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Microbiology of Food
CHAPTER OVERVIEW
This chapter discusses the microorganisms associated with foods. Some of these microorganisms cause food
spoilage, some are pathogens that are transmitted via foods, and some are used in the production of foods.
CHAPTER OBJECTIVES
After reading this chapter you should be able to:
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discuss the interaction of intrinsic (food-related) and extrinsic (environmental) factors related to food
spoilage
describe the various physical, chemical, and biological processes used to preserve foods
discuss the various diseases that can be transmitted to humans by foods
differentiate between food infections and food intoxications
discuss the detection of disease-causing organisms in foods
describe the fermentation of dairy products, grains, meats, fruits, and vegetables
discuss the toxins produced by fungi growing in moist corn and grain products
discuss the direct use of microbial cells as food by humans and animals
list foods that are made with the aid of microorganisms and indicate the types of microorganisms used in
their production
describe probiotics
CHAPTER OUTLINE
I.
Microorganism Growth in Foods
A. Intrinsic factors
1. Food composition
a. Foods rich in carbohydrates tend to support the growth of
fungi; once the protective skin is breached bacteria can grow
as well, releasing hydrolytic enzymes; molds prefer moist
foods
b. Proteins and/or fats result in bacterial growth; anaerobic breakdown of proteins
(putrefaction) leads to the release foul-smelling amines
c. Milk spoilage exhibits a succession of species where Lactococcus and Lactobacillus
produce lactic acid, which is later degraded by yeasts and molds, eventually leading to
bacterial breakdown of proteins
2. pH—low pH allows yeasts and molds to become dominant; higher pH allows bacteria to
become dominant; higher pH favors putrefaction
3. Physical structure affects the course and extent of spoilage
a. Grinding and mixing (e.g., sausage and hamburger) increases surface area, alters cellular
structure, and distributes microorganisms throughout the food
b. Vegetables and fruits have outer skins that protect against spoilage; spoilage
microorganisms have enzymes that weaken and penetrate such protective coverings
4. Many foods contain natural antimicrobial substances (e.g., herbs and spices)
B. Extrinsic factors
1. Temperature and relative humidity—at higher relative humidity, microbial growth is initiated
more rapidly, even at lower temperatures
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2.
Atmosphere—oxygen usually promotes growth and spoilage even in shrink-wrapped foods
since oxygen can diffuse through the plastic; high CO2 tends to decrease pH and reduce
spoilage
II. Controlling Food Spoilage
A. Removal of microorganisms—filtration of water, wine, beer, juices, soft drinks, and other liquids
can keep bacterial populations low or eliminate them entirely
B. Low temperature—refrigeration and/or freezing retards microbial growth but does not prevent
spoilage
C. High temperature
1. Canning
a. Canned food is heated in special containers called retorts to 115 C for 25–100 minutes to
kill spoilage microorganisms
b. Canned foods can undergo spoilage despite safety precautions; spoilage can be due to
spoilage prior to canning, underprocessing during canning, or leakage of contaminated
water through can seams during cooling
2. Pasteurization—kills pathogens and substantially reduces the number of spoilage organisms
a. Low-temperature holding (LTH)—62.8C for 30 minutes
b. High-temperature short-time (HTST)—71C for 15 seconds
c. Ultra-high temperature (UHT)—141C for 2 seconds
d. Shorter times result in improved flavor and extended shelf-life
3. Heat treatments are based on a statistical process involving the probability that the number of
remaining viable microorganisms will be below a certain level after a specified time at a
specified temperature
D. Water availability
1. Measured as water activity (aw); dehydration procedures (e.g., freeze-drying) remove water
and increase solute concentration and reduce spoilage
2. Osmophilic and xerophilic organisms prefer high osmotic concentrations and low aw,
respectively
E. Chemical-based preservation
1. Regulated by the U.S. Food and Drug Administration (FDA); preservatives are listed as
“generally recognized as safe” (GRAS); include simple organic acids, sulfite, ethylene oxide
as a gaseous sterilant, sodium nitrite, and ethyl formate
2. Nitrites protect against Clostridium botulinum, but are of some concern because of their
potential to form carcinogenic compounds when meats preserved with them are cooked
3. Low pH from addition of acetate or lactate inhibits microbial spoilage; the effectiveness of
some preservatives (e.g., sodium propionate) is affected by pH
F. Radiation
1. Nonionizing (ultraviolet or UV) radiation is used for surfaces of food-handling utensils, but
does not penetrate foods; ionizing (gamma radiation) penetrates well but must be used with
moist foods to produce peroxides, which oxidize sensitive cellular constituents
(radappertization)
2. Ionizing radiation is used for seafoods, fruits, vegetables, and meats; electron beams also can
be used irradiate foods
G. Microbial product-based inhibition
1. Bacteriocins—bactericidal proteins produced by bacteria; active against only closely related
bacteria (e.g., nisin); function by several mechanisms
2. Bacteriophages can be used to block Listeria monocytogenes growth
H. Packaging
1. Gases within packaging can affect microbial growth; modified atmosphere packaging (MAP)
typically excludes oxygen and increases carbon dioxide concentrations; high oxygen MAP
creates toxic chemical radicals
2. New packaging includes embedded nisin or biodegradable polylactic acid and pectin materials
III. Food-borne Diseases
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A.
