Food
Microbiology
Food Preservation
and
Microbial Growth
Microbial Growth and Food Spoilage
1. Foods often spoil due to growth of contaminating microorganisms.
2. Foods vary considerably in their sensitivity to microbial growth, depending
on their:
 nutrient content
 water availability (known as water activity, aw)
 pH.
3. Perishable and semiperishable foods have limited shelf life due to
spoilage.
4. Nonperishable (stable) foods have an extended shelf life and are
resistant to spoilage by microorganisms.
5. A variety of microorganisms induce food spoilage, and although most are
not, some food spoilage microorganisms are also potential pathogens.
Food Preservation
1. Food microbiology deals with methods for limiting spoilage & the growth
of disease-causing microorganisms in food during processing & storage.
2. The earliest methods of preserving foods included:
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drying
addition of salt or sugar
fermentation.
3. The growth of microorganisms in perishable foods can be controlled by:
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refrigeration or freezing
lyophilization (freeze-drying)
pasteurization
canning
fermentation or pickling
dehydration
chemical preservatives
irradiation.
Aseptic Packaging
Presterilized materials are assembled into packages and aseptically
filled with heat-sterilized liquid foods.
Radiation and Industrial Food Preservation
Gamma radiation can be used to sterilize food, kill insects and
parasitic worms, and prevent the sprouting of fruits and vegetables.
Irradiation logo
(radura)
Isn’t it friendly???
The intent?
To avoid consumer
images of Three Mile
Island or Chernobyl or
Fukushima!!
Foodborne Diseases
Manifestations and Causes of Foodborne Illnesses
1. The majority of cases of foodborne disease manifest as gastroenteritis,
most commonly with diarrhea and/or nausea and vomiting, but many
other signs and symptoms are possible, depending on the infecting
agent.
2. More than 200 different infectious agents, including bacteria, viruses,
protozoa and even prions can cause foodborne disease.
3. Viruses cause the vast majority of foodborne illnesses.
 Noroviruses (previously Norwalk and Norwalk-like viruses) are the
most common foodborne pathogens, causing more than 9 million
cases per year in the USA.
Foodborne Disease vs. Food Poisoning
Introduction and Clarification
1. Foodborne diseases include intoxications and foodborne infections.
 An intoxication is a disease caused by the ingestion of a
pre-formed toxin already present in food.
 A foodborne infection is established after ingesting food containing
pathogens that must first colonize and multiply before virulence factors,
which may include toxins, mediate foodborne disease.
IMPORTANT NOTE: Depending on the text that you consult, there are
differences in how food poisoning is defined.
2. For our purposes, we will define food poisoning as an intoxication.
A Single Virus is Responsible for
Vast Majority of Foodborne illnesses
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Noroviruses (previously Norwalk and Norwalk-like viruses) are the
most common foodborne pathogens, causing more than 9 million
cases per year in the USA.
Noroviruses are associated with large scale outbreaks (e.g., cruise
ships, hotels, etc.)
Virus remains infectious for long periods of time on surfaces
Following an outbreak, every surface from floor to ceiling must be
disinfected
Staphylococcal Food Poisoning
1. Staphylococcal intoxication results from the ingestion of preformed
enterotoxin produced by Staphylococcus aureus.
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S. aureus contaminates foods that are generally high in protein
content.
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Staphylococcal enterotoxin is a superantigen
2. In many cases, S. aureus cannot be cultured from the contaminated food.
Clostridial Food Poisoning and Botulism
1. The spore-forming bacterium, Clostridium perfringens can cause food
poisoning from ingestion of toxins (intoxication)
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Quite common
Usually self-limiting gastrointestinal disease associated with consumption of
contaminated meat.
2. Botulism (Clostridium botulinum) is a rare but very serious disease with
significant mortality due to a potent neurotoxin produced by microbial
growth in foods or by microbial growth and toxin production in the body.
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Sporadic cases are often associated with home canning.
Bacillus cereus Food Poisoning & Foodborne Infection
Bacillus cereus causes two clinical types of foodborne disease.
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Emetic (vomiting) form of gastroenteritis is an intoxication usually
associated with consumption of improperly refrigerated rice.
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Toxin is heat stable, so re-heating does not protect
Diarrheal form of gastroenteritis is usually associated with an
infection from consumption of contaminated meat or vegetables.
Campylobacteriosis
1.
Campylobacter infection was the most prevalent foodborne bacterial disease
2. Most cases are associated with improperly prepared poultry.
3. Campylobacteriosis affects nearly 2 million people per year in the USA
4. Usually a self-limiting gastroenteritis,
5. Infection with certain serotypes of Campylobacter may predispose certain
individuals to Guillain-Barre syndrome (GBS), an autoimmune disorder
of the peripheral nervous system.
Salmonellosis
(Non-typhoidal disease)
1. There are >1.3 million cases of salmonellosis every year in the U.S.
2. Most cases are associated with consumption of improperly prepared
poultry or raw eggs (e.g., Caesar salad)
3. The disease results from infection with ingested Salmonella introduced into
the food chain from food production animals or food handlers.
