General characteristics, classification and identification of yeasts, molds

Dr. Sujeet Kumar Mrityunjay, PhD
Assistant Professor
Department of Life Science
School of Sciences
ITM University,Gwalior (Turari Campus)
Madhya Pradesh-474001 (India)
Unit I General characteristics, classification and
identification of yeasts, molds and group of bacteria
important in food industry, sources of contaminations:
air, water, sewage, post processing contamination.
Factors influencing growth of microorganism on foods,
Intrinsic factors and Extrinsic factors

Food Microbiology Page 1 of 28
Unit I General characteristics, classification and identification of yeasts, molds and group of
bacteria important in food industry, sources of contaminations: air, water, sewage, post
processing contamination. Factors influencing growth of microorganism on foods, Intrinsic
factors and Extrinsic factors.
Food Microbiology: - Food microbiology comprehends the study of microorganisms that colonise,
modify, and process, or contaminate and spoil food. It is one of the most diverse research areas
within microbiology. It comprises a wide variety of microorganisms including spoilage, probiotic,
fermentative, and pathogenic bacteria, moulds, yeasts, viruses, prions, and parasites. It deals with
foods and beverages of diverse composition, combining a broad spectrum of environmental factors,
which may influence microbial survival and growth. Food microbiology includes microorganisms
that have beneficial or deleterious effects on food quality and safety and may therefore be of concern
to public health.
Yeast are Single-celled, but with Cellular Organization Similar to Higher Organisms
YEASTS: - Yeasts are generally unicellular and differ from bacteria in their large cell size and
morphology, and because they produce buds during the process of reproduction by division or
budding. Yeasts can be spread through the air, or other means, and alight on the surface of foodstuffs.
Yeast colonies are generally moist or slimy in appearance and creamy white colored. Yeasts prefer an
Aw of 0.90 - 0.94, but can grow below 0.90. These micro-organisms grow best in the intermediate
acid range, pH from 4.0 to 4.5. Food that is highly contaminated with yeasts will frequently have a
slightly fruity odour. Yeasts may be useful or harmful in foods. Yeast fermentations are involved in
the manufacture of foods such as bread, beer, wines, vinegar, and surface ripened cheese, and yeasts
are grown for enzymes and for food. Yeasts are undesirable when they cause spoilage of sauerkraut,
fruit juices, syrups; molasses, honey, jellies, meats, wine, beer, and other foods. There are hundreds
of economically important varieties of ascomycete yeasts; the types commonly used in the
production of bread, beer, and wine are selected strains of Saccharomyces cerevisiae. Some yeasts
are mild to dangerous pathogens of humans and other animals, especially Candida albicans,
Histoplasma, and Blastomyces.
GENERAL CHARACTERISTICS OF YEASTS: - Yeasts are classified primarily on their
morphological characteristics, while their physiological ones are more important to the food
microbiologist.
Dr. Sujeet Kumar Mrityunjay

Food Microbiology Page 2 of28
Saccharomyces cerevisiae, a type of budding yeast, is able to ferment sugar into
carbon dioxide and alcohol and is commonly used in the baking and brewing
industries.
Morphological Characteristics: - The morphological characteristics of yeasts are determined by
microscopic examination.
Form and Structure: - The form of yeasts may be spherical to ovoid, lemon shaped, pear-shaped,
cylindrical, triangular, or even elongated into a false or true mycelium. They also differ in size.
Visible parts of the structure are the cell wall, cytoplasm, water vacuoles, fat globules, and granules,
which may be metachromatic, albuminous, or Starchy. Special staining is necessary to demonstrate
the nucleus.
Budding yeast cells
Reproduction: - Yeasts reproduce by budding (asexual reproduction), when a small bud forms and
splits to form a new daughter cell, but under stress conditions they can produce spores (a form of
sexual reproduction). The bread yeast Saccharomyces cerevisiae uses the sugars in the flour to
produce energy, releasing the alcohol ethanol (which evaporates) and bubbles of the gas carbon
dioxide, which makes the bread dough rise. The bread yeast is also used to make some types of beer;
in this case the yeast uses the sugars from cereals like barley, to produce ethanol and carbon dioxide.
The bread yeast has been widely used by scientists to study important cellular processes.
Most yeasts reproduce asexually by multilateral or polar budding, a process in which some of
the protoplasm bulges out the cell wall; the bulge grows in size and finally walls off as a new yeast
cell. In some yeasts, notably some of the film yeasts, the bud appears to grow from a tube like
projection from the mother cell. Replicated nuclear material is divided between the mother and
daughter cells. A few species of yeasts reproduce by fission, and one reproduces by a combination of
fission and budding. Sexual reproduction of "true" yeasts (Ascomycotina) results in the production
of ascospores, the yeast cell serving as the ascus. The formation of ascospores follows conjugation of
two cells in most species of true yeasts, but some may produce ascospores without conjugation,
followed by conjugation of ascospores or small daughter cells. The usual number of spores per ascus
and the appearance of the ascospores are characteristic of the kind of yeast. The ascospores may
differ in color, in smoothness or roughness of their walls, and in their shape (round, oval, reniform,

bean- or sickle-shaped, Saturn- or hat~ shaped, hemispherical, angular, fusiform, or needle-shaped).
"False" yeasts, which produce no ascospores or other sexual spores, belong to the Fungi Imperfecti.
Cells of some yeasts become chlamydospores by formation of a thick wall about the cell, for
example, Candida, Rhodotorula, and Cryptococcus.
True yeasts: - These are unicellular round or oval fungi. Reproduction is by budding from the parent
cell. Cultures in vitro characteristically show 'creamy' colonies, e.g. Cryptococcus neoformans.
Yeast-like /"False" fungi: - These are like yeasts since they may appear as round or oval cells and
grow by budding. They may also form long non-branching filaments known as 'pseudohyphae', e.g.
Candida albicans.
Cultural Characteristics: - Growth as a film on the surface of liquid media suggests an oxidative or
film yeast, and production of a carotenoid pigment indicates the genus Rhodotorula. The appearance
of the growth is important when it causes colored spots on foods. It is difficult to tell yeast colonies
from bacterial ones on agar plates; the only certain way is by means of microscopic examination of
the organisms. Most young yeast colonies are moist and somewhat slimy but may appear mealy;
most colonies are whitish, but some are cream-colored or pink. Some colonies change little with age,
but others become dry and wrinkled. Yeasts are oxidative, fermentative, or both. The oxidative yeasts
may grow as a film, pellicle, or scum on the surface of a liquid and then are termed film yeasts.
Fermentative yeasts usually grow throughout the liquid and produce carbon dioxide.

