Chapter 34
Microbiology of Food
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Intrinsic and Extrinsic Factors
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Intrinsic Factors
•
•
•
•
food composition
pH
presence and availability of water
oxidation-reduction potential
– altered by cooking
• physical structure
• presence of antimicrobial substances
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• putrefaction
– proteolysis and anaerobic breakdown of proteins,
yielding foul-smelling amine compounds, such as
cadaverine and putrescine
• pH impacts make up of microbial community
and therefore types of chemical reactions that
occur when microbes grow in food
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Water availability
• measured in terms of water activity (aw)
• in general, lower water activity inhibits
microbial growth
• water activity lowered by:
– drying
– addition of salt or sugar
• osmophilic microorganisms
– prefer high osmotic pressure
• xerophilic microorganisms prefer low aw
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6
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Physical structure
• grinding and mixing increase surface
area and distribute microbes
– promotes microbial growth
• outer skin of vegetables and fruits slows
microbial growth
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Antimicrobial Substances
• coumarins: fruits and vegetables
• lysozyme: cow’s milk and eggs
• aldehydic and phenolic compounds:
herbs and spices
• allicin: garlic
• polyphenols: green and black teas
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Extrinsic Factors
• temperature
– lower temperatures retard microbial growth
• relative humidity
– higher levels promote microbial growth
• atmosphere
– oxygen promotes growth
– modified atmosphere packaging (MAP)
• use of shrink wrap and vacuum technologies to
package food in controlled atmospheres
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Microbial Growth and Food Spoilage
• food spoilage
– results from growth of microbes in food
• alters food visibly and in other ways, rendering it
unsuitable for consumption
– involves predictable succession of microbes
– different foods undergo different types of
spoilage processes
– toxins are sometimes produced
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Toxins
* ergotism
– toxic condition caused by growth of a fungus
in grains
• aflatoxins
– carcinogens produced in fungus-infected
grains and nut products
• fumonisins
– carcinogens produced in fungus-infected
corn
• algal toxins
– contaminate fish and shellfish
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Aflatoxins
Intercalate into DNA,
causing frameshift mutations
Four basic structures
- B1, B2, G1, and G2
M aflatoxins in the milk of
lactating animals that have
ingested type B aflatoxins
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Basic structure of fumonisins
Disrupt synthesis and metabolism
of sphingolipids
FB1, R = OH; FB2, R = H
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Controlling Food Spoilage
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Removal of Microorganisms
• usually achieved by filtration
• commonly used for water, beer, wine,
juices, soft drinks, and other liquids
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Low Temperature
• refrigeration at 5°C retards but does
not stop microbial growth
– microorganisms can still cause spoilage
– growth at temperatures below -10°C
has been observed
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High Temperature
• canning
• Pasteurization
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Canning
• food heated in special
containers (retorts) to
115°C for 25 to 100
minutes
• kills spoilage
microbes, but not
necessarily all
microbes in food
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Spoilage of Canned Goods
• spoilage prior to canning
• underprocessing
• leakage of contaminated water into
cans during cooling process
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Pasteurization
• kills pathogens and substantially
reduces number of spoilage
organisms
• different pasteurization procedures
heat for different lengths of time
– shorter heating times result in
improved flavor
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Water Availability
• dehydration
– e.g., lyophilization to produce freezedried foods is commonly used to
eliminate bacterial growth
– food preservation occurs as a result of
free-water loss and an increase in
solute concentration
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Chemical-Based
Preservation
• GRAS
– chemical agents “generally recognized
as safe”
• pH of food impacts effectiveness of
chemical preservative
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Radiation
• ultraviolet (UV) radiation
– used for surfaces of food-handling equipment
– does not penetrate foods
• radappertization
– use of ionizing radiation (gamma radiation) to extend
shelf life or sterilize meat, seafoods, fruits, and vegetables
– kills microbes in moist foods by producing peroxides
from water
• peroxides oxidize cellular constituents