Prevalence of
salmonellosis in
the USA
Escherichia coli Gastroenteritis
1. Most Escherichia coli are opportunistic pathogens, but some strains are
true pathogens and can cause serious foodborne diseases.
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Enterotoxigenic E.coli (ETEC) causes most cases of traveller’s
diarrhea.
Enterohemorrhagic E. coli (EHEC) (e.g., E. coli O157:H7) is associated
with undercooked ground beef and can cause hemolytic uremic
syndrome (HUS) and death in at-risk individuals.
Enteropathogenic E. coli causes infant diarrhea in underdeveloped
countries.
2. Large-scale processing methods for meats and meat products allow
contaminants from a small number of individual carcasses to contaminate
or infect large numbers of products.
3. Specific measures, such as radiation of ground beef, have been
implemented to curb the spread of these pathogens.
Listeriosis
1. Listeria monocytogenes is an environmentally ubiquitous microorganism
that is found occasionally as a contaminant of:
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soft cheeses
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luncheon meats
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hot dogs.
2. Large-scale food recalls are not uncommon.
3. Listeria is one of the few pathogens that can multiply at refrigerator
temperatures (4oC).
4. In normal individuals, Listeria seldom causes infection.
5. In immunocompromised individuals or pregnant women, Listeria can
cause serious disease and even death.
Common Diseases
that may be both
Water and Foodborne
Shigellosis (Bacillary Dysentery)
1. Water and foodborne disease, also known as bacillary dysentery
 Cramping and diarrhea, often with blood and mucus
 Frequent, low-volume stools
 Straining to defecate, called tenesmus
2. Fecal-oral transmission
3. Humans are primary reservoir, most commonly fecal contamination of
water sources or unsanitary practices by food handlers
4. Extremely low infectious dose; < 100 bacterial cells can cause disease
5. Common sources include:
 Fecally-contaminated water
 Salads
 Raw vegetables
 Milk, dairy, and poultry.
6. The genus Shigella has only four species
Typhoid Fever
1. Water and foodborne disease caused by Salmonella typhi, but quite
unique from ‘garden-variety’ salmonellosis
2. Caused by fecal-oral transmission by human carriers
 Typhoid Mary was first recognized human carrier
 In about 1 of every 20 cases, the organisms colonize the gallbladder
which serves as the reservoir
 Organisms move through the biliary duct into the small intestine and
are shed in feces
3. 10-14 day incubation period with
 Initial signs of sepsis/bacteremia as organisms goes systemic
 Prolonged fever with delirium for > one week
 Returns to intestine via gallbladder with abdominal pain and
gastrointestinal symptoms
4. Humans are primary reservoir, most commonly fecal contamination of
water sources or food via unsanitary practices by food handlers that are
carriers
Cholera
1. Water and foodborne disease, caused by Vibrio cholerae
 Frequent epidemics and pandemics from fecal-oral transmission
 Often associated with poor public health and sanitation
 Easily treatable, but often goes untreated leading to death in 10-30%
of untreated cases
2. 2-3 days after ingestion of vibrios, about 20% of infected cases have an
abrupt onset of
 Vomiting
 Severe, life-threatening, watery diarrhea
 Patients may lose 15-20 liters (~4-5 gallons) of fluid per day
 Stool becomes colorless, odorless, and devoid of protein and is
referred to as rice-water stool
 Stool may be ‘flecked’ with mucus
 Loss of fluids and electrolytes leads to death by
 Hypovolemic shock (low blood volume)
 Metabolic acidosis (due to loss of buffering capacity)
3. Other vibrios, Vibrio vulnificus and Vibrio parahaemolyticus cause
serious disease or even death from consumption of contaminated shellfish
or raw fish
Additional
Material
Food Preservation
and
Microbial Growth
Gamma-ray irradiation facility
Industrial Food Canning
1. Commercial sterilization heats canned foods to the minimum
temperature necessary to destroy endospores of Clostridium botulinum
while minimizing alteration of the food.
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Sufficient heat to reduce a population of C. botulinum by 12 logarithmic
cycles (12D treatment).
Endospores of thermophiles can survive commercial sterilization.
2. Canned foods stored above 45°C can be spoiled by thermophilic
anaerobes.
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Thermophilic anaerobic spoilage may be accompanied by gas production
If no gas is formed, the spoilage is called flat sour spoilage.
3. Spoilage by mesophilic bacteria is usually from improper heating
procedures or leakage.
4. Acidic foods can be preserved by heat of 100°C because
microorganisms that survive are not capable of growth in a low pH.
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Acid-tolerant and heat-resistant microbes can spoil acidic foods.
 Byssochlamys, Aspergillus, and Bacillus coagulans
Construction of a metal can
Commercial sterilization process in
industrial canning
Commercial sterilization of food is accomplished
by steam under pressure in a retort.
