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Chapter 40
Microbiology of Food
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Microbial Growth and 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
2
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Microbial Growth and Food
Spoilage…
• microbial growth is controlled by
– intrinsic factors
• factors related to the food itself
– extrinsic factors
• environment where food stored
3
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Figure 40.1
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Intrinsic Factors
• food composition - carbohydrates
– mold predominates
• degrades food by hydrolysis, tomatoes
particularly susceptible
• little odor
• ergotism
– hallucinogenic alkaloids released by Claviceps
purpurea
– may cause death
5
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Table 40.1
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Figure 40.2
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Intrinsic Factors…
• food composition – proteins or fats
– bacterial growth predominates
• putrefaction
– proteolysis and anaerobic breakdown of
proteins, yielding foul-smelling amine
compounds
• unpasteurized milk spoilage
– acid production followed by putrification
• butter
– short chained fatty acid production results in
rancid butter
8
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Figure 40.3
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Figure 40.4
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Intrinsic Factors…
• pH
– impacts make up of microbial
community and therefore types of
chemical reactions that occur when
microbes grow in food
– e.g., low pH favors yeast and mold
• presence and availability of water
– in general, lower water activity inhibits
microbial growth
11
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Intrinsic Factors…
• oxidation-reduction potential
– altered by cooking
– lower redox – more bacteria and
anaerobes
• physical structure
– grinding and mixing distribute
microbes; promotes microbial growth
– outer skin of vegetables and fruits
slows microbial growth
12
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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
13
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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
14
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Controlling Food Spoilage
• modern era of food microbiology
established by Louis Pasteur in 1857
• methods of preservation
– goal is to eliminate or reduce the
populations of spoilage and disease
causing microbes while maintaining
food quality
15
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Removal of Microorganisms
• water, wine, beer, juices, soft drinks,
and other liquids usually by
filtration
• may better preserve flavor and
aroma
16
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Low Temperature
• refrigeration at 5°C retards but does
not stop microbial growth
– microorganisms can still cause spoilage
with extended spoilage
– growth at temperatures below -10°C
has been observed
17
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High Temperature: 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
• spoilage of canned foods
– spoilage prior to canning
– underprocessing
– leakage of contaminated water into cans
during cooling process
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Figure 40.5
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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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Table 40.2
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Chemical-Based Preservation
• GRAS
– chemical agents “generally recognized
as safe”
– agents include organic acids, sulfite,
ethylene oxide gas, ethyl formate
– sodium nitrite – inhibits spore
formation in meats, forms nitrosamines
• pH of food impacts effectiveness of
chemical preservative
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Table 40.3
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Radiation
• radappertization
– use of ionizing radiation (gamma
radiation) to extend shelf life or
sterilize meat, seafoods, fruits, and
vegetables
– excellent penetrating power – food not
radioactive
– kills microbes in moist foods by
producing peroxides from water
• peroxides oxidize cellular constituents
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Radiation…
• 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
27
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Microbial Product-Based
Inhibition
• e.g., nisin from Lactococcus lactis
– used in low-acid foods to inactivate
Clostridium botulinum during canning
process
• e.g., bacteriophages that kill Listeria
monocytogenes
– sprayed onto ready-to-eat meats prior
to packaging
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Packaging
• modified atmosphere packaging (MAP)
– gases in stored food affect microbial growth
– shrink wrap materials and vacuum technology
control atmosphere
• impermeable to oxygen
• superoxide radicals inhibit growth
– polylactic acid
• green alternative to plastic made from wood and
corn
• embedded with nisin which is slowly released
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Types of Food-Borne Disease
• about 76 million cases/yr in U.S.
– 18% attributed to known pathogens
– at least 5,000 deaths/yr in U.S.
