132 ◾ Fundamental Food Microbiology
see Table 11.1). Hexoses are metabolized through the Embden-Meyerhoff-Parnas (EMP) pathway (Figure 12.3). These species have fructose diphosphate (FDP) aldolase, which is necessary to
hydrolyze a 6C hexose to yield two molecules of 3C compounds. They also lack a key enzyme,
phosphoketolase (in HMS pathway), present in species that are heterolactic fermentors. 2,5
In the EMP pathway, with glucose as the substrate, two ATP molecules are used to convert
glucose to fructose-1,6-diphosphate (Figure 12.3). Hydrolysis of these molecules generates two
molecules of 3C compounds. Subsequent dehydrogenation (to produce NADH + H + from NAD),
phosphorylation, and generation of two ATP molecules lead to production of PEP (can be used
in PEP-PTS sugar transport). Through the generation of substrate-level ATP, PEP is converted to
pyruvate, which, by the action of lactate dehydrogenase, is converted to lactic acid. The ability of
a lactic acid bacterial species to produce l(+)-, d(–)-, or dl-lactic acid is determined by the type of
lactate dehydrogenase (L, D, or a mixture of both, respectively) it contains. The overall reaction
involves the production of two molecules each of lactic acid and ATP from one molecule of hexose.
The lactic acid is excreted into the environment.
Other hexoses, such as fructose (transported as fructose or from hydrolysis of sucrose), galactose (from hydrolysis of lactose), and galactose-6-phosphate (from hydrolysis of lactose-phosphate
following transport of lactose by the PEP-PTS system), undergo different molecular conversion
table 12.1 Fermentation of Monosaccharides by Some Starter-Culture Bacteria
to Produce Different Byproducts
Genus
Monosaccharide Fermentation Pathway a
Main Products
Lactococcus
Hexoses
Homolactic
EMP
Lactate
Streptococcus
Hexoses
Homolactic
EMP
Lactate
Pediococcus
Hexoses
Homolactic
EMP
Lactate
Leuconostoc
Hexoses
Heterolactic
HMS
Lactate, CO 2 , acetate/
ethanol (1:1:1)
Pentoses
Heterolactic
PP
Lactate, acetate/ethanol
(1:1)
Lactobacillus
Group I
Hexoses
Homolactic
EMP
Lactate
Group II
Hexoses
Homolactic
EMP
Lactate
Pentoses
Heterolactic
PP
Lactate, acetate/ethanol
(1:1)
Group III
Hexoses
Heterolactic
HMS
Lactate, CO 2 , acetate/
ethanol (1:1:1)
Pentoses
Heterolactic
PP
Lactate, acetate/ethanol
(1:1)
Bifidobacterium
Hexoses
Heterolactic
BP
Lactate, acetate (1:1.5)
a EMP, Embden-Meyerhoff-Parnas; HMS, hexose monophosphate shunt (also called phosphogluconate-phosphoketolase); PP, pentose-phosphate; BP, bifidus (also called fructose ketolase).
see Table 11.1). Hexoses are metabolized through the Embden-Meyerhoff-Parnas (EMP) pathway (Figure 12.3). These species have fructose diphosphate (FDP) aldolase, which is necessary to
hydrolyze a 6C hexose to yield two molecules of 3C compounds. They also lack a key enzyme,
phosphoketolase (in HMS pathway), present in species that are heterolactic fermentors. 2,5
In the EMP pathway, with glucose as the substrate, two ATP molecules are used to convert
glucose to fructose-1,6-diphosphate (Figure 12.3). Hydrolysis of these molecules generates two
molecules of 3C compounds. Subsequent dehydrogenation (to produce NADH + H + from NAD),
phosphorylation, and generation of two ATP molecules lead to production of PEP (can be used
in PEP-PTS sugar transport). Through the generation of substrate-level ATP, PEP is converted to
pyruvate, which, by the action of lactate dehydrogenase, is converted to lactic acid. The ability of
a lactic acid bacterial species to produce l(+)-, d(–)-, or dl-lactic acid is determined by the type of
lactate dehydrogenase (L, D, or a mixture of both, respectively) it contains. The overall reaction
involves the production of two molecules each of lactic acid and ATP from one molecule of hexose.
The lactic acid is excreted into the environment.
Other hexoses, such as fructose (transported as fructose or from hydrolysis of sucrose), galactose (from hydrolysis of lactose), and galactose-6-phosphate (from hydrolysis of lactose-phosphate
following transport of lactose by the PEP-PTS system), undergo different molecular conversion
table 12.1 Fermentation of Monosaccharides by Some Starter-Culture Bacteria
to Produce Different Byproducts
Genus
Monosaccharide Fermentation Pathway a
Main Products
Lactococcus
Hexoses
Homolactic
EMP
Lactate
Streptococcus
Hexoses
Homolactic
EMP
Lactate
Pediococcus
Hexoses
Homolactic
EMP
Lactate
Leuconostoc
Hexoses
Heterolactic
HMS
Lactate, CO 2 , acetate/
ethanol (1:1:1)
Pentoses
Heterolactic
PP
Lactate, acetate/ethanol
(1:1)
Lactobacillus
Group I
Hexoses
Homolactic
EMP
Lactate
Group II
Hexoses
Homolactic
EMP
Lactate
Pentoses
Heterolactic
PP
Lactate, acetate/ethanol
(1:1)
Group III
Hexoses
Heterolactic
HMS
Lactate, CO 2 , acetate/
ethanol (1:1:1)
Pentoses
Heterolactic
PP
Lactate, acetate/ethanol
(1:1)
Bifidobacterium
Hexoses
Heterolactic
BP
Lactate, acetate (1:1.5)
a EMP, Embden-Meyerhoff-Parnas; HMS, hexose monophosphate shunt (also called phosphogluconate-phosphoketolase); PP, pentose-phosphate; BP, bifidus (also called fructose ketolase).