Physiological Characteristics: - Most common yeasts grow best with a plentiful supply of available
moisture. But since many yeasts grow in the presence of greater concentrations of solutes (such as
sugar or salt) than most bacteria. Most yeast requires more moisture than molds, however. On the
basis of water activity or aw, yeasts may be classified as ordinary if they do not grow in high
concentrations of solutes, i.e., in a low aw, and as Osmophilic if they do. Lower limits of aw for
ordinary yeasts range from 0.88 to 0.94. Osmophilic yeasts have been found growing slowly in
media with aw as low as 0.62 to 0.65 in syrups, although some Osmophilic yeasts are stopped at
about 0.78 in both salt brine and sugar syrup. The aw values will vary with the nutritive properties of
the substrate, pH, temperature, availability of oxygen, and presence or absence of inhibitory
substances. The range of temperature for growth of most yeasts is 25ºC to 30ºC and the maximum
about 35 to 47º C. Some kinds can grow at 0ºC or less. The growth of most yeasts is favored by an
acid reaction in the vicinity of pH 4 to 4.5, and they will not grow well in an alkaline medium unless
adapted to it. Yeasts grow best under aerobic conditions, but the fermentative types can grow
anaerobically, although slowly. Yeasts may change in their physiological characteristics, especially
the true, or ascospore-forming, yeasts, which have a sexual method of reproduction. These yeasts can
be bred for certain characteristics or may mutate to new forms. Most yeasts can be adapted to
conditions which previously would not support good growth. Illustrative of different characteristics
within a species is the large number of strains of Saccharomyces cerevisiae suited to different uses,
e.g., bread strains, beer strains, wine strains, and high-alcohol-producing strains or varieties.
CLASSIFICATION AND IDENTIFICATION OF YEASTS: - Yeast taxonomy began in 1837
with Meyer's assigning of the genus Saccharomyces (from the Greek sakehar, meaning sugar and
mykes, meaning fungus) to yeast. The true yeasts are in the subdivision Ascomycotina, and the false,
or asporogennous, yeasts are in the subdivision Fungi Imperfecti or Deuteromycotina. Certain
yeasts are actually represented in two different genera based on whether they reproduce sexually.
The principal bases for the identification and classification of genera of yeasts are as follows:-
1. Whether ascospores are formed.
2. If they are spore-forming:-
A. The method of production of ascospores: -
i. Produced without conjugation of yeast, cells (parthenogenetically). Spore
formation may be followed by: - (a) Conjugation of ascospores. (b) Conjugation
of small daughter cells.
ii. Produced after isogamic conjugation (conjugating cells appear similar).

iii. Produced by heterogamic conjugation (conjugating cells differ in appearance).
B. Appearance of ascospores: shape, size, and color. Most spores are spheroidal or ovoid,
but some have odd shapes, e.g., most species of Hansenula, which look like derby hats.
C. The usual number of ascospores per ascus: one, two, four, or eight.
2. Appearance of vegetative cells: shape, size, color, inclusions.
3. Method of asexual reproduction: -
i. Budding.
ii. Fission.
iii. Combined budding and fission.
iv. Arthrospores (oidia).
4. Production of a mycelium, pseudo mycelium, or no mycelium.
5. Growth as a film over surface of a liquid (film yeasts) or growth throughout medium.
6. Color of macroscopic growth.
7. Physiological characteristics (used primarily to differentiate species or strains within a
species):-
i. Nitrogen and carbon sources.
ii. Vitamin requirements.
iii. Oxidative or fermentative: film yeasts are oxidative; other yeasts may be fermentative
or fermentative and oxidative.
iv. Lipolysis; urease activity, acid production, or formation of starch like compounds.
GENERAL CHARACTERISTICS, CLASSIFICATION AND IDENTIFICATION OF MOLDS
Molds: - Molds are multicellular micro-organisms with mycelial (filamentous) morphology. These
microbes are also characterized by their display of a variety of colors and are generally recognized
by their mildewy or fuzzy, cotton like appearance. Molds can develop numerous tiny spores that are
found in the air and can be spread by air currents. These spores can produce new mold growth if they
are transferred to a location that has conditions conducive to germination. Molds generally withstand
greater fluctuation in pH than bacteria and yeasts and can frequently tolerate more temperature
fluctuation. Although molds thrive best at or near a pH of 7.0, a pH range of 2.0 to 8.0 can be
tolerated, even though an acid to neutral pH is preferred. Molds thrive better at ambient temperature
than in a colder environment, even though growth can occur below 0°C. Although mold growth is
optimal at a water activity (Aw) of approximately 0.85, growth can and does occur below 0.80. At an

Aw of 0.90 or higher, bacteria and yeasts grow more effectively and normally utilize available
nutrients for growth at the expense of molds. When the Aw goes below 0.90, molds grow more
effectively. That is why foodstuffs, such as pastries, cheeses, and nuts, that are low in moisture
content are more likely to spoil from mold growth.
General characteristics of molds: - The term "mold" is a common one applied to certain
multicellular filamentous fungi whose growth on foods usually is readily recognized by its fuzzy or
cottony appearance. The main part of the growth commonly appears white but may be colored or
dark or smoky due to the production of pigment I some filamentous fungi. Colored spores are typical
of mature mold of some kinds and give color to part or all of the growth. The thallus, or vegetative
body, is characteristic of thallophytes, which lack true roots, stems, and leaves, e.g. Trichophyton
mentagrophytes.
Morphological Characteristics: - The morphological characteristics i.e. form and structure, of
molds are determined by macroscopic and microscopic examination.
Hyphae and Mycelium: - The mold thallus consists of a mass of branching, intertwined filaments
called hyphae (singular hypha), and the whole mass of these hyphae is known as the mycelium. The
hyphae may be submerged, or growing within the food, or aerial, or growing into the air above the
food. Molds are divided into two groups: septate, i.e., with cross walls dividing the hypha into cells;
and noncoenocytic, septate with the hyphae apparently consisting of cylinders without cross walls.
The non-septate hyphae have nuclei scattered throughout their length and are considered
multicellular. Special, mycelial structures or parts aid in the identification of molds. Examples are
the rhizoids, or "holdfasts," of Rhizopus and Absidia, the foot cell in Aspergillus, and the
dichotomous, or Y-shaped, branching in Geotrichum.
Reproductive Parts or Structures: - Reproduction in fungi is complex and involves a great
diversity of structures. At the most fundamental level we can say that most moulds reproduce by
spores. Spores are like seeds; they germinate to produce a new mould colony when they land in a
suitable place. Unlike seeds, they are very simple in structure and never contain an embryo or any
sort of preformed offspring. Spores are produced in a variety of ways and occur in a bewildering
array of shapes and sizes. In spite of this diversity, spores are quite constant in shape, size, colour and
form for any given mould, and are thus very useful for mould identification. The most basic
difference between spores lies in their method of initiation, which can be either sexual or asexual.

Rhizopus Digrame of simple conidial head of Aspergillus
Geotrichum
Asexual Spores: - The asexual spores of molds are produced in large numbers and are small, light,
and resistant to drying. They are readily spread through the air to alight and start new mold thallus
where conditions are favorable. The three principal types of asexual spores are (1) conidia (singular
conidium), (2) arthrospores or oidia (singular oidium), and (3) sporangiospores. Conidia are cut off,
or bud, from special fertile hyphae called conidiophores and usually are in the open, i.e., not
enclosed in any container, in contrast to the sporangiospores, which are in sporangium (plural
sporangia), or sac, at the tip of a fertile hypha, the sporangiophore. Arthrospores are formed by
fragmentation of a hypha, so that the cells of the hypha become arthrospores. Examples of these
three kinds of spores will be given in the discussion of important genera of molds. A fourth kind of
asexual spore, the chlamydospore, is formed by many species of molds when a cell here and there in
the mycelium stores up reserve food, swell, and forms a thicker wall than that of surrounding cells.
This chlamydospore, or resting cell, can withstand un-favorable conditions better than ordinary mold
mycelium can and later, under favorable conditions, can grow into a new mold.
Sexual Spores: - Sexually initiated spores result from a mating between two different organisms or
hyphae, whereas asexual spores result from a simple internal division or external modification of an
individual hypha. The recognition of a mating and subsequent spore formation is often difficult for

