Unit 5: Microbiology of Foods
Food Microbiology: the study of microorganisms in foods: bacteria, yeasts, molds,
protozoa and viruses.
Microorganisms: living entities that are too small to be seen with the naked eye.
o Organisms belonging to the same species share many phenotypic
characteristics as well as being genetically similar (>70% similarity in
Gram +/ ;
• Gram +ve have a thick cell wall and no outer membrane
• Gram –ve have a thin cell wall and an outer membrane
• Spherical coccus (ex. Staphylococcus aureus).
Diplococcus arrangements also occur, where two cells
remain attached after division.
• Rodshaped (ex. Escherichia coli)
• Spirillum (ex. Campylobacter is a curvedrod shaped
bacterium, which is a variation of the spirillum).
Spores: some species can develop spores by coating their
membrane and cell wall with extra layers in a process known as
sporulation. This occurs in response to unfavorable growth
conditions (lack of nutrients/water etc.). In this state, cells are
unable to germinate. When conditions become favorable spores
will germinate into vegetative cells.
o Fungi : includes molds and yeasts
Molds : multi or unicellular organisms, found in decaying organic
• They grow in the form of mycelium (a tangled mass that
spreads quickly). Mycelium is composed of filaments
called hyphae: nucleated tubes containing cytoplasm and
• Conidia: important in foods. Nonmotile spores found on
top of hyphae (make molds look dusty).
• Do not grow in the form of mycelium. Grow as single cells
that are spherical or ovalshaped = greater surface areato
volume of medium, therefore cells distribute and grow
faster than molds.
1 o Protozoa: (Parasites, ex. Giardia lamblia)
Classified by morphology, locomotion, and life cycle
They do not grow in food. Instead they require at least one animal
host to grow.
Most are phagotrophic; able to ingest particulate food.
Some species (ex. Giardia lamblia) are ingested in the form of
cysts. They germinate to yield trophozoites (motile parasites) that
migrate to the small intestine.
Obligate parasites and hostspecific
Composed of a protinaceous capsule surrounding nucleic acid.
RNA viruses are typically associated with food. Most food viruses
are temperate viruses, which insert their nucleic acid into the
host’s DNA, leaving the host cell intact.
Factors affecting microbial growth
o Chemotrophic organisms require chemicals for metabolism. They can be
either lithotrophs (require inorganic compounds, ex. minerals) or
organotrophs (require organic compounds, ex. carbohydrates)
Foodborne organisms are chemotrophic.
o Phototrophic organisms require energy in the form of light
o The organisms that predominate in food are those that can utilize the food
Ex. Pectinolyic organisms occur in pectincontaining foods
o Some organisms require the presence of other organisms in order to grow
in that environment.
Ex. bacteria digest food components into low molecular weight
compounds that can be used as a nutrient source by other
o Bacteria require water to survive.
o The amount of unbound water, not involved in chemical reactions with
food components, that is available for microbial growth is termed water
activity, or w .
o Most bacteria require an a of 0.90, with 1.0 being the maximum possible.
(Exception: S. aureus requires 0.83)
At low aw i.e. high solute outside cell), bacteria die because water
inside the cell diffuses out to balance osmotic pressure = cell death
due to dehydration shrinkage
2 o Most yeasts require an a ofw0.80 and molds require 0.70. (Exception:
Saccharomyces cervisiae – a yeast, requires 0.90)
o Most bacteria live between pH 4.59.0, with optimum around 6.57.5
o Yeasts can live between pH 3.08.5, and can therefore survive in
environments where other bacteria cannot.
o Molds can live between pH 2.011.0
o The pH level alters a microorganism’s ability to transport molecules
into/out of the cell across the membrane.
In acidic environment (low pH): protons (H ) saturate the
membrane, making it difficult for cations to move in/out.
