOOK THE ALCOHOL TEXTBOOK

Understanding yeast fundamentals 93
Carbohydrate (g/100 ml)
14
12
10
8
6
4
2
Maltotriose
Glucose
Ethanol
Maltose
Glycerol
0
0 10 20 30 40 50
Fermentation time (h)
Figure 10. Uptake of sugars from a laboratory 19.3ºPlato/Brix whisky mash.
CH O +2P+2ADP
2CHOH+2CO +2ATP + 2H O
6 12 6 i 2 5 2 2
Glucose
2 carbon dioxide + 2 ethanol + energy
Mol wt 180 2 x 44 2 x 46
This equation, named after the French scientist
Gay-Lussac, shows that glucose yields almost
equal parts of carbon dioxide and ethanol as well
as the production of energy. Part of the energy
is used for cell metabolism and some of it will
be lost as heat. What this equation does not
address is that some of the sugar will be used
for yeast growth and that there are other
metabolites produced (glycerol, lactic and
succinic acid, etc.). These other metabolites are
small in quantity compared to the amount of
ethanol produced.
The glycolytic (EMP) pathway operates in the
presence or the absence of oxygen to convert
glucose to pyruvic acid, energy and reduced
nicotinamide adenine dinucleotide (NADH +
H + ). When oxygen is abundant and the sugar
level is kept low, little or no ethanol is made by
the growing yeast. The yeast uses all of the sugar
as energy for new yeast cell production (i.e. the
baker’s yeast propagation process). However
when oxygen is reduced and/or the glucose
levels exceed 0.1% (w/v), then ethanol is made.
Under low oxygen conditions less than 5% of
the sugar is metabolized for growth and the total
cell yield is approximately 10% of what is
possible under aeration and non-repressing
glucose concentrations.
Not all glucose is metabolized by the glycolytic
(EMP) pathway described above. Some of the
sugar will be metabolized by a route called the
hexose monophosphate pathway. This pathway
also yields energy, but more importantly it yields
pentose sugars, which are important in the
synthesis of yeast nucleotides and yeast nucleic
acids.
From glucose to pyruvate: glycolysis in
yeast
Glucose is phosphorylated in two stages. Two
ATPs are used to produce fructose 1,6
disphosphate, which is then split by the enzyme
aldolase to form two 3-carbon triose phosphates.
Inorganic phosphate is assimilated to form two
triose diphosphates from which four H atoms
are accepted by two molecules of oxidized NAD.
Lastly, four ATPs are formed by transfer of
phosphate from the triose diphosphates to ADP