The Role
of
Microorganisms
in
Food Production
Mushrooms
1. The most important food produced from a microorganism is the mushroom
which are any of a variety of fungi of the order Agaricalles.
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Edible mushrooms are fleshy, umbrella-shaped fruiting bodies of the fungus.
Poisonous species are properly called toadstools.
2. The study of fungi is called mycology.
Fermented Foods
Microbial fermentation is an important process used to produce, preserve
and/or enhance a number of foods, including:
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breads
dairy products
meats and vegetables
alcoholic beverages
vinegar and other organic acids
sauerkraut, pickles, olives and soy sauce.
Nondairy Fermentations
1. Sugars in bread dough are fermented by yeast to ethanol and C02
 C02 causes the bread to rise and produces the “holes” in the bread.
2. Sauerkraut, pickles, olives, and soy sauce are the products of microbial
fermentations.
Cheese
1. The milk protein casein curdles because of the action by lactic acid
bacteria or the enzyme rennin or chymosin.
 The growth of microorganisms in cheeses is called ripening.
2. Cheese is the curd separated from the liquid portion of milk, called whey.
 Hard cheeses are produced by lactic acid bacteria growing in the
interior of the curd.
 Semisoft cheeses are ripened by bacteria growing on the surface
 Soft cheeses are ripened by Penicillium growing on the surface.
Other Dairy Products
1. Old-fashioned buttermilk was produced by lactic acid bacteria growing
during the butter-making process.
2. Commerical buttermilk is made by letting lactic add bacteria grow in skim
milk for 12 hours.
3. Sour cream, yogurt, kefir, and kumiss are produced by lactobacilli,
streptococci, or yeasts growing in low-fat milk.
Alcoholic Beverages and Vinegar
1. Carbohydrates obtained from grains, potatoes, or molasses are
fermented by yeasts to produce ethanol and CO2 in the production of
beer, ale and distilled spirits.
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Beer is made from fermentation of malted grain (brewing).
Distilled beverages are made from distillation of fermented solutions.
2. The sugars in fruits such as grapes are fermented by yeasts to produce
wines.
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In wine-making, lactic acid bacteria convert malic acid into lactic acid in
malolactic fermentation in fruits with high acidity.
3. Acetic acid (vinegar), an organic acid, oxidized from ethanol in fruit
juice/wine by Acetobacter and Gluconobacter.
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Adequate aeration is the most important consideration in ensuring a
successful vinegar process.
Basic steps in red winemaking
Citric Acid and Other Organic Compounds
1. A number of organic chemicals are manufactured commercially using
fermentation by microorganisms.
2. Citric acid is an economically important product of the Aspergillus niger
fungus, which, in the absence of iron, overproduces citric acid as a
chelator to scavenge iron.
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Over 550,000 tons of citric acid are produced per year worldwide with a total
value of $1 billion U.S.
Historically, lessons learned from perfecting citric acid fermentation were
applied to penicillin and other antibiotic fermentations.
Citric acid is commonly used as an antioxidant & flavoring agent in
beverages, confectionaries & other foods and as a leavening agent in bread.
3. Lactic acid, used to acidify foods and beverages, is fermented by various
types of lactic acid bacteria.
4. Sorbose, used in the manufacture of ascorbic acid (vitamin C), is
produced when Acetobacter oxidizes sorbitol.
Citric acid fermentation
Sucrose is converted to glucose and fructose by the enzyme sucrase.
As the Aspergillus niger culture reaches stationary phase, these sugars
are converted to citric acid, a secondary metabolite, to scavenge iron.
Yeast as a Food and Food Supplement
1. Yeast cells are grown for use in the baking and food industries.
2. Commercial yeast is produced in large-scale aerated fermentors using
molasses as the main carbon and energy source.
Foodborne Diseases
Foodborne Disease:
Infection vs. Intoxication
Introduction and Clarification
1. Foodborne diseases include foodborne intoxications and foodborne
infections.
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Depending on the text that you consult, there are differences in how food
poisoning is defined. Some textbooks consider food poisoning the result of
ingesting microbial toxins or microorganisms that produce toxins.
This interpretation is problematic in that:
i) many microorganisms that are ingested, establish an infection and cause
disease by producing toxins and/or other virulence factors
ii) an infection, by definition, is colonization and/or invasion with
multiplication of a pathogen with or without disease.
4. For our purposes, we will define food poisoning as an intoxication.
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An intoxication is a disease caused by the ingestion of a pre-formed toxin
already present in food.
Thus, food poisoning is one form of foodborne disease, but the ingestion of
pathogens in/on food that must first establish an infection to cause foodborne
disease, whether or not they do so by the production of toxin(s), we will not
consider to be food poisoning.
FP = foodborne intoxication
FI = nonintoxication foodborne disease
Learning Objectives
1. Describe thermophilic anaerobic spoilage and flat sour spoilage by
mesophilic bacteria.
2. Compare and contrast food preservation by industrial food canning, aseptic
packaging, and radiation.
3. Name four beneficial activities of microorganisms in food production.