an observer, and is usually reserved for patient specialists. The molds which can produce sexual
spores are classified on the basis of the manner of formation of these spores and the type produced,
namely, the four kinds of sexually determined spores that appear in mould fungi:-
A.
A. Oospores
B. Zygospores
C. Ascospores, and
D. Basidiospores.
Oospores: - Oospores are termed Oomycetes. These molds are mostly aquatic; however,
included in this group are several important plant pathogens. Oospores are produced when male
gametes (reproductive nuclei) enter a large spherical cell (oogonium) and fertilize the eggs within.
The result, as seen in routine examination, is numerous oogonia containing one to several spherical
and often brownish eggs. The oogonia are usually penetrated by one or more hyphae (antheridia) that
give rise to the male nuclei. (The oospores are formed by the union of a small male gamete and a
large female gamete).
B.Zygospores: - Zygospores do not occur inside any kind of enclosing structure, but are produced
by the direct fusion of two hyphal protrusions (suspensors) from neighbouring filaments. Usually
zygospores are recognized as large, nearly spherical, often dark brown or black, rough- walled
spores with two connecting hyphae, representing the two mating gametangia. Sometimes the
zygospore may be surrounded by several finger-like extensions from the two gametangia.
(Zygomycetes form zygospores by the union of the tips of two hyphae which often appear similar and
which may come from the same mycelium or from different mycelia. Both Oospores and zygospores
are covered by a tough wall and can survive drying for long periods).
C.Ascospores: - Ascospores are produced within spherical to cylindrical cells called asci, most often
in groups of four or eight. Usually the asci are produced within some kind of enclosing structure and
thus are not found exposed on the hyphae. In a few cases the asci may be borne among hyphae and
resemble oogonia with eggs, but they will never be penetrated by any sort of fertilizing hypha.
Fertilization occurs early in the life cycle and is not evident at the time ascospores are produced.
D.Basidiospores: - Basidiospores are always produced externally on a structure called a basidium.
Basidia come in a variety of forms, but those commonly encountered on moulds will be club-shaped
and bear four or eight spores on sharp projections at the apex. At first it may be difficult to
distinguish between a basidiospores and one of the asexually initiated spore types, but one should
always suspect the presence of basidia when externally produced spores consistently occur in groups

of four or eight. As with ascospores, basidiospores are the result of an early fertilization that is not
easily observed.
CULTURAL CHARACTERISTICS: - Some molds are loose and fluffy; others are compact. Some
look velvety on the upper surface, some dry and powdery, and others wet or gelatinous. Definite
zones of growth in the thallus distinguish some molds, e.g. Aspergillus niger. Pigments in the
mycelium-red, purple, yellow, brown, gray, black, etc - are characteristic, as are the pigments of
masses of asexual spores; green, blue-green, yellow, orange, pink, lavender, brown, gray, black, etc.
The appearance of the reverse side of a mold on an agar plate may be striking, like the opalescent
blue-black or greenish-black color of the underside of Cladosporium.
PHYSIOLOGICAL CHARACTERISTICS: - The physiological characteristics of molds will be
discussed briefly.
Temperature requirement - Most molds grow well at ordinary temperature. A number of molds
grow well at refrigeration temperatures. A few can grow at a high temperature. The optimal
temperature for most molds is around 25 to 30º C, but some grow well at 35 to 37ºC or above, e.g.,
Aspergillus spp., and some at still higher temperatures. A number of molds are psychrotrophic; i.e.,
they grow fairly well at temperatures of refrigeration, and some can grow slowly at temperatures
below freezing. Growth has been reported at as low as - 5 to - 10ºC. A few are thermophilic; i.e., they
have a high optimal temperature.
Moisture Requirements: - In general molds require less moisture to grow than yeast and bacteria. If
dried food has a moisture content below 14 to 15%, it will prevent or delay mold growth.
Oxygen and pH Requirements: - Molds are aerobic, so they require oxygen for their growth. Most
molds can grow over a wide range of hydrogen-ion concentration (pH 2 to 8.5), but the majority are
favored by an acid pH.
Food Requirements: - Molds in general can utilize many kinds of foods, ranging from simple to
complex. Most of the common molds possess a variety of hydrolytic enzymes, and some are grown
for their amylases, pectinases, proteinases, and lipases.
Inhibitors: - Compounds inhibitory to other organisms are produced by some molds, such as
penicillin from Penicillium chrysogenum and clavacin from Aspergillus clavatus. Certain chemical
compounds are mycostatic, inhibiting the growth of molds (sorbic acid, propionates, and acetates are
examples), or are specifically fungicidal, killing molds.

CLASSIFICATION AND IDENTIFICATION OF MOLDS: - Molds are part of the kingdom
Myceteae. They have no roots, stems, or leaves and are devoid of chlorophyll. They belong to the
Eumycetes, or true fungi, and are subdivided further to subdivisions, classes, orders, families, and
genera. The following criteria are used mainly for differentiation and identification of molds:-
1. Hyphae septate or non-septate
2. Mycelium clear or dark (smoky)
3. Mycelium colored of colorless
4. Whether sexual spores are produced and the type: oospores, zygospores, or ascospores
5. Characteristics of the spore head
a) Sporangia: size, color, shape, and location
b) Spore heads bearing conidia: single conidia, chains, budding conidia, or masses;
shape and arrangement of sterigmata or phialides; gumming together of conidia
6. Appearance of sporangiophores or conidiophores: simple or branched, and if branched the
type of branching; size and shape of columella at tip of sporangiophore; whether
conidiophores are single or in bundles
7. Microscopic appearances of the asexual spores, especially of conidia: shape, size, color;
smooth or rough; one-, two-, or many-celled
8. Presence of special structures (or spores):- stolons, rhizoids, foot cells, apo-physis,
chlamydospores, sclerotia, etc.
GROUPS OF BACTERIA IMPORTANT IN FOOD INDUSTRY
1. Lactic acid – forming bacteria or lactics: - The lactic acid bacteria are a group of Gram-
positive bacteria, non-respiring non-spore-forming, cocci or rods, which produce lactic acid
as the major end product of the fermentation of carbohydrates. These bacteria ferment sugars
to lactic acid. This may be desirable in making products such as sauerkraut and cheese. But
undesirable in terms of spoilage of wines because they usually form acid rapidly. Ex:
Leuconostoc, Lacto bacillus, Streptococcus and Pediococcus.
2. Acetic acid forming bacteria or acetics: - Acetic acid bacteria (AAB) are a group of Gram-
negative bacteria which oxidize sugars or ethanol and produce acetic acid during
fermentation. The acetic acid bacteria consist of 10 genera in the family Acetobacteraceae.
Several species of acetic acid bacteria are used in industry for production of certain foods and
chemicals. All acetic acid bacteria are rod-shaped and obligate aerobes. Acetic acid bacteria
are airborne and are ubiquitous in nature. They are actively present in environments where

ethanol is being formed as a product of the fermentation of sugars. Acetic acid bacteria are
some of the most common wine spoilage organisms and, as such, they represent a genuine
threat to the winemaker. The most common reaction associated with wine spoilage is the
oxidation of ethanol to form acetic acid, which leads to the production of wine commonly
referred to as “pricked.” The growth of Acetobacter in wine can be suppressed through
effective sanitation, by complete exclusion of air from wine in storage, and by the use of
moderate amounts of sulfur dioxide in the wine as a preservative. Characteristics that make
acetic acid bacteria important are:-
iii.
i. Their ability to oxidize ethanol to acetic acid.
ii. Their strong oxidizing power, result in oxidation of desired product like acetic acid,
by desirable sps or undesirable sps under favorable conditions.
Excessive sliminese of some species Ex: Acetobacter acetic sub sp. suboxydans. This
bacteria clog vinegar generators.
3. Butyric acid forming bacteria or butyrics: - Most bacteria of this group are spore forming
anaerobes of the genus clostridium.
4. Propionic acid – forming bacteria or propionics: - The bacteria that produce propionic
acid were named Propionibacterium by Orla-Jensen in 1898 and their fermentation was
studied by Pasteur and Fitz. Propionic acid bacteria are usually found in Swiss-type cheeses
where they grow during ripening and contribute to the characteristic flavour and appearance
of these cheeses. Ex: Propionibacterium. (Propionibacterium is a gram-positive, anaerobic,
rod-shaped genus of bacteria named for their unique metabolism: They are able to synthesize
propionic acid by using unusual transcarboxylase enzymes.)
5. Lipolytic Bacteria: - This bacteria produce lipases which catalyze the hydrolysis of fats to
fatty acids and glycerol. Many of the aerobic, actively proteolytic bacteria also are lipolytic.
Pseudomonas fluorescens – Strongly lipolytic, Pseudomonas, Alcaligenes, Staphylococcus,
Serratia and Micrococcus are genera that contain lipolytic bacteria.
6. Saccharolytic bacteria: - These bacteria hydrolyze disaccharides or polysaccharides to
simpler sugars. Amylolytic bacteria possess amylase to bring about the hydrolysis of starch
outside the cell. Amylolytic bacteria are Bacillus subtilis and Clostridium butyricum.
7. Pectinolytic Bacteria: - Pectins are complex carbohydrates that are responsible for cell wall
rigidity in vegetables and fruits pectic substances derived from citrus fruits can be used in
commercial products as gelling agents. Ex: Erwinia, Bacillus, Clostridum, Achromobacter,
Aeromonas, Arthrobacter, Flavobacterium.

