Fundamental Food Microbiology, Third Edition - Fuad Fathir

BIOCHEMISTRY OF SOME BENEFICIAL TRAITS 143
lactic acid in large amounts (mainly some Lactobacillus spp.) are commercially used
for this purpose. At present, genetic studies are being conducted to develop strains
by inactivating the D-lactate dehydrogenase system in species that have both the L
and D systems and produce large amounts of a mixture of L(+)- and D(–)-lactic acid.
Some of the species that produce L(+)-lactic acid (>90% or more) are Lactococcus
lactis ssp. lactis and cremoris, Streptococcus thermophilus, Lactobacillus. amylovorus,
Lab. amylophilus, Lab. casei spp. casei, Lab. casei spp. rhamnosus, and Lab.
divergens. Some of the common species used in food fermentation, namely Pediococcus
acidilactici, Ped. pentosaceus, Lab. delbrueckii spp. bulgaricus and helvaticus,
Lab. acidophilus, Lab. reuteri, and Lab. plantarum, produce a mixture of D(–)and
L(+)-lactic acids, with 20 to 70% being L(+)-lactic acid. 2
E. Heterolactic Fermentation of Carbohydrates
Hexoses are metabolized to produce a mixture of lactic acid, CO 2, and acetate or
ethanol by heterofermentative lactic acid bacteria (Table 11.1). Species from genera
Leuconostoc and Group III Lactobacillus lack fructose diphosphoaldolase (of EMP
pathway), but have glucose phosphate dehydrogenase and xylulose phosphoketolase
enzymes, which enable them to metabolize hexoses through phosphogluconatephosphoketolase
pathway (or hexose monophosphate shunt) to generate energy. 3–8
This pathway has an initial oxidative phase followed by a nonoxidative phase
(Figure 11.3). In the oxidative phase, glucose following phosphorylation is oxidized
to 6-phosphogluconate by glucose phosphate dehydrogenase and then decarboxylated
to produce one CO 2 molecule and a 5C compound, ribulose-5-phosphate. In
the nonoxidative phase, the 5C compound is converted to xylulose-5-phosphate,
which, through hydrolysis, produces one glyceraldehyde-3-phosphate and one acetyl
phosphate. Glyceraldehyde-3-phosphate is subsequently converted to lactate. Acetyl
phosphate can be oxidized to yield acetate, or reduced to yield ethanol (depending
on the O–R potential of the environment). Species differ in their abilities to produce
ethanol, acetate, or a mixture of both. The end products are excreted into the
environment.
F. Metabolism of Pentoses
The species in genera Leuconostoc and Group III Lactobacillus can ferment different
pentose sugars by the pentose-phosphate pathway to produce ATP, lactate, and
acetate, because they have the phosphoketolase enzyme. In Group II Lactobacillus,
this enzyme is inducible and is produced only when a pentose is present in the
environment. Although Ped. pentosaceus, Ped. acidilactici, and Lac. lactis can
metabolize some pentoses, the pathways are not clearly known. 4–6,8
The metabolizable pentoses by the Leuconostoc and Lactobacillus (Group II and
Group III) are first converted to xylulose-5-phosphate by several different ways.
Xylulose-5-phosphate is then metabolized to produce lactate and acetate or ethanol
by the mechanisms described in the nonoxidizing portion of metabolism of hexoses
by heterofermentative lactic acid bacteria (Figure 11.3). No CO 2 is produced from
the metabolism of pentoses through this pathway.
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