BIOCHEMISTRY OF FOOD
BORNE MICROORGANISMS
Submitted by: Irshad Ahmad
Roll No: 0240-BH-MB-16
Submitted to: Dr. Asad-ur-Rehman
Subject: Advances in Food Microbiology
HALEEB FOODS
Biochemistry of food borne microorganisms
Advances in Food Microbiology
Contents
1 Introduction …………………………………………………………………………………………………………….. 2
2 Structure and morphology of microorganisms present in food ………………………………………. 2
2.1 Molds and yeasts ……………………………………………………………………………………………….. 2
2.1.1 Important Genera of Molds …………………………………………………………………………… 3
2.1.2 Important Genera of Yeast ……………………………………………………………………………. 4
2.2 Bacteria…………………………………………………………………………………………………………….. 5
2.2.1 Important genera in bacteria …………………………………………………………………………. 6
2.3 Viruses ……………………………………………………………………………………………………………… 8
2.3.1 Important Genera in Viruses …………………………………………………………………………. 8
3 Transport mechanism of nutrients ……………………………………………………………………………… 9
4 Carbohydrates, transport and their metabolism ………………………………………………………….. 10
4.1 Permease system in L. Acidophillus for lactose ……………………………………………………. 10
4.2 Carbohydrates inside cell for metabolism ……………………………………………………………. 11
4.3 Homolactic fermentation of carbohydrates ………………………………………………………….. 11
4.4 Heterolytic fermentation of carbohydrates …………………………………………………………… 13
4.5 Pentose metabolism ………………………………………………………………………………………….. 14
4.6 Fermentation of hexose from Bifidobacterium …………………………………………………….. 14
4.7 Diacetyl production from citrate ………………………………………………………………………… 15
4.8 Production of propionic acid from Propionibacterium ………………………………………….. 15
5 Amino acids and proteinaceous compounds, transport and their metabolism …………………. 16
6 Lipid compounds, their transport and metabolism …………………………………………………….. 16
7 Conclusion ……………………………………………………………………………………………………………. 17
8 References …………………………………………………………………………………………………………….. 18
Biochemistry of food borne microorganisms
Advances in Food Microbiology
Biochemistry of food borne microorganisms
1 Introduction
Food borne organisms include bacteria, viruses, and parasites that can cause infectious or
harmful diseases in the environment. They enter the body by ingesting contaminated food or
water. Everyone is at risk of foodborne illness, although infants, adults, misbehavior, and
malnutrition are at risk. Foodborne illness can be debilitating, severe, or even fatal. Illness is
often manifested by diarrhea, vomiting, or both, but may affect other organs, such as the nervous
system. Outbreaks of foodborne illness are often the result of inadequate cooking, insufficient
heat, contamination, unsafe food, and poor hygiene
(Whitlock et al., 2000).
2 Structure and morphology of microorganisms present in food
2.1 Molds and yeasts
Both yeast and molds are eukaryotic, but yeast is not the same as mold. Eukaryotic cells are
generally much larger (20-100 pm) than prokaryotic cells (1-10 pm). Eukaryotic cells have
strong cell walls and small cell membranes. The cell wall is peptidoglycan-free, strong, and high
in carbohydrates. The plasma membrane contains sterols. The cytoplasm contains organelles
(mitochondria, vacuoles) bound by embryos. Ribosomes are type 80S and are attached to the
endoplasmic reticulum. DNA is a line (chromosome), contains histones, and is embedded in a
nuclear membrane. Cell division by mitosis (i.e., reproduction); sexual reproduction, when it
occurs, is included in meiosis. The embryo is motile, filamentous, and branched. The cell walls
are made of cellulose, chitin, or both
(Vanderzand & Splittoesser, 1992). Thallus is made up of a
large number of filaments called hyphae. The hyphae combination is called mycelium. Hyphae
can be picked up, extracted uninucleate, or septate-multinucleate. Hyphae can be a plant or a
breed. Reproduction of hyphae continues to grow in the air and form exospores, both free
(conidia) and sacs (sporangium). The shape, size and color of the seeds are used for taxonomic
classification. The leaven spread throughout the land. The cells are oval, round, or long. They
don’t have a car. The cell wall contains polysaccharides (glycans), proteins, and lipids
(Thompson et al., 1988). Walls can have tricks that show growth. The membrane is under the
wall. The cytoplasm has a beautiful round appearance of ribosomes and organelles. The nucleus
is well defined by a nuclear union
(Thomas & Pritchard, 1987).