8. Thermophilic Bacteria or Thermophiles: - Optimum temperature required for these bacteria
45°C - 55°C. Bacillus stearothermophilus – thermophilic flat sour spoilage of low acid
canned foods.
9. Thermoduric Bacteria: - Thermoduric bacteria are usually defined as those which can
survive a heat treatment such as pasteurization. Ex: Bacillus sps, Micrococci, Enterococci
can survive pasteurization of liquid eggs. Fungi like Byssochlamys fulva, Aspergillus and
Penicillium are thermoduric. Some thermoduric bacteria like Bacillus and enterococci can
also be psychrotrophic.
10. Psychrotrophic Bacteria or psychrotrophs: - These bacteria are able to grow at
commercial refrigeration temperatures. Unlike psychrophiles, psychrotrophs do not have
their optimal temperature for growth at refrigeration temperature and their optimum between
25°C and 30°C. Ex: Pseudomonas, Flavobacterium, Achromobacter and Alcaligenes,
Micrococcus, Lactobacillus etc.
11. Halophilic Bacteria or Halophiles: - Halophiles are organisms that thrive in high salt
concentrations. They are a type of extremophile organism. The name comes from the Greek
word for "salt-loving". Halophilic bacteria are categorized based on their requirements to
salt. They apply some strategies in adaptation to the high saline environment. Their low
nutritional requirements and resistance to high concentrations of salt, introduce as potent
agents in wide range of biotechnological applications. Halophilic bacteria are very divergent
and more than 150 species introduce in 70 genera of halophilic bacteria are reported.
Halophilic bacteria are suitable for developing open and unsterile fermentation process as
they can be grown under high salt concentration to prevent contamination from nonhalophilic
microorganisms. Halophilic Bacteria require certain minimal concentrations of dissolved
sodium chloride for growth. Ex: - Pseudomonas, Moraxella, Acinetobacter,
Flavobacterium, Vibrio sps. which grow best in media with 0.5 – 3.0 percent salt. These
microorganisms are isolated from fish shell fish. These are slightly halophilic. Extreme
halophiles grow in the heavily brined foods 15 – 30% salt. Ex: - Halobacterium,
Halococcus.
12. Osmophilic or Saccharophilic Bacteria: - Osmophilic organisms are microorganisms
adapted to environments with high osmotic pressures, such as high sugar concentrations.
Osmophiles are similar to halophilic (salt-loving) organisms because a critical aspect of both
types of environment is their low water activity, aw. High sugar concentrations represent a
growth-limiting factor for many microorganisms, yet osmophiles protect themselves against

this high osmotic pressure by the synthesis of osmoprotectants such as alcohols and amino
acids. Osmophilic bacteria are those which grow in high concentrations of sugar. Ex: -
Leuconostoc. Many osmophilic microorganisms are yeasts (Zygosaccharomyces bailii,
Wallemia sebi, Debaryomyces sp.).
13. Pigmented Bacteria: - Colors produced by pigmented bacteria growing on or in foods.
Flavobacterium – Yellow to orange; Serratia – Red; Halobacterium – Pink.
14. Slime or rope forming bacteria: - Alcaligenes viscolactis, Enterobacter aerogenes &
Klebsiella oxytoca causes ropiness of milk and Leuconostoc spp., producing slime in sucrose
solutions and slimy surface growth of various bacteria occurring on foods. Streptococcus &
Lactobacillus make milk slimy or ropy. Micrococcus makes curing solutions for meats ropy.
Lactobacillus plantarum and Lactobacilli may cause ropiness in various fruit, vegetable and
grain products e.g. in cider, sauerkraut and beer.
15. Gas forming Bacteria: - Many kinds of bacteria produce small amounts of gas and yield it
slowly. Ex: Leuconostoc, Lactobacillus (heterofermentative), Propionic bacterium,
Escherichia, Enterobacter, Proteus, Bacillus and Clostridium.
16. Coliform and fecal coliform group: - Coliforms are short rods that are defined as aerobic
and facultative anaerobic, gram negative, non-spore forming bacteria. Ex: Escherichia coli,
Enterobacter aerogenes. Fecal coliform group includes coliforms capable of growing at 44 -
45°C. Geotrichum candidum is the machinery mold and as an indicator of plant sanitation
and contaminated equipment. Some of the characteristics that make the coliform bacteria
important in food spoilage are :-
1. Their ability to grow well in a variety of substrates and synthesize most of the
necessary vitamins.
2. Their ability of the group to grow well over a fairly wide range of temperatures from
below 10°C to about 46°C.
3. Their ability to produce considerable amounts of acid and gas from sugars.
4. Their ability to cause off – flavours often described as unclean or barny.
5. Their ability of E. aerogenes to cause sliminess or ropiness of foods.
SOURCES OF CONTAMINATION: - Food contamination is the introduction or occurrence of a
contaminant in food. A contaminant is any biological or chemical agent, foreign matter, or other
substance unintentionally added to food that may compromise food safety or suitability. Among
these contaminants are biological, chemical or physical agents in, or condition of, food with the
potential to cause an adverse health effect. Major contamination sources are water, air, dust,

equipment, sewage, insects, rodents, and employees. Contamination of raw materials can also occur
from the soil, sewage, live animals, external surface, and the internal organs of meat animals.
Additional contamination of animal foods originates from diseased animals, although advances in
health care have nearly eliminated this source. Contamination from chemical sources can occur
through accidental mixing of chemical supplies with foods. Ingredients can contribute to additional
microbial or chemical contamination. Contamination can be reduced through effective housekeeping
and sanitation, protection of food during storage, proper disposal of garbage and litter, and protection
against contact with toxic substances. Microorganisms from various natural sources act as source of
contamination.
1. From green plants and fruits
2. From animals
3. From sewage
4. From soil
5. From water
6. From air
7. During handling and processing.
1. From sewage: - Raw, untreated sewage can contain pathogens that have been eliminated
from the human body, as well as other materials of the environment. Examples are
microorganism causing typhoid and paratyphoid fevers, dysentery, and infectious hepatitis.
Sewage may contaminate food and equipment through faulty plumbing. When untreated
domestic sewage is used to fertilize plant crops, there is a chance that raw plant foods will be
contaminated with human pathogens especially those causing gastrointestinal diseases. The
use of “night soil” as a fertilizer still persists in some parts of the world. In addition to the
pathogens, coliform bacteria, anaerobes, enterococci, other intestinal bacteria and viruses can
contaminate the foods from this source. Natural water contaminated with sewage contributes
their microorganisms to shell fish, fish, and other seafood. If raw sewage drains or flows into
potable water lines, wells, rivers, lakes, and ocean bays, the water and living organisms such
as seafood are contaminated. To prevent this contamination, privies and septic tanks should
be sufficiently separated from wells, streams, and other bodies of water. Raw sewage should
not be applied to fields where fruits and vegetables are grown.
2. From soil: - Soil contains greatest variety of microorganisms. They are ready to contaminate
the surfaces of plants growing on or in them and the surfaces of animals roaming over the

