Microbial Metabolism of Food Components ◾ 85
Pyruvate H CO Acetyl CoA
Acetyl CoA
Acetate, B
→
+
+
→
2
2
∼
∼
u utyrate, Butanol, and Acetone
Acetyl CoA is use
∼
d d to produce different compounds
Diaetyl, Acetoin, and Butanediol Fermentation
This type of fermentation is carried out by some lactic acid bacteria and some members of
Enterobacteriaceae.
2 Pyruvate
Pyruvate Acetaldehyde TPP]
-Acetol
→
+
→
[
∼
α
a actate
CO
↓
2
CO
H
H
Diacetyl Acetoin
, -Butanediol
2
2
2
2 3
+
+
↓
↓
↓
→
→
→
Anaerobic Respiration
Sulfate-reducing Desulfotomaculum nigrificans metabolizes glucose as an energy source, primarily through the EMP pathway, to produce pyruvate, which is then decarboxylated to generate
acetate (or ethanol) and CO 2 . Sulfate, acting as an electron acceptor, is reduced to generate
H 2 S. NO 3 -reducing bacteria, containing nitrate reductase (such as the species in the family
Enterobacteriaceae, some Bacillus spp., and Staphylococcus spp.), degrade metabolizable carbohydrates through EMP, HMS, and ED (also mixed-acid fermentation) pathways. Pyruvate produced through these pathways can act as an effective electron donor and, depending on the
species, may be converted to lactate, acetate, ethanol, formate, CO 2 , H 2 , butanediol, acetoin,
and succinate.
Aerobic Respiration
Aerobes (Bacillus spp., Pseudomonas spp., molds, and yeasts) and many facultative anaerobes
(Enterobacteriaceae, Staphylococcus spp.) under aerobic conditions can use molecular oxygen as
the terminal electron acceptor during metabolism of carbohydrates to produce pyruvate by one
or more of the major pathways mentioned earlier. Pyruvate, as well as other carboxylic acids,
can be oxidized completely through oxidative decarboxylation to generate CO 2 , H 2 O, and large
quantities of ATP. The pathway (designed as the Krebs cycle, the tricarboxylic acid cycle, or the
citric acid cycle) also generates large numbers of intermediates that are utilized to synthesize cell
materials. Initially, pyruvate is decarboxylated to generate acetyl~CoA and CO 2 . Acetyl~CoA then
combines with oxaloacetate (4C compound) to produce citrate (6C compound). Through successive reactions, citrate is metabolized to a 5C compound (and CO 2 ) and a 4C compound (and
Pyruvate H CO Acetyl CoA
Acetyl CoA
Acetate, B
→
+
+
→
2
2
∼
∼
u utyrate, Butanol, and Acetone
Acetyl CoA is use
∼
d d to produce different compounds
Diaetyl, Acetoin, and Butanediol Fermentation
This type of fermentation is carried out by some lactic acid bacteria and some members of
Enterobacteriaceae.
2 Pyruvate
Pyruvate Acetaldehyde TPP]
-Acetol
→
+
→
[
∼
α
a actate
CO
↓
2
CO
H
H
Diacetyl Acetoin
, -Butanediol
2
2
2
2 3
+
+
↓
↓
↓
→
→
→
Anaerobic Respiration
Sulfate-reducing Desulfotomaculum nigrificans metabolizes glucose as an energy source, primarily through the EMP pathway, to produce pyruvate, which is then decarboxylated to generate
acetate (or ethanol) and CO 2 . Sulfate, acting as an electron acceptor, is reduced to generate
H 2 S. NO 3 -reducing bacteria, containing nitrate reductase (such as the species in the family
Enterobacteriaceae, some Bacillus spp., and Staphylococcus spp.), degrade metabolizable carbohydrates through EMP, HMS, and ED (also mixed-acid fermentation) pathways. Pyruvate produced through these pathways can act as an effective electron donor and, depending on the
species, may be converted to lactate, acetate, ethanol, formate, CO 2 , H 2 , butanediol, acetoin,
and succinate.
Aerobic Respiration
Aerobes (Bacillus spp., Pseudomonas spp., molds, and yeasts) and many facultative anaerobes
(Enterobacteriaceae, Staphylococcus spp.) under aerobic conditions can use molecular oxygen as
the terminal electron acceptor during metabolism of carbohydrates to produce pyruvate by one
or more of the major pathways mentioned earlier. Pyruvate, as well as other carboxylic acids,
can be oxidized completely through oxidative decarboxylation to generate CO 2 , H 2 O, and large
quantities of ATP. The pathway (designed as the Krebs cycle, the tricarboxylic acid cycle, or the
citric acid cycle) also generates large numbers of intermediates that are utilized to synthesize cell
materials. Initially, pyruvate is decarboxylated to generate acetyl~CoA and CO 2 . Acetyl~CoA then
combines with oxaloacetate (4C compound) to produce citrate (6C compound). Through successive reactions, citrate is metabolized to a 5C compound (and CO 2 ) and a 4C compound (and