Food-borne illnesses impact the entire world; are either infections or intoxications; are associated
with poor hygiene practices
B. Food-borne infections—Due to ingestion of microorganisms, followed by growth, tissue invasion
and/or release of toxins; widespread outbreaks are possible
C. Food intoxications
1. Ingestion of microbial toxins in foods
2. Staphylococcal food poisoning is caused by exotoxins released by Staphylococcus aureus,
which is frequently transmitted from its normal habitat (nasal cavity) to food by person’s
hands; improper refrigeration leads to growth of bacterium and toxin production
3. Clostridium botulinum, C. perfringens, and B. subtilis also cause food intoxication
4. Aflatoxins are produced in foods by Aspergillus flavus and leads to cancers primarily in the
liver; fumonisins produced by Fusarium moniliforme in corn products cause a variety of
cancers and diseases in humans and animals; algal toxins from harmful blooms are a concern
IV. Detection of Food-borne Pathogens
A. Methods need to be rapid; therefore, traditional culture methods that might take days to weeks to
complete are too slow; identification also is complicated by low numbers of pathogens compared to
normal microflora; chemical and physical properties of food can make isolation of food-borne
pathogens difficult
B. Molecular methods are valuable for three reasons
1. They can detect the presence of a single, specific pathogen
2. They can detect viruses that cannot be conveniently cultured
3. They can identify slow-growing or nonculturable pathogens
C. Food-borne pathogen fingerprinting is an integral part of an initiative by the Centers for Disease
Control (CDC) to control food-borne pathogens; the CDC has established a procedure (PulseNet) in
which pulsed-field gel electrophoresis is used under carefully controlled and standardized
conditions to detect the distinctive DNA patterns of nine major food pathogens; these pathogens are
being followed by a surveillance network (FoodNet)
V. Microbiology of Fermented Foods
A. Fermented milks—at least 400 different fermented milks are produced throughout the world;
fermentations are carried out by mesophilic, thermophilic, and therapeutic lactic acid bacteria, as
well as by yeasts and molds
1. Lactic acid bacteria—used for majority of fermented milks; gram-positive, acid-tolerant
bacteria with a strictly fermentive metabolism
2. Mesophilic—acid produced from microbial activity at temperatures lower than 45°C causes
protein denaturation (e.g., cultured buttermilk and sour cream)
3. Thermophilic—fermentations carried out at about 45°C (e.g., yogurt)
4. Probiotics—fermented milks may have beneficial therapeutic effects
a. Acidophilus milk contains L. acidophilus; improves general health by altering intestinal
microflora; may help control colon cancer
b. Bifidbacterium spp. fermented milk products improve lactose tolerance, possess
anticancer activity, help reduce serum cholesterol levels, assist calcium absorption, and
promote the synthesis of B-complex vitamins; may also reduce or prevent the excretion
of rotaviruses, a cause of diarrhea among children
5. Yeast-lactic fermentations—include kefir, which is made by the action of yeasts, lactic acid
bacteria, and acetic acid bacteria
6. Mold-lactic fermentation—used to make viili, a Finnish beverage; carried out by the mold
Geotrichium candidum and lactic acid bacteria
B. Cheese production
1. Initial fermentation with starter culture of lactic acid bacteria followed by coagulation of curd
by the stomach enzyme renin; watery whey is squeezed out and ripening continued by
microbial fermentation with nonstarter lactic acid bacteria and fungi
2. Cheese can be internally inoculated or surface ripened; water content (ripening time)
determines the hardness of the cheese; fungal spores (for brie or blue cheese) or
Propionibacterium (for Swiss cheese) can be added
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C.