• electron beams
– electrons are electrically generated, so can be turned on
only when needed
– does not generate radioactive waste, but also does not
penetrate foods as deeply as gamma radiation
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Microbial Product-Based
Inhibition
• bacteriocins
–
–
–
–
–
bactericidal proteins active against related species
some dissipate proton motive force of susceptible bacteria
some form pores in plasma membranes
some inhibit protein or RNA synthesis
e.g., nisin
• used in low-acid foods to inactivate Clostridium botulinum during
canning process
• bacteriophages
– in 2006, bacteriophages that kill Listeria monocytogenes were
approved for application to ready-to-eat meats
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Food-Borne Diseases
• two primary types
– food-borne infections
– food intoxications
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Food-Borne Infection
• ingestion of microbes, followed by
growth, tissue invasion, and/or
release of toxins
• raw foods (e.g., sprouts, raspberries,
and seafood) are important sources
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Food Borne Infections
• Salmonellosis (S. typhimurium, S. enterica, S. typhi)
– gastroenteritis from ingestion of contaminated meats,
poultry or eggs
• Campylobacter jejuni
– transmitted by uncooked or poorly cooked poultry products,
or raw milk
– gastroenteritis
• Listeriosis (L. monocytogenes)
– pregnant women, the young and old and
immunocompromised individuals most vulnerable
– responsible for the largest meat recall in U.S.
– at risk people should not eat soft cheeses, refrigerated
smoked meats, deli meats and undercooked hot dogs
– fever, muscle aches, vomitting, nausea, diarrhea
– menigitis
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Food Borne Infections…
• Escherichia coli
– diarrhea caused by enteropathogenic,
enteroinvasive and enterotoxigenic
types
– E. coli 0157:H7 is thought to have
acquired enterohemorrhagic genes
from Shigella, including genes for the
shigalike toxin
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Food Borne Infections…
• other organisms
– viruses
– protozoan pathogens
– prions
• other concerns
– foods that are transported and consumed in
uncooked state
• sprouts
• shellfish and finfish
• raspberries
33
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Food-Borne Intoxications
• ingestion of toxins in foods in which
microbes have grown
• include staphylococcal food
poisoning, botulism, Clostridium
perfringens food poisoning, and
Bacillus cereus food poisoning
34
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Detection of Food-Borne Pathogens
• must be rapid and sensitive
• methods include:
– culture techniques – may be too slow
– immunological techniques - very sensitive
– molecular techniques
• probes used to detect specific DNA or RNA
• sensitive and specific
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Molecular Probes and Food Microbiology
Autoradiogram of a radioactively-labeled
Listeria monocytogenes DNA probe
against 100 Listeria spp. cultures.
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PCR-based Pathogen Detedtion
Comparison of PCR and
growth for detection of
Salmonella
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Surveillance for food-borne
disease…
• PulseNet
– established by Centers for Disease Control
and Prevention, USA
– uses pulsed-field gel electrophoresis under
carefully controlled and duplicated
conditions to determine distinctive DNA
pattern of each bacterial pathogen
– enables public health officials to link
pathogens associated with disease outbreaks
in different parts of the world to a specific
food source
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Surveillance…
• FoodNet
– active surveillance network used to follow
nine major food-borne diseases
– enables public health officials to rapidly
trace the course and cause of infection in
days rather than weeks
39
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Microbiology of Fermented Foods
• major fermentations
- lactic, propionic, and ethanolic fermentations
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Fermented Milks
• majority of fermented milk products rely
on lactic acid bacteria belonging to the
genera Lactobacillus, Lactococcus,
Leuconostoc, and Streptococcus
– gram-positives that tolerate acidic conditions,
are non-spore forming, and are aerotolerant
with a strictly fermentative metabolism
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Insert Table 34.5
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Lactobacillus helveticus
Lactobacillus delbrueckii
sub. bulgaricus
Lactobacillus lactis
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Fermented Milks…
• mesophilic
– Lactobacillus and Lactococcus
– buttermilk and sour cream
• thermophilic
– Lactobacillus and Streptococcus
– yogurt
• probiotics
– Lactobacillus and Bifidobacterium
– addition of microbes to the diet to improve
health beyond basic nutritive value (see later
slides)
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A light microscope of Bifidobacterium sp.