land. Soil dust is whipped up by air currents and soil particles are carried by running water to
get into or onto foods. Soil is an important source of heat resistant spore forming bacteria.
3. From water: - Water serves as a cleaning medium during the cleaning operation and is an
ingredient added in the formulation of various processed foods. It can also serve as a source
of contamination. Natural water contain not only their natural flora but also microorganisms
from soil and possibly from animals or sewage. Surface waters in streams or pools and stored
waters have low microbial content because self-purification of quiet lakes and ponds or of
running water. Ground waters from springs or wells have passed through layers of rock and
soil to a definite level hence most of the bacteria, suspended material have been removed.
Kinds of bacteria in natural waters are chiefly of in Pseudomonas, Chromobacterium,
Proteus, Micrococcus, Bacillus, Streptococcus, Enterobacter and Escherichia coli.
4. From Air: - Contamination can result from airborne microorganisms in food processing,
packaging, storage, and preparation areas. This contamination can result from unclean air
surrounding the food plant or from contamination through improper sanitary practices. The
most effective methods of reducing air contamination are through sanitary practices, filtering
of air entering the food processing and preparation areas, and protection from air by
appropriate packaging techniques and materials. Air does not contain a natural flora of
microorganisms, but accidentally they are present on suspended solid material or in moisture
droplets. Microorganisms get into air on dust or lint, dry soil, spray from stream, lakes or
oceans, droplets of moisture from coughing, sneezing or talking and growth of sporulating
molds on floors, etc.
Microorganisms in air have no opportunity for growth but merely persist there and the
organisms resistant to dessications will live longer. Mold spores because of their small size,
resistance to drying and large numbers of per mold plant are usually present in air. Cocci are
more numerous than rod shaped bacteria. Yeasts especially asporogenous chromogenic ones
are found in most samples of air. Number of microorganisms in air at any given time depend
on factors like amount of movement, sunshine, humidity, location and the amount of
suspended dust or spray. No. of microorganisms vary from mountains to dusty air. Less on
mountains and more in dusty air. Direct rays from the sun kill microorganisms suspended in
air and hence reduce numbers. Dry air contains more organisms than moist air. Rain or snow
removes organisms from the air. Number of microorganisms in air may be reduced under
natural conditions by sedimentation, sunshine and washing by rain or snow.

Filters in ventilating or air conditioning systems prevent the spread of organisms from one
part of a plant to another.
5. During handling and processing: - Additional contamination may come from equipment
coming in contact with foods, from packaging materials and from personnel. A sick person
can pass on germs, ranging from flu to gastroenteritis. A chopping board used for meat that is
not washed and then used for vegetables is another source of possible contamination.
Unwashed hands, dirty kitchen spaces, insects and rodents in the kitchen etc. are all possible
sources of food contamination.
POSTHARVEST CONTAMINATION: - Postharvest contamination is the unintended
contamination of a food product with microorganisms or particles after a processing step has been
used to remove or kill them.
POST-HARVEST/SLAUGHTER CONTAMINATION/ POST PROCESSING CONTAMINATION
Food processing and preparation are both practices used to make a change to a food to alter its eating quality
or shelf life. For example, the milling of wheat into flour or fermentation of maize into ‘ogi‘ is food processing
whereas cooking the wheat in the kitchen or cooking and mixing the ‘ogi‘ with other ingredients are part of
their preparation for consumption. However, the term food processing is broader than preparing and cooking
foods. It involves applying scientific and technological principles to preserve foods, by allowing changes to the
eating quality of foods to be made in a predictable and controlled way. Modern food processing is sometimes
defined as taking place at a plant or factory. This is distinct from food preparation which usually takes place in
kitchens. However, many activities (such as cooking, washing, slicing, peeling, juicing, shredding, mixing etc.)
and use of several equipment are common to both processing and food preparation. Food preparation or
processing, whether in the home, at a restaurant or quick service, other types of food service quality (such as
cafeteria, nursing home or hospital) or food companies or industries can introduce pathogens into a product if
not done properly. This is possible through a process of cross- contamination defined as the physical
movement or transfer of harmful microorganisms or microbial toxins from one food to another either directly
(food to food) or indirectly (equipment/utensils or food contact surfaces to food and people to food).
Pathogenic microorganisms can be found more or less everywhere and consequently may be found in raw
foods, e.g. meat or poultry to be cooked prior to consumption. L. monocytogenes can be transferred from
processing surfaces to foods. Though thorough cooking of such fresh foods will render the small numbers of
microbes harmless, it then constitutes a danger when microbes are spread from raw or contaminated food to
another food, especially ready-to-eat prepared dishes.
Food contact equipment factors: - One important factor of microbial contamination during food processing
and preparation is food contact equipment which may harbor and introduce pathogens into food. Processes
such as trimming, slicing, milling, shredding, peeling mechanical abrasion and various methods of

disintegration if done with contaminated equipment may introduce contaminants from the equipment involved.
When cutting boards used for raw meat, poultry or seafood come in contact with other foods, microorganisms
can be transferred. Another example is slicing ready-to-eat produce, e.g. lettuce with a knife previously used to
cut raw poultry without proper washing and sanitation of the knife in between the processes. Other equipment
such as conveyor belts and aprons, filters, blanchers, presses, screens and wooden surfaces are difficult to
clean and sanitize and therefore are especially likely to be sources of contamination. Listeria monocytogenes
has been found on equipment and process surfaces, which are difficult to clean.
Unhygienic practices of food handlers: - Another avenue through which foods get contaminated during
processing and preparation is infected food handlers and their unhygienic practices. Humans (their skin,
mucous membranes and cuts, open sores or a skin infection) can serve as reservoirs of pathogens, e.g. S.
aureus and from where foods get contaminated if handled under unhygienic condition, especially through
unwashed hands. Direct contamination can happen by contact between the body and the food product. In
indirect contamination, people act as vectors and transfer contamination from one area or surface to another.
When infected food handlers fail to wash their hands after using the toilet, handling raw meat poultry or fish,
taking out garbage, cleaning up spills, handling money, touching other contaminated surfaces such as
blemishes, pimples, the nose, boils, open wounds and soiled tissues, food contamination takes place. Viruses
such as hepatitis A and Norwalk are easily transmitted to shellfish, salads, vegetables and fruits when infected
food handlers fail to wash their infected or contaminated hands. Failure to wear gloves or touching
contaminated surface even while wearing gloves, sneezing or coughing into a gloved hand, food, food contact
equipment or surfaces are ways foods also get contaminated during processing and preparation. Handlers who
do not wear gloves commonly spread Staph bacteria to meat, cream-filled desserts, potato salads and egg
products.
Biofilm formation: - During food processing, microbial communities including pathogens can inhabit or
accumulate on critical places such as food contact and environmental sites on equipment to form biofilms
(microbial cell clusters with a network of internal channels or voids in the extracellular polysaccharide and
glycoprotein matrix, which allows nutrients and oxygen to be transported from the bulk liquid to the cells).
Microbes inhabiting contact and environmental sites in food processing are mostly harmful. The biofilms
contribute to food contamination, especially in foods passing through the same processing line where the
process equipment is not hygienically integrated in the process line or the cleaning and disinfection procedures
are not properly designed to remove the organic soil from the process surfaces. Any microbe can form biofilm
under suitable conditions. However some microbes naturally have a higher tendency to produce biofilm than
others. Foodborne pathogens that readily form biofilms include Bacillus cereus, S. aureus, M.
paratuberculosis, C. perfringens, E. coli 0157:H7, S. typhimurium, C.jejunii, Yersinia enterocolitica and L.
monocytogenes. L. monocytogenes has been found to form biofilms on common food contact surfaces e.g.

