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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).

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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

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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

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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

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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

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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

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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,

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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)

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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

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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).

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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

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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).

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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 described in the section on Non-

oxidation on heterolactic fermentation (Pool, 2002).

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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.

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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

Biochemistry of food borne microorganisms

Advances in Food Microbiology

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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Advances in Food Microbiology

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