Biochemistry of food borne microorganisms
Advances in Food Microbiology
2.1.1 Important Genera of Molds
Molds are important in food because they can grow even in situations where many bacteria are
ineffective, such as low pH, low water function (Aw), and high osmotic pressure. There are
many types of molds found in food
(Vanderzand & Splittoesser, 1992). Many species produce
mycotoxins and have been implicated in food addiction. Some mycotoxins are carcinogenic or
mutagenic and cause certain diseases in the body such as hepatotoxic (toxic) or nephrotoxic
(toxic to the kidneys). It is widely used for food distribution
(Thompson et al., 1988). Finally,
most of it is used to produce food additives and enzymes. Some common types of molds are
found here
Sr
No
Genera Examples Hyphae Spores Characters Spoilage
1. Rhizopus Rhizopus
stolonifer
Aseptate Sporangiophores
in sporangium
Fruits and
vegetables
2. Penicillium Penicillium
roquefortii
Penicillium
roquefortii
Septate Blue-green~
brush-like
conidia
Produce
mycotoxins
(e.g.,
Ochratoxin A)
Fungal rot
in fruits and
vegetables
3. Mucor Mucor rouxii nonseptate Sporangiophores Cottony
colonies, are
used in food
fermentation
Vegetables
4. Geotrichum Geotrichum
candidum.
Septate Rectangular
anthrospores
Form yeast
like cottony
colony, grow
on dairy
products
Dairy
5. Fusarium Fusarium
verticillioides,
Fus.
graminearum,
Fus.
Septate Sickle shaped
conidia
Produce
mycotoxins
such as
ticothecenes,
deoxynivalonel
Grains,
potatoes,
citrus fruits
Biochemistry of food borne microorganisms
Advances in Food Microbiology
proliferatum (DON)
6. Asperillus Aspergillus
flavus
Aspergillus
niger
Aspergillus
niger
Septate Black-colored
sexual spores in
conidia
Xerophilic, can
grow in grains,
produce
aflatoxins
Jam, nut,
fruit and
vegetables
7. Alternaria Alternaria
citri
Septate Dark color
spores on
conidia
Produce
mycotoxins
Rot in
tomatoes
and rancid
flavor in
dairy
products
2.1.2 Important Genera of Yeast
Yeast is important in food because it can cause spoilage. Mostly used for food preparation. Some
are used to produce food additives
(Tevel et al., 2013). Many of the important types are briefly
described here
Sr No Genera Species Cells Characters
Saccharomyces Saccharomyces
cerevisiae
round,
oval, or
elongated
Cause spoilage of food
by producing CO2 and
alcohol
Pichia Pichia
membranaefaciens
oval to
cylindrical
form pellicles in beer,
wine, and brine
Rhodotorula Rhodotorula
glutinis
Form pigments,
discoloration of food
Torulopsis Torulopsis
versitalis
spherical to
oval
Milk spoilage
Biochemistry of food borne microorganisms
Advances in Food Microbiology
Candida Candida
lipolyticum
Rancidity in butter and
dairy products, spoils
food with high sugar,
acid and salt
Zygosaccharomyces Zygosaccharomyces
bailii
Spoil high acid foods,
and less salty and
sugary foods
2.2 Bacteria
Bacteria are unicellular and have three morphological patterns: round (cocci), rod-shaped
(bacillus), and curved (comma). They can form organizations such as groups, chains (two or
more cells), or tetrads. They can be motile or nonmotile. The cytoplasmic material is covered by
a sturdy wall. Nutrients in molecular and ionic forms are transferred to the environment through
membranes in certain ways. The membrane contains substances that produce energy. It also
makes intrusions into the cytoplasm (mesosome). The cytoplasmic substance is immobile and
does not contain organelles that are wrapped around different organs (Sospedra et al., 2015).