plastic, polypropylene, rubber, stainless steel and glass. Also inclusion of decayed parts of foods e.g. fruits
increases the numbers of microorganisms in such food products such as fruits juices.
Use of poor wash water :- Prior to reaching the consumer, most foods, especially fresh produce are washed at
least once and usually several times during processing. Raw water contains bacteria. Most water supplies are
sourced from rivers and lakes which can be contaminated by run-off from land, sewage and slurry. Use of poor
water for washing in food processing and preparation can introduce microorganisms into food.
Packaging factors: - Packaging serves as a major defense against external hazards. However undesirable
interactions between packaging material and food can give rise to potential problems. The risk connected to
packaging material is the potential transfer of food spoilage or pathogenic organisms to the packed food. Most
packaging materials have proven to be completely impervious to microorganisms. A low number of bacteria in
the packaging material could be of concern for e.g. aseptic foods; if bacteria migrate across the package. The
routes of contamination from the packaging material to food include the surface, cutting dust or direct contact
with the raw edge of the paperboard.
Distribution, marketing and storage factors: - These also play important roles in microbial contamination of
food. Contamination can occur via storage in the market in contaminated bins and other containers, possible
contact with decaying products. During food storage in the refrigerator, cross contamination takes place when,
for e.g., juice from raw meat, poultry or fish drips onto vegetables or other ready-to-eat foods on the shelf
below. Inclusion of raw foods with ready to eat foods during shopping or storage leads to cross contamination.
Contamination also occurs when foods are not kept at the right temperature, thus promoting the temperature
danger zone (i.e. the temperature in which bacteria and such can be the most widely spread). This includes the
trip home from the store, the time the food is on the table and the time you have it in your bag for lunch at
work. In addition, handling by sales people or customers and spraying with contaminated water (for fruits and
vegetables) are also important factors. The spraying gives a fresh appearance to the vegetables but also adds
organisms. Inclusion of decayed food part during distribution, marketing and storage increases the number of
microorganisms in foods.
FACTORS INFLUENCING GROWTH OF MICROORGANISM ON FOODS, INTRINSIC
FACTORS AND EXTRINSIC FACTORS.
Foods are mainly composed of biochemical compounds which are derived from plants and animals.
Carbohydrates, proteins and fats are the major constituents of food. In addition, minor constituents
such as minerals, vitamins, enzymes, acids, antioxidants, pigments, flavors are present. Foods are
subject to physical, chemical, and biological deterioration. The factors that affect microbial growth
in foods, and determine the nature of spoilage and any health risks can be cauterized in four groups.
The major factors affecting microbial growth in foods are as follows:-

Factors affecting the development of microbial associations in food
1. Nutrients
2. pH and buffering capacity
1 Intrinsic Factors
3.
4.
Redox potential
Water activity
5. Antimicrobial constituents
6. Antimicrobial structures
Food Microbiology
Page 19 of28
1. Intrinsic Factors (Physico-chemical properties of the food itself)
2. Extrinsic Factors (conditions of the storage environment)
3. Implicit Factors (properties and interactions of the microorganisms present); and
4. Processing Factors.
2
Extrinsic Factors /Environmental
factors
1. Relative humidity
2. Temperature
3. Gaseous atmosphere
3 Implicit factors
1. Specific growth rate
2. Mutualism
3. Antagonism
4. Commensalism
4 Processing factors
1. Slicing
2. Washing
3. Packing
4. Irradiation
5. Pasteurization
Extrinsic Factors /Environmental factors That Influence Microbial Growth: - Extrinsic factors
are external to the food. Temperature and gas composition are the main extrinsic factors influencing
microbial growth. The influence of temperature on microbial growth and physiology is huge. While
the influence of temperature on the growth rate is obvious and is covered in some detail here, the
influence of temperature on gene expression is equally important. Cells grown at refrigeration
temperature do not just grow more slowly than those grown at room temperature, they express
different genes and are physiologically different. A rule of thumb in chemistry suggests that reaction
rates double with every 10°C increase in temperature. This simplifying assumption is valid for
bacterial growth rates only over a limited range of temperatures.
Extrinsic Factors/ Environmental factors:- The main extrinsic parameters influence the foods
are:
1. Relative Humidity: - Humidity is the concentration of water vapour in the atmosphere. Relative
humidity is the ratio expressed as the percentage of moisture in air to the moisture present in food
under the saturation condition at temperature and pressure. Relative humidity and water activity
are inter-related i.e. when food with low water activity are stored in the environment ofhigh

humidity, water will transfer from gas phase (air) to the food and thus increases water activity of
food, leading to spoilage by microorganisms. Microorganisms also produced water as an in
product of respiration (metabolism).Thus, they increases the water activity (aw) of their own
immediate environment. So, that eventually microorganisms requiring high water activity are able
to grow and spoiled food, which was initially consider to be microbiological safe. The storage of
fresh fruits and vegetables required very careful control of relative humidity. If it is too low than
many vegetables will lose water and become flaccid or shrinkage. If it is too high condensation
may occurs. Hence, RH should be chosen such that there will be neither condensation nor
desiccation i.e. food and environment should be in equilibrium state. There is also relationship
between temperature and humidity .i.e. higher the temperature lower will be the RH and vice
versa. The highest humidity near saturation is required for bacterial growth, less by yeast and even
less by mold. Food that undergo surface spoilage from mold, yeast and certain bacteria should be
stored under condition of low relative humidity (RH). Improperly wrapped food (such as meat
item) suffer from surface spoilage in the refrigerator due to higher RH.
2. Temperature: - Microbes have an optimum temperature as well as minimum and maximum
temperatures for growth. These are known as cardinal temperatures and are, to a large extent,
characteristic of an organism, although they are influenced by other environmental factors such as
nutrient availability, pH and aw. Therefore, the environmental temperature determines not only the
proliferation rate but also the genera of micro-organisms that will thrive and the extent of
microbial activity that occurs. For example, a change of only a few degrees in temperature may
favor the growth of entirely different organisms and result in a different type of food spoilage and
food poisoning. These characteristics have been responsible for the use of temperature as a
method of controlling microbial activity. The optimal temperature for the proliferation of most
micro-organisms is from 15ºC to 40ºC. However, many genera of microbes are capable of growth
from 0ºC to 15ºC and other even micro-organisms will grow at subzero temperatures. Still other
genera will grow at temperatures up to and exceeding 100ºC. Micro-organisms can be classified
into several physiological groups based on their cardinal temperatures (according to temperature
of optimal growth include):-
A. Thermophiles (high-temperature-loving micro-organisms), with growth optima at
temperatures above 45ºC (e.g., Bacillus stearothermophilus, Bacillus coagulans,
and Lactobacillus thermophilus).
B. Mesophiles (medium-temperature-loving micro-organisms), with growth optima
between 20º and 45ºC (e.g., most Bactobacilli and Staphylococci).