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Fermented Milks…
• yeast-lactic fermentation
– yeasts, lactic acid bacteria, and acetic
acid bacteria combine to produce kefir,
a product with an ethanol content up
to 2%
• mold-lactic fermentation
– filamentous fungi and lactic acid
bacteria combine to produce vili, a
product with a velvety top layer
46
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Cheese Production
milk
lactic acid bacteria and rennin
curd
removal of whey 
ripening by microbial action 
cheese
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Figure 34.12
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Fermented Meat and Fish
•
•
•
•
•
sausages
hams
bologna
salami
izushi – fish, rice, and vegetables
(Lactobacillus spp.)
• katsuobushi – tuna (Aspergillus glaucus)
50
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Production of Alcoholic Beverages
• begins with formation of liquid
containing carbohydrates in readily
fermentable form
– must
• juice from crushed grapes
– mashing
• hydrolysis of complex carbohydrates in cereals
by addition of water and incubation
• yields wort – clear liquid containing fermentable
carbohydrates
51
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Wine making
or
racking – removes sediments
Wine vinegar: produced from microbial oxidation of ethanol to acetic acid
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Beers and ales
• malt
– germinated barley grains having activated
enzymes
• mash
– the malt after being mixed with water in
order to hydrolyze starch to usable
carbohydrates
• mash heated with hops
– hops provide flavor and assist in clarification
of wort
– heating inactivates hydrolytic enzymes
53
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Beers and ales…
• wort is then inoculated (pitched) with desired
yeast
– bottom yeasts (e.g. Saccharomyces pastorianus,
S. carlsbergensis)
• used in production of lager beers
• Fermentation: 8-14 days at 6-12oC
• Aged several weeks at -1oC
– top yeasts (Saccharomyces cerevisiae)
• used in production of ales
• Fermentation: 5-7 days at 14-23oC
• Aged for short periods at 4-8oC
• CO2 usually added at bottling
• beer can be pasteurized or sterilized by
filtration
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Beer Production
(pitched)
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Brew kettles used for preparation of wort
Insert Figure 34.15
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Distilled Spirits
• similar to beer-making process
– begins with sour mash
• mash inoculated with homolactic
bacterium
– following fermentation, is distilled to
concentrate alcohol
57
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Production of Breads
• involves growth of Saccharomyces
cerevisiae (baker’s yeast) under aerobic
conditions
– maximizes CO2 production, which leavens
bread
• other microbes used to make special
breads (e.g., sourdough bread)
• can be spoiled by Bacillus species that
produce ropiness
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Other Fermented Foods…
• sufu – from fermentation of tofu
• sauerkraut (sour cabbage) – from
wilted, shredded cabbage
• pickles – from cucumbers
• silage – from grass, chopped corn
and other fresh animal feeds
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Table 34.7
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Microorganisms as Foods and
Food Amendments
• variety of bacteria, yeasts, and other
fungi (mushrooms) are used as animal
and human food sources
• probiotics
– microbial dietary adjuvants
: e.g. Spirulina, lactic acid bacteria, etc.
– microbes added to diet in order to provide
health benefits beyond basic nutritive value
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Mushroom Farming
Insert Figure 34.16
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Benefits of probiotics
•
•
•
•
immunodilation
control of diarrhea
anticancer effects
possible modulation of Crohn’s disease
(intestine)
• in beef cattle
– Lactobacillus acidophilus decrease E. coli (O157:H7)
• in poultry
– Bacillus subtilis limit Salmonella colonization of the
gut by the process of competitive exclusion
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* Prebiotics
• oligosaccharide polymers that are not
processed until they enter large intestine
* synbiotic system
– combination of prebiotics and probiotics
– results in production of certain acids (e.g.
butryic and propionic acids) that may be
responsible for possible beneficial effects of
probiotics
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