Ribosomes are type 70S and distributed in the cytoplasm. Nucleic substances (DNA structures
and plasmids) are circular, do not contain a nuclear membrane, and do not contain basic proteins
like history. Genetic engineering and genetic reincarnation are possible but do not involve the
design of genes or zygotes (Sospedra et al., 2012). The cell division is carried out by the selected
instructions. Prokaryotic cells may have flagella, capsules, surface-surrounding proteins, and
fimbriae for specific functions. Some form endospores (one in each cell). Based on the Gram
color behavior, bacterial cells are classified as either Gram -ve or Gram +ve (Sneath, 1986).
Gram -ve cells have a complex cell wall that contains an outer layer (OM) and a middle layer
(MM). OM contains lipopolysaccharides (LPS), lipoproteins (LP), and phospholipids. The
phospholipid molecule is bilayer formulated, with hydrophobic (fatty acids) inside and
hydrophilic components (glycerol and phosphate) on the outside. The LPS and LP molecules
enter the phospholipid layer. OM has limited transport operations (Thomas and Pritchard, 1987).
The resistance of gram -ve viruses to many enzymes (lysozyme, contains peptidoglycan
hydrolysis), hydrophobic molecules [sodium dodecyl sulfate (SDS) and bile salts], and
Biochemistry of food borne microorganisms
Advances in Food Microbiology
antibiotics (penicillin) due to the barrier properties of OM. LPS molecules also have
antigenic properties. Beneath the OM is MM, which consists of a thin layer of peptidoglycan or
mucopeptide embedded in a periplasmic material containing many types of proteins, enzymes,
and toxins. Beneath the periplasmic material is the plasma or internal membrane (IM), which
includes the phospholipid bilayer in which most of the protein is incorporated (Senoh et al.,
2012). Gram +ve cells have thick cell walls that contain several layers of peptidoglycan
(mucopeptide; responsible for solid structure) and two types of teichoic acid. Peptidoglycan is a
polymer consisting of muramic acid N-acetyl (NAM) and N-acetyl glucosamine (NAG)
combined with a short peptide chain. Some types also have a layer on top of the cell surface,
called a protein layer (SLP) layer (Samson et al., 2000). The teichoic acid molecules in the walls
are linked to the mucopeptide layer, and the lipoteichoic acid molecules are linked to the
mucopeptide membrane and cytoplasm. Acids are poorly charged (due to phosphate groups) and
can bind or regulate the movement of cationic molecules inside and outside the cell. Acidic acid
has antigenic properties and can be used for serological detection to identify Gram from good
bacteria. Due to the complexity of the chemical structure in the cell wall, Gram +ve bacteria are
thought to have originated before Gram -ve bacteria (Alam et al., 2012).
2.2.1 Important genera in bacteria
Among the microorganisms found in food bacteria are an important group. This is not only
because many different species can be present in food, but also because of its fast growth, the
ability to consume nutritious foods, and the ability to grow under temperature, pH, and water
functions, and live better lives in bad condition. For convenience, the essential bacteria in food
are divided into groups based on certain characteristics (Axelsson, 1998). These groups have no
taxonomic significance. Some of these groups and the importance of food are listed here.