C. Psychrotrophs (cold-temperature-tolerant micro-organisms), which tolerate and
thrive at temperatures below 20ºC (e.g., Pseudomonas and Acinetobacter).
Cardinal Temperatures for Microbial Growth
Bacteria, molds and yeasts each have some genera with temperature optima in the range
characteristic of thermophiles, mesophiles, and psychrotrophs. Molds and yeasts tend to be less
thermophilic than bacteria. As temperature approaches 0ºC, fewer micro-organisms can thrive and
their proliferation is slower. As temperature falls below approximately 5ºC, proliferation of spoilage
micro-organisms is retarded as the growth of nearly all pathogens ceases.
3. Gaseous Atmosphere: - Presence of different gases and its concentration may significantly affects
the growth of microorganisms on the food i.e. surface spoilage is prevented by altering gaseous
composition. Oxygen is one of the most important gas which affects both food products as well as
microorganisms. Oxygen gas when come in contact with food, influence redox potential of food
and finally the microbial growth. Carbon dioxide (CO2) is the one of most important atmospheric
gas that is used to control microorganisms in foods. The storage of food in atmosphere containing
increased amount of co up to 10% is referred as "controlled atmospheric storage"(CAS). The
inhibitor effects of co on microbial growth is applied in modified atmospheric packing of food
and food drinks. Mold and oxidative gram negative bacteria are more sensitive and gram positive
bacteria like Lactobacillus tends to more resistant to CO2. The inhibitory effects of CO2 increases
with decreases with decrease in temperature. This is because CO2 is highly soluble at lower
temperature. Some microorganisms are killed by prolong exposure to CO2. CO2 when come in
contact with water, form weak acid i.e. H2CO3 (carbonic acid). H2CO3 penetrate the plasma
membrane and acidity the cells interior. Ozone (O3) is the other atmospheric gas that has
antimicrobial properties, and it has been tried over a number of decades as an agent to extend the
shelf life of certain foods. It has been shown to be effective against a variety of microorganisms,
but because it is a strong oxidizing agent, it should not be used on high-lipid-

content foods since it would cause an increase in rancidity. Ozone added to food as a preservative
action on certain food. Ozone has given GRAS (generally recognized as safe) status in US,
effective range is 1-5 ppm. However, it has some demerits like strong oxidizing agents, causes the
rancidity of high lipid containing food. Moulds are aerobic, most yeast grows best aerobically,
and bacteria of different kinds may be aerobic, anaerobic or facultative. The bacteria which have
the ability to thrive in the absence of oxygen or free air are known as 'strict' or 'obligate
anaerobes'. These organisms die when exposed to air or oxygen. But there are only few obligate
anaerobes. Many bacteria are referred thus, because they tolerate extremely low levels of oxygen.
There is another category of bacteria, which can survive either in the presence or absence of
oxygen. They are known as “facultative anaerobes” The other kinds of bacteria which cannot
survive in the absence of oxygen are called 'obligate aerobes'. There is yet another category of
bacteria known as 'microaerophiles', which require oxygen 'oxygen for survival albeit at low
concentrations than present in air.
Intrinsic Factors That Influence Microbial Growth: - Characteristics of the food itself are called
intrinsic factors. These include naturally occurring compounds that influence microbial growth,
compounds added as preservatives, the oxidation-reduction potential, water activity, and pH.
1. Nutrients
2. pH and buffering capacity
3. Redox potential
4. Water activity
5. Antimicrobial constituents
6. Antimicrobial structures
1. Nutrients: - The kinds and proportional of nutrient in food are all important in determining
which microorganism is most likely to grow. Microorganisms can use foods as a source
of nutrients and energy. From them, they derive the chemical elements that constitute
microbial biomass, those molecules essential for growth that the organism cannot
synthesize, and a substrate that can be used as an energy source. The inability of an
organism to utilize a major component of a food material will limit its growth and put it
at a competitive disadvantage compared with those that can. Therefore, the ability to
synthesize Amylolytic (starch degrading) enzymes will favor the growth of an organism
on cereals and other farinaceous products. Protein-rich food like meat, egg, fish etc. are
always spoiled by proteolytic organism because they can utilize protein as a source of
energy if sugar is not available. In fact, protein-rich food promotes more growth of

bacteria then yeast and mould. Similarly, in the general mould can grow in the higher
concentration of sugar, yeast in fairly high concentration but most bacteria grow best in
the low concentration of sugar.
pH and buffering capacity: - Every organism has a minimal, maximal and optimal pH
for growth. Some organism can grow better at low pH or acidic pH, some can grow in
alkaline pH and while other grow at somewhat neutral pH. pH influence both the growth
rate and types of organism that will predominant the food. In general yeast and mould are
more acid tolerant than bacteria. The acidity and alkalinity of an environment affects
growth and metabolism of microorganisms as the activity and stability of
macromolecules, enzymes and nutrient transport is influenced by pH. Generally, bacteria
grow fast at pH 6-8. But bacteria that produce acids have optimum pH between pH 5 and
6 (Ex: Lactobacillus and Acetic acid bacteria). Yeast grows best at pH 4.5-6.0 and Fungi
at 3.5 – 4.0. In low pH foods (Ex. Fruits), spoilage is mainly by yeasts and fungi than
bacteria. Fishes with pH around neutrality (6.5-7.5) favor bacterial growth and spoil
rapidly than meat (pH: 5.5 – 6.5). Ability of low pH to restrict microbial growth has been
employed as a method of food preservation (Ex: use of acetic and lactic acid). Buffering
capacity refers to the ability of foods to withstand pH changes. Microorganisms have
ability to change pH of the surrounding environment to their optimal level by their
metabolic activity. Decarboxylation of amino acids releases amines which increases
surrounding pH. Deamination of amino acids by enzyme deaminizes release organic
acids causing decease in pH. Thus, protein rich foods like fish and meat have better
buffering capacity than carbohydrate rich foods.
Redox potential (also known as redox potential, oxidation / reduction potential, ORP, pe, ε,
or Eh): - The oxidation – reduction (redox) potential is an indication of the oxidizing and
reducing power of the substrate. To attain optimal growth, some microorganisms require
reduced conditions while others need oxidized conditions. Thus, the importance of the
oxidation-reduction potential is apparent. Aerobic micro-organisms grow more readily
under a high oxidation – reduction potential (oxidizing reactivity). A low potential
(reducing reactivity) favors the growth of anaerobes. Facultative microorganisms are
capable of growth under either condition. Micro-organisms can alter the oxidation-
reduction potential of food to the extent that the activity of other microorganisms is
restricted. For example, anaerobes can decrease the oxidation – reduction potential to
such a low level that the growth of aerobes can be inhibited.
2.
3.

Redox or oxidoreduction potential is defined as the sum of all the oxidizing
(dissolved oxygen, free radicals, hydrogen peroxide, some oxidized metal ions…) and
reducing (some vitamins, some reduced metal ions, thiol-containing molecules,
hydrogen…) couples found in the medium. This means when the concentration of
oxidizing molecules increases the redox potential value of the medium increases
(determined in millivolts or volts) but when the concentration of the reducing molecule
increases the redox value of the medium decreases. For simplifying the term of redox, we
can say that it expresses the concentration of mobile electrons (that can move from
molecule to another) in the medium, whereas the pH value expresses the concentration of
protons (H+) in the medium.
How can the redox potential affect the growth of microorganisms?
To respond to this question we must keep in the head the relationship between the
oxygen as oxidant found in the atmosphere and the redox potential. When a
microorganism is found in aerobic medium (containing dissolved oxygen) it means this
medium is favorable for the growth of aerobic microorganisms which can use the oxygen
as a final acceptor of electrons produced from the substrate via the metabolism.
It is important to indicate here also that every microorganism possesses a favorable
value of redox for growth (this behavior likes that found for pH, water activity and
temperature; and a microorganism found in a medium “swims” to a location with a
favorable redox value. Furthermore, it was reported that the redox potential value affects
the pH intracellular of a microorganism, which forms a component of the proton-motive
force. This effect of the redox on the pH intracellular was attributed to the change of the
permeability of the cellular membrane, which increased by the decrease of the redox
value of the medium. Other studies reported the effect of redox potential on the surface
properties of the cellular membrane such as the hydrophobicity and the character of
donor/acceptor of electrons. The ability of redox potential of the medium on the toxin
production capacity and spore germination in some microbial strains was also reported. It
is important here to remind that the bacterial membranes have an essential function in
energy conservation as the location about which a proton motive force is generated. This
proton motive force (also called electrochemical gradient) forms a source of energy for
bacteria.
We can summarize the mechanism of the effect of redox potential on the growth of
microorganisms as follows:

