BIOCHEMISTRY OF FOOD BORNE MICROORGANISMS Submitt

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
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).

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
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
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), lipoprotei ns
(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

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

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

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,

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)
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
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).
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

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).
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
described in the section on Non-
oxidation on heterolactic fermentation (Pool, 2002).
Figure 04: Diacetyl production by citrate

Figure 05: Propionic acid metabolism
4.8 Diacetyl production from citrate
Compound 4C is important in many fermented dairy products
because of its diacetyl, aroma or
salty taste (buttery taste). It is also used separately in many
foods to give a buttery flavor. Many
lactic acid bacteria can produce small amounts of pyruvate
produced by metabolizing
carbohydrates (Rajkovic, 2014). But Lac. lactis ssp. Lactis
biovar diacetylactis and Leuconostoc
strains can produce large amounts of diacetyl from citrate.
Citrate, a compound of 6C, is
transported from outside to the cells by the citrate permease
system. It is then metabolized via
pyruvate to acetaldehyde "-'TPP (thiamine pyrophosphate). It
then combines with pyruvate to
form alpha-acetolactate, which is converted to diacetylation
with the formation of CO2 (Ray and
Sandine, 1992). Under reduced conditions, diacetyl can be
converted to acetoin with the desired
loss of flavor, engineering metabolism) can be genetically
modified to produce.
4.9 Production of propionic acid from Propionibacterium

Propionibacterium milk product is used as one of the starter
cultures for some cheeses (such as
Swiss), the desired taste is propionic acid. Propionibacterium
produces pyruvate from hexose via
the EMP pathway and pyruvate is used to produce propionic
acid. This produces pyruvate and
methylmalonyl-CoA, propionyl-CoA and oxalate. Propionyl-
CoA is then converted into
propionate. Oxalacetate is recycled via succinyl-CoA to produce
methylmalonyl-CoA.
Propionibacterium also produces acetate and CO2 (CO2
contributes to eye formation in
Swiss cheese) from pyruvate (Mata and Ritzenhaler, 1988).
5 Amino acids and proteinaceous compounds, transport and
their
metabolism
Most of the lactic acid bacteria used in food fermentation have
an active transport system for

transporting small amino acids and peptides (about 8-10 amino
acids in length) (Samson et al.,
2000). Peptides and large proteins which can be metabolized in
the environment are converted
into small peptides and amino acids by being hydrolyzed mainly
by the proteinases and
peptidases found in the cell walls of these species, and then
transported mainly by active groups
within the cells transportation system (Hettinga and Reinbold,
1972). Inside the cell acids,
peptides are converted into amino acids and then the amino is
metabolized differently to produce
many different end products which are discharged into the
environment. Several metabolic
processes include decarboxylation (production of amines, some
of which are biologically active,
such as histamine from histidine), deamination, oxidative
reduction, and anaerobic reduction
(Reddy et al., 2014).
Amino acid metabolism during fermentation by various lactic
acid bacteria and related bacteria
can produce a variety of products, many of which have specific
taste characteristics. Some of

these include ammonia, hydrogen sulfide, amines and disulfides
(Sospedra et al., 2015). Many of
these, in low concentrations, contribute to the desired taste of
various fermented foods. In many
fermented foods (such as cheese), proper hydrolysis of dietary
protein by proteolytic enzymes
initiates the microorganisms to obtain the desired texture (Mata
and Ritzenhaler, 1988). The
rapid hydrolysis of protein can lead to the formation and
accumulation of several specific
hydrophobic ebbs that give the product a bitter taste (as in some
sharp cheddar cheeses) (Tevel et
al., 2013).
6 Lipid compounds, their transport and metabolism
Many starter culture bacteria metabolize lipids poorly.
However, mushrooms have a better lipid
metabolism system. Triglycerides and phospholipids are
hydrolyzed by lipases produced by
extracellular microorganisms to release fatty acids and glycerol
and glycerides (mono- or
diglycerides) (Whitlock et al., 2000). Fatty acids can diffuse
into cells across membranes and be

metabolized. There are several fatty acids in the membrane. The
hydrolysis of glycerides,
especially those containing minor fatty acids such as butyric
acid, can cause the hydrolytic
rancidity of the product. The oxidation of unsaturated fatty
acids by microorganisms, especially
molds, can produce many desired and unwanted flavoring
compounds (Hettinga and Reinbold,
1972).
7 Conclusion
Beneficial microorganisms metabolize certain components
found in starting materials (i.e. foods
such as milk or meat) to produce and regenerate energy and
cellular materials. In this process,
they produce some end products that are no longer needed for
cells, and therefore these by-
products are discharged into the environment. Some of these by-
products give the residue from
the starting material unique properties (mostly texture and
flavor). It is a processed or fermented

product and is in demand by consumers. Some by-products of
fermentation can also be purified
and used as food additives. The production of some of these by-
products by the desired
microorganisms is discussed in this section. Before identifying
the metabolic pathways these
microbes use, it is helpful to review the food ingredients
(substrates) used in fermentation. The
substrate must be transported from the external environment
before it can be metabolized into
microbial cells. It is useful to know in a nutshell. Cellular
components are involved in the
transport of this substrate. Important substrates found in the
starting material for fermentation
include various carbohydrates, nitrogenous proteinaceous and
non-proteinaceous compounds
(NPN), and lipids. Important fermentable carbohydrates in
foods are starch glycogen (in meat),
lactose (in dairy products), sucrose, maltose (from the
breakdown of starch), glucose-fructose
and pentose (from plant sources). Protein and NPN components
mainly include large proteins
(structural and functional), peptides of different sizes, and
amino acids. It can be in the form of

lipids, triglycerides, phospholipids, fatty acids and sterols.
Microorganisms differ greatly in their
ability to transport these components from outside and
metabolize them inside the cell.
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