Sr No Group Species Characters
1. Lactic Acid
Bacteria
Streptococcus thermophilus,
Lactobacillus, Leuconostoc
Produce lactic acid from
carbohydrates
2. Acetic acid
bacteria
Acetobacter aceti Produce acetic acid
3. Propionic acid
bacteria
Propionibacterium
freudenreichii
Used in dairy fermentation,
Produce propionic acid
Biochemistry of food borne microorganisms
Advances in Food Microbiology
4. Butyric acid
bacteria
Clostridium butyricum Produce butyric acid
5. Proteolytic
bacteria
Bacillus, Staphylococcus,
Micrococcus, Pseudomonas,
Alcaligenes,
Flavobacterium
Hydrolize proteins
6. Lipolytic
bacteria
Staphylococcus,
Micrococcus, Pseudomonas,
Flavobacterium
produce extracellular lipases,
hydrolyze triglycerides
7. Saccharolytic
bacteria
Bacillus, Pseudomonas,
Enterobacter, Aeromonas
hydrolyze complex
carbohydrates
8. Thermophylic
bacteria
Lactobacillus,
Streptococcus, Pediococcus
Grow at 50°C or above
9. Psychrotrophic
bacteria
Clostridium, Listeria,
Leuconostoc, Alteromonas,
Flavobacterium,
Aeoromonas, Serratia,
Less than 5°C
10. Thermoduric
Bacteria
Bacillus, Micrococcus,
Clostridium, Lactobacillus
Can survive pasteurization
temperature
11. Halotolerant
bacteria
Staphylococcus,
Corynebacterium, Bacillus,
Micrococcus,
Can grow in high salt
concentration
12. Aciduric bacteria Streptococcus,
Lactobacillus, Pediococcus,
enterococcus,
Can live in pH lower than 4
13. Osmophilic
bacteria
Lactobacillus,
Staphylococcus,
Leuconostoc
Grow at high osmotic
environment
14. Gas Producing
bacteria
Leuconostoc, Lactobacillus,
Clostridium, Enterobacter,
Produce gases like CO2, H2S,
H2
15. Slime producers Lactococcus, Enterobacter, Produce slime, synthesize
Biochemistry of food borne microorganisms
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Alcaligenes, Xanthomonas,
Leuconostoc
polysaccharides
16. Spore formers Clostridium, Bacillus Produce spores
17. Aerobes Bacillus, Pseudomonas,
Flavobacterium
Require oxygen for growth
18. Anaerobes Clostridium Grow in absence of oxygen
19. Facultative
anaerobes
Leuconostoc, Pediococcus,
Lactobacillus
Can grow in presence and
absence of oxygen
20. Enteric
pathogens
Yersinia, Vibrio,
Camphylobacter, Shigella,
Salmonella, Escherichia,
Hepatitis A
Cause gastrointestinal
infections
2.3 Viruses
Viruses are considered non-cellular. Bacteriophages which are important in the microbiology of
food are found in nature. They contain nucleic acid (DNA or RNA) and lots of protein. Protein
from the head (around DNA or RNA) and tail. Some viruses carry adhesions or soil molecules to
attach to the recipient cells. Bacteriophage attaches to the surface of the host bacterial cell and
secretes its own cell acid to the host cell (Biswas & Rolain, 2013). Furthermore, most of the cells
are made inside the captured cells and released outside after cell lysis. Many infectious diseases
have been identified as causes of foodborne illness in humans. The most important viruses
involved in food poisoning are hepatitis A and viruses like Norwalk. Both contain unmarried
RNA viruses. Hepatitis A is a small, naked, polyhedral enteric virus, ca. 30 nm in diameter. The
RNA strand is closed as a capsid (Bohmed et al., 2012).