• pathogens
– Noroviruses, Campylobacter jejuni,
Salmonella are major causes
– E. coli and Listeria are also important
pathogens
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Types of Food-Borne
Disease…
• two primary types
– food-borne infections
– food intoxications
• transmission
– breakdown in hygiene
– fecal-oral route key
– fomites also important
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Food-Borne Infection
• ingestion of pathogen, 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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Table 40.4
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Food-Borne Infections…
• Listeriosis
– 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
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Food-Borne Intoxications
• ingestion of toxins in foods in which
microbes have grown
• produce symptoms shortly after the food
is consumed because growth of the
disease-causing microorganism is not
required
• include staphylococcal food poisoning,
botulism, Clostridium perfringens food
poisoning, and Bacillus cereus food
poisoning
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Other Food Disease
• enterohemorrhagic E. coli (O157:H7)
– normal inhabitant of intestines of
mammals including cattle, swine
• contamination of meat during slaughter
• in salad vegetables contaminated in field
– shiga toxin from horizontal gene
transfer from Shigella
• may result in life-threatening hemolytic
uremic syndrome, most common in
children and elderly
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Other Food Disease
• fungus-derived toxins
– aflatoxins
• carcinogens produced in fungus-infected
grains and nut products
– fumonisins
• carcinogens produced in fungus-infected
corn
• algal toxins
• contaminate fish and shellfish
37
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Figure 40.6
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Figure 40.7
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Detection of Food-Borne
Pathogens
• must be rapid, sensitive and simple and
for prevention
• methods include:
– culture techniques – may be too slow but are
most likely to detect food pathogens
– immunological techniques - very sensitive
– molecular techniques
• probes used to detect specific DNA or RNA
• sensitive and specific
40
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Detection of Food-Borne
Pathogens…
• PulseNet
– established by Centers for Disease Control
– 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
41
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Figure 40.8
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Detection of Food-Borne
Pathogens…
• in addition to PFGE, PCR and
RFLP are increasingly developed
and used
– amplify small quantities
– species specific
– microarray techniques directly from
food or water
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Figure 40.9
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Microbiology of Fermented
Foods
• chemical changes in food brought
about microbial action
• major fermentations used are lactic,
propionic, and ethanolic
fermentations
45
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Fermented Milks
• majority of fermented milk products rely
on lactic acid bacteria (LAB) belonging to
the genera Lactobacillus, Lactococcus,
Leuconostoc, and Streptococcus
– gram-positives that tolerate acidic conditions,
non-spore forming, aerotolerant with a strictly
fermentative metabolism
46
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Figure 40.10
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Table 40.5
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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
49
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Probiotics and
Standardization
• probiotics
– live microorganisms, which when
administered in adequate amounts,
confer a health benefit to the host
– specific requirements should be met
• microorganisms
– Lactobacillus, Bifidobacterium
50
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Figure 40.11
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Benefits of Probiotics
•
•
•
•
•
immunomodulation
control of diarrhea
anticancer effects – colon cancer
possible modulation of Crohn’s Disease
treatment of enteric disease
– competition with pathogens
– antibiotic resistance is continually rising
52
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Benefits of Probiotics…
• Lactobacillus acidophilus and
Bifidobacterium
– improve lactose intolerance
– improves general intestinal health and
balance
– may lower serum cholesterol
– may have anti-tumor activity
53
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Other Milk Fermentations
• yeast lactic
– yeasts, lactic acid bacteria, and acetic
acid bacteria
– kefir
• mold lactic
– filamentous fungi and lactic acid
bacteria
– villi
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Cheese Production
• approximately 2,000 distinct
varieties representing 20 general
types
• classified based on
– texture, hardness (soft, semi-soft, hard,
very hard)
• all from lactic acid fermentation
– molds may further enhance
55
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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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Table 40.6
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Figure 40.12
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Meat and Fish
•
•
•
•
•
•
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sausages
hams
bologna
salami
izushi – fish, rice, and vegetables
katsuobushi – tuna
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Wines and Champagnes
• enology (wine production)
– crushed grapes
• separation and storage of liquid (must)
before fermentation
– fresh must
• treated with sulfur dioxide fumigant
• Saccharomyces cerevisiae or S. liposideus
added for consistent results
• fermented for 3–5 days at 20–28oC
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Figure 40.13
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Wines and Champagnes…
• dry or sweet wines
– controlled by regulating initial must
sugar content
• racking
– removes sediment produced during
fermentation
62
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Beers and Ales
• cereal grains used for fermentation
– 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
63
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Figure 40.14
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Beers and Ales…
• wort inoculated (pitched) with desired
yeast
– bottom yeasts
• used in production of beers
– top yeasts
• used in production of ales
• freshly fermented beers are aged or
lagered
– CO2 usually added at bottling
• beer can be pasteurized or sterilized by
filtration
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Figure 40.15
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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
67
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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
68
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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
69
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Table 40.7
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Microorganisms as Foods and
Food Amendments
• mushrooms (e.g., Agaricus bisporus)
• cyanobacterium Spirulina dried cakes
• probiotics
– Lactobacillus acidophilus in beef cattle
• decrease E. coli O157:H7
– Bacillus subtilis (PREEMPT) in poultry
• limit colonization of gut by the process of
competitive exclusion
• reduces Salmonella and Campylobacter
71
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Figure 40.16
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