4.
1) Its effect on the structural composition of some sensitive-components/molecules
found on the surface of the cell. These redox-sensitive components/molecules could be
enzymes located in the surface of the cell, which its protein part (Apoenzyme) could be
composed of sulfur-containing amino acids making it sensitive to the redox potential of
the medium. Furthermore, the cofactors of these enzymes such as Fe, Zn, Mg and Cu
could be found in oxidized or reduced form, which means the susceptibility of these
metal ions to the redox potential of the substrate or/and the medium.
By the same manner, we can discuss the effect of redox potential on several redox-
sensitive molecules located in the surface of the cell such as proteins, phospholipids,
saturated and unsaturated fatty acids, which could be affected directly by the redox
potential of the medium. The modification in the structural or the composition of these
molecules affects different cellular systems such as the transport and energetic ones.
2) If the energetic system of the cell (proton motive force) changes the ATP content of
the cell will change which can affect many essential functions of the cell.
Moisture content or water activity (aw):- Microorganism has an absolute demand for
water, however, the exact amount of water needed for growth of microorganisms varies.
This parameter helps us to understand the movement of water from the environment to
the cytoplasm or from cytoplasm to the environment. The water requirement of
microorganisms is expressed in physical form, called water activity (aw).
Water activity is the ratio of the vapour pressure of water present in food substrate (solution) to
the vapour pressure of pure water at the same temperature. The unit of measurement for
water requirement of microorganism is usually expressed as water activity (aw).
Bacteria generally require higher value of aw than fungi
G – ve bacteria require higher aw than G +ve bacteria
Most spoilage bacteria do not grow at aw below 0.91
Spoilage molds grow at aw of 0.80
Halophilic bacteria grow at aw of 0.75
Xerophilic and osmophilic yeasts grow at of 0.61
Microorganisms like halophiles, osmophiles and xerophiles grow better at reduced
aw. Microorganisms cannot grow below aw 0.60, and in such situations spoilage
of food is not microbiological but due to chemical reactions (Ex: oxidation).
Relationship between aw, temperature and nutrition

Growth of microorganisms decreases with lowering of aw
The range of aw at which the growth is greatest occurs at optimum temperature for growth
The presence of nutrients increases the range of aw over which the organisms can survive
Antimicrobial constituents: - The stability of some foods against attack by
microorganisms is due to the presence of certain naturally occurring substances that have
been shown to have antimicrobial activity. Some species are known to contain essential
oils that possess antimicrobial activity. Among these are eugenol in cloves, allicin in
garlic, cinnamic aldehyde and eugenol in cinnamon, allyl isothiocyanate in mustard,
eugenol and thymol in sage, and carvacrol (isothymol) and thymol in oregano. Cow's
milk contains several antimicrobial substances, including lactoferrin, conglutinin, and the
lactoperoxidase system (see below). Raw milk has been reported to contain a rotavirus
inhibitor that can inhibit up to 106
pfu (plaqueforming units)/mL. It is destroyed by
pasteurization. Milk casein as well as some free fatty acids have been shown to be
antimicrobial under certain conditions. Eggs contain lysozyme, as does milk, and this
enzyme, along with conalbumin, provides fresh eggs with a fairly efficient antimicrobial
system. The hydroxycinnamic acid derivatives (p-coumaric, ferulic, caffeic, and
chlorogenic acids) found in fruits, vegetables, tea, molasses, and other plant sources all
show antibacterial and some antifungal activity. Lactoferrin is an ironbinding
glycoprotein that is inhibitory to a number of foodborne bacteria. Its antimicrobial
activity is antagonized by citrate. Ovotransferrin appears to be the inhibitory substance in
raw egg white that inhibits Salmonella enteritidis. Cell vacuoles of cruciferous plants
(cabbage, Brussels sprouts, broccoli, turnips, etc.) contain glucosinolates, which upon
injury or mechanical disruption, yield isothiocyanates.
Antimicrobial structures: - The natural covering of some foods provides excellent
protection against the entry and subsequent damage by spoilage organisms. In this
category are such structures as the testa of seeds, the outer covering of fruits, the shell of
nuts, the hide of animals, and the shells of eggs. In the case of nuts such as pecans and
walnuts, the shell or covering is sufficient to prevent the entry of all organisms. Once
cracked, of course, nutmeats are subject to spoilage by molds. The outer shell and
membranes of eggs, if intact, prevent the entry of nearly all microorganisms when stored
under the proper conditions of humidity and temperature. Fruits and vegetables with
damaged covering undergo spoilage much faster than those not damaged. The skin
covering offish and meats such as beef and pork prevents the contamination and spoilage
5.
6.

of these foods, partly because it tends to dry out faster than freshly cut surfaces. These six
intrinsic parameters (pH, Moisture content, Oxidation-reduction potential (Eh),
Nutrient content, Antimicrobial constituents, Biological structures) represent nature's
way of preserving plant and animal tissues from microorganisms.
Implicit factors That Influence Microbial Growth (Physiological properties of
microorganisms):- The implicit factors are the factors related to the microorganisms themselves,
including interactions between the microorganisms contaminating the food and between these
microorganisms and the food.
1. Specific growth rate:-
2. Mutualism: - Mutualism is defined as an interaction between individuals of different species
that results in positive (beneficial) effects on per capita reproduction and/or survival of the
interacting populations. Or, Mutualism is an interaction between two populations that
positively affects the fitness of both populations, where the fitness of a population is defined
as the success of a population in propagating its genetic material. Digestive bacteria and
humans - Human beings have what are often called "good" bacteria in their digestive
systems. This "good" bacteria exists in order to help the human to digest food.
3. Antagonism: - Antagonism, in ecology, an association between organisms in which one
benefits at the expense of the other.
4. Commensalism: - Commensalism benefits the symbiont without significantly affecting the
host. This is a relatively rare type of interaction because few hosts can be considered to be
completely unaffected by their symbionts. In commensalism, one organism benefits while the
other is unaffected. For example, one organism can provide an essential growth factor, such
as a vitamin, for another organism. This type of cross-feeding is common in soil organisms.
The opposite of commensalism is amensalism, where one organism is harmed while the other
is unaffected. A good example of this interaction is when one organism produces an
antibiotic against another organism. Such an interaction is often the basis of biological
control. For example, some isolates of the bacterium Pseudomonas fluorescens can suppress
the fungal pathogen Gaeumannomyces graminis, responsible for ‘take-all’ in wheat
(Triticum aestivum).
Microbial Antagonisms: - Some microorganisms can inhibit other microorganisms or reduce their
growth in medium thanks to their metabolites, indirect (by changing pH, osmotic pressure and
surface tension) or direct (by producing toxic component, antimicrobial component,bacteriocin,

antibiotic etc.), this situation is called as antagonistic relation. For example, organic acids produced
by microorganisms have antimicrobial effect on sensitive microorganisms to acids. Also some
bacteria prevent growth of some other bacteria in media by producing metabolite called as
bacteriocin. The most intensive studies and commercial practices about microbial antagonisms have
focused on lactic acid bacteria. In addition to lactic acid bacteria, it was reported that probiotics,
yeasts, fungus and bacteriophages have antagonistic characteristics.

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