2.3.1 Important Genera in Viruses
Viruses are important in food for a variety of reasons. Some of these can cause bowel disease
and hence cause foodborne illness if present in food. Hepatitis A and Norwalk-like or Norovirus
play a role in foodborne outbreaks. Some other enteric viruses such as poliovirus, adenovirus,
echo virus and Coxsackie virus can cause foodborne illness. In some countries the level of
sanitation is not very high, it can contaminate food and cause illness. Various bacterial viruses
(bacteriophages) are used to identify specific pathogens (Salmonella spp., Listeria,
Biochemistry of food borne microorganisms
Advances in Food Microbiology
Staphylococcus aureus) based on the sensitivity of cells to various bacteriophages in
appropriate dilutions. Bacteriophages are used to transfer genetic traits to certain bacterial strains
or strains through a process called transduction (for example, Escherichia coli or Lactococcus
lactis) (Cogan & Jordan, 1994). Finally, some bacteriophages can be very important because
they can cause fermentation failure. Most lactic acid bacteria used as starter cultures in food
fermentation are sensitive to different bacteriophages. They can infect and destroy starter culture
bacteria, causing crop failure. Among the lactic acid bacteria, bacteriophages have been isolated
for many species in the genus Streptococcus, Lactococcus, Pediococcus, Leuconostoc and
Lactobacillus. Methods are being developed for genetic engineering of lactate starter cultures to
make them resistant to many bacteriophages (Cousin et al., 2005).
3 Transport mechanism of nutrients
Food molecules have to cross the cell barrier – the cell wall and cell membrane. However, in
most Gram-positive lactic acid bacteria, the main barrier is the cytoplasmic membrane. The
cytoplasmic membrane consists of two layers of lipids in which protein molecules are embedded;
some extend the lipid bilayer from the side of the cytoplasm to the side of the cell wall. Many
transport proteins are involved in the transport of nutrient molecules from outside to cells (also
removing many byproducts from cells to the environment) (Dowsen, 2005). In general, small
molecules such as mono- and disaccharides, amino acids, and small peptides (up to 8-10 amino
acids) are transported within cells by certain transport systems, almost unchanged individually or
in groups. Fatty acids (glyceride-free or hydrolyzed) can dissolve and diffuse in the lipid bilayer.
On the contrary, large carbohydrates (polysaccharides such as starch), large peptides and proteins
(such as casein, albumen) cannot be directly transported into cells (Deak and Beuchet, 1987). If
the cell is able to produce specific extracellular hydrolysis enzymes that are present on the
surface of the cell wall or released into the environment, large nutrient molecules can be split
into smaller molecules and then transported by appropriate transport systems. Mono and
disaccharides, amino acids and small peptides are transported across membranes by different
active transport systems such as primary transport systems (eg ATP binding cassette or ABC
transporter), secondary transport systems (eg Uniport), symport and anti-port systems (using
proton motive forces) and the phosphoenolpyruvate-phosphotransferase (PEP-PTS) system
(Frank et al., 2011). A system for a molecular type or similar group of molecules (group transfer)
Biochemistry of food borne microorganisms
Advances in Food Microbiology
can be defined and transported against a substrate concentration gradient, and the transport
process requires energy. For PTS sugar in PEP-PTS system, energy is obtained from PEP; In a
permeable system (for permease sugars, amino acids, and possibly small peptides) the energy
comes from the driving force of the protons (Gerba, 1988).
4 Carbohydrates, transport and their metabolism
In lactic acid bacteria and other bacteria used in food fermentation, disaccharide and
monosaccharide (both hexoses and pentoses) molecules can be transported by PEP-PTS as well
as by permease systems. The same carbohydrate can be transported by the PEP-PTS system in
one species and by the permease system in another species. Similarly, in a species, some
carbohydrates are transported by the PEP-PTS system, whereas others are transported by the
permease system (Gupta, 2002).
4.1 Lactose transport in Lactococcus lactis (PEP-PTS system)
The high-energy phosphate from PEP is transferred sequentially to EnzI, HPr (both in the
cytoplasm and nonspecific for lactose), FacⅢLac, and_EnzIILac (both on the membrane and
specific for lactose), and finally to lactose. Lactose from the environment is transported in the
cytoplasm as lactosephosphate (galactose-6-phosphate-glucose) (Hait et al., 2014). The PEP-PTS
pathway is shown in figure
Figure 01: PEP-PTS pathway for lactose transport
4.2 Permease system in L. Acidophillus for lactose
The lactose molecule carries with it the H
+
in the permeaseLac molecule (especially for lactose).