Meat and fish
1. Meat products include sausages, country-cured hams, bologna, and salami; these fermentations
frequently involve Pediococcus cerevisiae and Lactobacillus plantarum
2. Fish products include izushi (fresh fish, rice, and vegetables incubated with Lactobacillus spp.)
and katsuobushi (tuna incubated with Aspergillus glaucus)
D. Wines and champagnes
1. Grapes are crushed and liquids that contain fermentable substrates (musts) are separated; musts
can be fermented immediately, but the results can be unpredictable; usually must is sterilized
by pasteurization or with sulfur dioxide fumigant; to make a red wine, the skins of a red grape
are left in contact with the must before the fermentation process; if must was sterilized, the
desired strain of Saccharomyces cerevisiae or S. ellipsoideus is added, and the mixture
fermented (10 to 18% alcohol)
2. Another important fermentative process that occurs is the malo-lactic fermentation carried out
by Leuoconostoc spp.; this fermentation reduces the amount of organic acids (e.g., malic acid)
in the wine, improving its flavor, stability, and “mouth feel”
3. For dry wine (no free sugar), the amount of sugar is limited so that all sugar is fermented
before fermentation stops; for sweet wine (free sugar present), the fermentation is inhibited by
alcohol accumulation before all sugar is used up; in the aging process flavoring compounds
accumulate
4. Racking—removal of sediments accumulated during the fermentation process
5. For champagnes, fermentation is continued in bottles to produce a naturally sparkling wine
E. Beers and ales
1. Malt is produced by germination of the barley grains and the activation of their enzymes; mash
is produced from malt by enzymatic starch hydrolysis to accumulate utilizable carbohydrates;
mash is heated with hops (dried flowers of the female vine Humulus lupulis) to provide flavor
and clarify the wort; hops inactivate hydrolytic enzymes so that wort can be pitched
(inoculated with yeast)
2. Beer is produced with a bottom yeast, such as Saccharomyces pastorianus, and ale is produced
with a top yeast, such as S. cerevisiae; freshly fermented (green) beers are lagered (aged),
bottled, and carbonated; beer can be pasteurized or filtered to remove microorganisms and
minimize flavor changes
F. Distilled spirits—beerlike fermented liquid is distilled to concentrate alcohol; type of liquor
depends on composition of starting mash; flavorings also can be added; a sour mash involving
Lactobacillus delbrueckii-mediated fermentation is often used
G. Production of breads
1. Aerobic yeast fermentation is used to increase carbon dioxide production and decrease alcohol
production; other metabolic products add flavors
2. Bread products can be spoiled by Bacillus species that produce ropiness
H. Other fermented foods
1. Sufu, fermented tofu (a chemically coagulated soybean milk product) and tempeh, made from
soybean mash, are made by the action of molds
2. Sauerkraut—fermented cabbage; involves a microbial succession mediated by Leuconostoc
mesenteroides, Lactobacillus plantarum, and Lactobacillus brevis
3. Pickles are cucumbers fermented in brine by a variety of bacteria; fermentation process
involves a complex microbial succession
4. Silages—animal feeds produced by anaerobic, lactic-type mixed fermentation of grass, corn,
and other fresh animal feeds
VI. Microorganisms as Foods and Food Amendments
A. Microbes that are eaten include a variety of bacteria, yeasts, and other fungi (e.g., mushrooms,
Spirulina)
B. Probiotics are being used with poultry to increase body weight and feed conversion; also reduce
coliforms and Campylobacter; may be useful in preventing Salmonella from colonizing gut due to
competitive exclusion
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TERMS AND DEFINITIONS
Place the letter of each term in the space next to the definition or description that best matches it.
____ 1.
____ 2.
____ 3.
____ 4.
____ 5.
____ 6.
____ 7.
____ 8.
____ 9.
____ 10.
____ 11.
____ 12.
____ 13.
____ 14.
____ 15.
____ 16.
a.
b.
c.
d.
e.
The food spoilage process that results in the release of foulsmelling amine compounds
Microorganisms that grow in media with high salt or sugar
concentrations
Microorganisms that grow in media with low water activity
A commercial process that preserves food by heating it in
containers at about 115°C for up to 100 minutes
A process that reduces the total microbial population and usually
eliminates all pathogenic microorganisms
Chemicals used to preserve foods that are generally recognized as
safe
The process of irradiating moist foods to kill contaminating
microorganisms
Small, labeled nucleic acids used to detect specific sequences of
DNA or RNA
Fruit juices that contain readily fermentable substrates
The process by which complex polysaccharides found in grains
are hydrolyzed to form fermentable substrates
A clear liquid containing fermentable sugars derived from grains
The science of wine production
The process by which sediments in wine are removed
The product formed when Acetobacter and Gluconobacter are
allowed to oxidize the ethanol in wine to acetic acid
A mash that has been inoculated with a homolactic acid
bacterium in order to decrease the pH of the mash
An animal feed created by lactic acid fermentation of grass,
chopped corn and other plant materials
canning
enology
GRAS
mashing
musts
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f.
g.
h.
i.
j.
k.
l.
m.
n.
o.
p.
osmophilic
microorganisms
pasteurization
probes
putrefaction
racking
radappertization
silage
sour mash
wine vinegar
wort
xerophilic
microorganisms
FILL IN THE BLANK
1.