In basic environment (high pH): hydroxide (OH ) ions saturate the
membrane, making it difficult for anions to move in/out.
o Proteins are also denatured in extreme pH
o When referring to oxygen content of a medium, we are really talking
about the oxidationreduction potential of that medium (redox potential)
This redox potential depends on the ratio of total oxidizing
(electronaccepting) molecules to the total reducing (electron
donating) molecules in the medium
o An oxidized environment means that the molecules have a relatively high
affinity for electrons
o A reduced environment means that the molecules have a relatively low
affinity for electrons
o Aerobes require oxidized conditions, thus oxygen, to be present
Bacteria that cause food spoilage are notably strict aerobes
Aerobic microorganisms require oxygen to produce adenosine
triphosphate (ATP), and will therefore die without oxygen.
o Most bacteria that cause disease are facultative anaerobes, as they prefer
an aerobic environment but can grow without oxygen (i.e. in a reduced
o Microaerophilic bacteria require some oxygen, but cannot tolerate levels
present in aerobic environments. Ex. Campylobacter requires 6% oxygen.
o Strict anaerobes cannot tolerate any oxygen (require a completely
Anaerobes cannot remove oxygenderived radicals (O supero2ide
radical) from the cell. These radicals disturb metabolism by
replacing essential molecules in chemical reactions. Anaerobes will
therefore die in the presence of oxygen.
3 o The maximum growth temperature is the temperature that causes
inactivation of an organism’s enzymes and structure damage to the extent
that these outbalance the enhanced ability to synthesize new cell material
o Optimum growth temperature is the temperature corresponding to the
shortest generation time (time for cell division). Usually minutes.
o Minimum growth temperature is the temperature corresponding to the
longest generation time. Usually 1000 minutes +.
o Psychrophiles prefer low temperatures
o Psychrotrophs prefer higher temperatures, but can grow at low
o Thermophiles prefer high temperatures
o Thermotrophs tolerate high temperatures, but don’t prefer them.
o Mesophiles can tolerate extreme temperatures, but prefer the levels found
in the tissues of humans/animals. Most diseasecausing organisms are
o Cell death occurs at low temperatures because reaction rates of
metabolism slow and because there is a decrease in cell membrane
fluidity, which slows down transport of nutrients into the cell.
o Cell death occurs at low temperatures because of inactivation of enzymes
and denaturation of cell structural components.
o Foods can have different levels of antimicrobial substance (ex.
phosphates), which can affect microbial growth
o Foods with high protein content exert a buffering effect, such that
microorganisms are able to live and grow in the food, even if the pH is
below the minimum level necessary for survival.
o The poising effect refers to the fact that some characteristics of food can
prevent a change in the redox potential, despite the oxygen content of the
atmosphere in which it is packaged.
o The more alkali the pH, the more negative the redox potential, thus the
food is more reduced and anaerobic.
o Oxygenation of the product can differ depending on whether the product
has to be agitated (difference in redox potential)
o Highprotein and highfat foods can have an insulating/shielding effect,
which enables organisms to live in environments that are designed to
inhibit their growth.
o Some bacteria can switch their metabolism to synthesize more unsaturated
fatty acids/shortchain fatty acids, which may allow for survival at lower
temperatures (strengthens their cell membrane to protect against damage
from ice crystals).
o Some bacteria can synthesize enzymes that function at low temperatures
The hurdle concept: lowlevel hurdles are combined to prevent microbial growth
4 o Ex. in fermented dairy products; low pH from lactic acid is combined with
Sources of microorganisms:
Splashing of water containing microorganisms against surfaces forms aerosols,
causing microbial cells to become airborne.
o The more shallow a body of water, the more contamination it has because
oxygen from the atmosphere is readily available for microbial growth.
o Bacteria degrade excess organic material exerted by algae and remains
from dead animals. Marine bacteria tolerate the salty environment.
o Most bacteria in the soil are spore formers because of the poor nutrient
content of soil.
Microbial flora of foods originate from
o Microorganisms associated with the raw food
Contact with air, water or soil
o Microorganisms acquired during handling/processing
Humans harbor microorganisms on their skin, hair, mucous
membranes, and intestines as well as on their clothing.
Utensils/equipment can carry contaminants
o Microorganisms that survived the preservation/storage tr