Once inside, there is a conformational change in the permease molecule that causes the release of
lactose and H
+
molecules in the cytoplasm (Hettinga and Reinbold, 1972). The release of lactose
and H
+
causes the permease to change to its original structure as shown below
Biochemistry of food borne microorganisms
Advances in Food Microbiology
Lactose + [H
+
] PermeaseLac [Permease:Lactose:H
+
] Lactose + [H
+
]
4.3 Carbohydrates inside cell for metabolism
Mono and disaccharides are transported from the media in cells by a permease system or PEP-
PIS. Fermented foods usually contain small amounts of pentose and glucose, fructose, sucrose,
maltose and lactose. Pentose and hexose are metabolized in several different ways, as will be
explained later (Hill, 1993). Sucrose, maltose, and lactose, three disaccharides, are hydrolyzed to
hexose by the enzymes sucrase, maltase and lactase CB-galactosidase in cells. Lactose-
(galactose-6-phosphate-glucose) is hydrolyzed with phospho- (3-galactosidase to produce
glucose and galactose-6-phosphate before further metabolism (Holt et al., 1994).
4.4 Homolactic fermentation of carbohydrates
The hexose in the cytoplasm, which is transported as hexose or derived from disaccharides, is
fermented mainly by homolactic lactic acid bacteria to produce lactic acid. Theoretically, one
hexose molecule will produce two lactate molecules. These species include Lactococcus,
Streptococcus, Pediococcus, and Group I and Group II Lactobacillus. Hexose is metabolized via
the Embden-Meyerhoff-Parnas (EMP) pathway (Hugenholtz et al., 2002). This strain contains
fructose diphosphate (FDP) aldose, which is required to hydrolyze 6C hexose to produce two
molecules of the 3C compound. They also lack phosphositolase (in the HMS pathway), a key
enzyme found in species that are heterolactic fermenters. In the EMP pathway, with glucose as
the substrate, two ATP molecules are used to convert glucose to fructose-1,6-diphosphate (Drew
and Schlegel, 1999) . The hydrolysis of this molecule produces two molecules of the 3C
compound. Subsequent dehydrogenation (to produce NADH + H + from NAD) t
phosphorylation and formation of two ATP molecules leads to the production of PEP (PEP-PTS
can be used to transport sugar) (King et al., 2012). Through the formation of ATP at the substrate
level, PEP is converted to pyruvate, which is converted to lactic acid by the action of lactic
dehydrogenase. The ability of lactic acid bacterial species to produce L (+) -, D (-) – or DL-lactic
acid is determined by the type of lactic dehydrogenase (L, D or a mixture of the two) they
contain. The common reaction involves the production of two molecules, lactic acid and ATP
respectively, from one hexose molecule. Lactic acid is discharged into the environment (Knadler,
1983).
Biochemistry of food borne microorganisms
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Other hexoses, such as fructose (transported as fructose or from the hydrolysis of sucrose),
galactose (from lactose hydrolysis), and galactose-6-phosphate (from lactose-phosphate
hydrolysis after the transport of lactose by the PEP-PTS system) have previously undergone
different molecular transformations (Kreig, 1984). It can be metabolized in the EMP pathway.
Therefore, fructose is phosphorylated by ATP to fructose-6-phosphate before being used in the
EPM pathway.
Figure 02: Homolactic fermentation by EMP pathway
Galactose is first converted to galactose I-phosphate, then glucose-I-phosphate and finally
glucose-6-phosphate via the Leloir pathway before entering the EMP pathway. Galactose-6-
phosphate is first converted to tagatose-6-phosphate, then to tagatose-1,6-diphosphate, and then
hydrolyzed via tagatose to dihydroxyacetone phosphate and glyceraldehyde-3-phosphate before
entering the EMP pathway (Kull et al., 2010). Besides being an essential ingredient in the
production of fermented foods (eg yoghurt, cheese, sausage and fermented vegetables) lactic acid
is used as an ingredient in many foods (eg processed meat products) (Hettinga and Reinbold,
1972). For this purpose, L (+) · lactic acid is preferred and approved by regulatory agencies as a
food additive (since it is also produced by muscle). Lactic acid bacteria that can produce large
quantities of L (+) -lactic acid (particularly some Lactobacillus spp.) Are used commercially for
this purpose. Genetic studies are still ongoing to develop species with the D Land system by
Biochemistry of food borne microorganisms
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deactivating the D-lactic dehydrogenase system and producing a broad mixture of L (+) –
and D (-) -lactic acids (Kunj et al., 1996).
Various species producing L (+) -lactic acid (> 90% or more) Lactococcus lactis ssp. lactis and
cremoris, Streptococcus thermophilus, Lactobacillus amylovorus, Lab. amylophilus, Lab. casei
spp. Casei, Lab. casei spp. rhanmoslls. Various species commonly used in food fermentation,
Pediococcus acidilactici, Ped. pentosacells, Lab. delbrueckii spp. blllgariclls and helvaticus,
Lab. acidophilus, Lab. reuteri and Lab. Plantarum produces a mixture of DC -) – and L (+) –
lactic acid, which is 20-70% L (+) – lactic acid (Lengeler et al., 2000).
4.5 Heterolytic fermentation of carbohydrates
Hexose is metabolized by heterofermentative lactic acid bacteria to produce a mixture of lactic
acid, CO2 and acetic or ethanol. Strains from Leuconostoc and group III Lactobacillus strains
lacked fructose diphosphoaldolase (EMP pathway) but had glucose phosphate dehydrogenase
and xylulose phosphocetoase enzymes (Liang et al., 2015); this enzyme allows them to
metabolize hexose via the phosphogluconate-phosphosphate pathway (or hexose monophosphate
shunt). This pathway has an initial oxidative phase followed by a non-oxidative phase. In the
oxidative phase, after phosphorylation, glucose is oxidized to 6-phosphogluconate by glucose
phosphate dehydrogenase and then decarboxylated to produce CO molecules and a 5C
compound, ribulose-5-phosphate. In the non-oxidative phase, compound 5e is converted by
hydrolysis to xylulose-5-phosphate, which produces glyceraldehyde-3-phosphate and acetyl
phosphate (Lin et al., 2015). Glyceraldehyde-3-phosphate is then converted into lactate. Acetyl
phosphate can be oxidized to acetate or reduced to ethanol (depending on the O-R strength of the
medium) (Mata and Ritzenhaler, 1988). The breed differs in the ability to produce ethanol,
acetate, or “a mixture of the two”. The final product is discharged into the environment
(Muratovic et al., 2015).
Biochemistry of food borne microorganisms
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Figure 03: Heterolactic fermentation by HMS pathway
4.6 Pentose metabolism
Pentose can be metabolized by Leuconostoc and Lactobacillus (Group and Group Ⅲ) is first
converted to xylulose-5-phosphate in several different ways. The xylulose-5-phosphate is then
metabolized by heterophenentative lactic acid bacteria to produce lactate and acetate or ethanol
by the mechanisms described in the non-oxidation section of hexose metabolism. In this way,
CO2 is not generated from pentose metabolism (Pinchuk et al., 2010).
4.7 Fermentation of hexose from Bifidobacterium
Bifidobacterium species metabolize hexose to produce lactate and acetate via the fructose-
phosphate pathway or the Bifidus pathway. For every two hexose molecules, two lactate and
three acetate molecules are produced without producing CO2 (Piras et al., 2015). Two glucose
molecules are produced from two fructose-6-phosphate molecules. One molecule is converted to
produce erythrose-4-phosphate 4C and acetyl-phosphate (later converted to acetate). Another
fructose-6-phosphate molecule combines with erythrose-4-phosphate to produce two xylulose-5-
phosphate 5C molecules at some intermediate stage. The xylulose-5-phosphate is then
metabolized to produce lactate and acetate by the method …
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