Food spoilage is dependent on food-related or __________ factors, such as pH, moisture, water activity
or availability, oxidation-reduction potential, physical structure of the food, available nutrients, and the
possible presence of antimicrobial compounds; and on environmental, or __________, factors, such as
temperature, relative humidity, gases present and the types and levels of microorganisms added to the
food.
2. Recently, the CDC has established a program called __________, in which pulsed-field gel
electrophoresis is used under carefully controlled conditions to determine the distinctive DNA patterns of
bacterial pathogens. Using data generated in this way, an active surveillance network called __________
is monitoring disease outbreaks caused by nine major food-borne pathogens.
3. If a food-borne disease requires ingestion of the pathogen, growth of the organism, and release of toxins
in the intestine, it is referred to as a food-borne ____________; if no growth of the microorganism is
required after ingestion because the toxic substances are already present in the food as a result of
previous growth, then it is referred to as a food-borne ____________.
4. Infection of grains by the ascomycete Claviceps purpura can cause ____________, in which
hallucinogenic alkaloids produced by the fungus lead to altered behavior, abortion, and death after their
consumption. Another group of fungi that can develop in grains and nuts produces powerful carcinogens
known as ____________. Another group of fungal toxins called __________ can contaminate corn, and
finfish and shellfish can be contaminated with __________ toxins.
5. The liquid extracted from crushed grapes has readily fermentable substrates and is referred to as
____________. However, if cereals and other starchy materials are used, the complex carbohydrates must
be depolymerized by a process known as ____________ to produce a liquid that has fermentable
substrates. In this case, the liquid is referred to as a ____________.
6. The production of beer and ale begins with the germination of barley grains and the activation of their
enzymes to produce a __________. This is then mixed with water and the desired grains and the activated
enzymes hydrolyze starch, producing utilizable carbohydrates. This mixture is called a ____________.
Heating this mixture with hops yields the __________, which is inoculated with the desired yeast. This is
called __________. If a __________ yeast is used, the product is beer, and if a top yeast is used, the
product is ale. Freshly fermented beers are then aged or __________.
7. A patented blend of bacteria is currently being used as a __________ for poultry. The blend is sprayed on
young chicks, and as they preen, they ingest the mixture. Once ingested the bacterial blend helps establish
a functional microbial community in the cecum, which limits the colonization of the gut by Salmonella.
The basis of this effect is called __________ __________.
8. Many fermented milks are made worldwide, using numerous types of microorganisms. Thermophilic
microorganisms such at Streptococcus thermophilus and Lactobacillus bulgaricus are used to make
__________. Lactic acid producing yeasts are used to make __________, a frothy, foamy beverage that
originated in the Caucasus Mountains.
9. Large quantities of sugar or salt added to foods cause microorganisms to dehydrate. However, food
spoilage can still result from the action of ____________ microorganisms that grow best at high osmotic
concentration, and from ____________ microorganisms that grow best in a low water activity
environment.
10. One new approach to packaging foods is __________ __________ packaging, which creates a specific
environment within the package that extends shelf life of the food.
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MULTIPLE CHOICE
For each of the questions below select the one best answer.
1.
2.
3.
4.
5.
6.
Which of the following helps preserve foods
by controlling availability of water?
a. drying
b. addition of salt
c. addition of sugar
d. All of the above help preserve foods by
controlling the availability of water.
Which of the following reduces spoilage of
the foods involved?
a. grinding and mixing of foods such as
sausage and hamburger
b. peeling off the skins of fruits and
vegetables
c. Both (a) and (b) are correct.
d. Neither (a) nor (b) is correct.
Which of the following products seems to aid
a variety of intestinal processes and promote
general health?
a. acidophilus milk
b. bifid-amended fermented milk products
c. Both (a) and (b) are correct.
d. Neither (a) nor (b) is correct.
Gamma irradiation has been used to sterilize
certain types of foods. There are, however,
certain limitations on its effectiveness. One
of these is that it will only work on what
types of food?
a. moist foods
b. dry foods
c. foods that are not in metal cans
d. Actually, it will work well on all foods.
Microbial fermentations are used in the
production of cheeses in order to do which of
the following steps?
a. coagulate the milk solids to form a curd
b. ripen the cheese to give a characteristic
texture and flavor
c. Microbial fermentations are used to
accomplish both (a) and (b).
d. Microbial fermentations are not used to
accomplish either (a) or (b).
Which term refers to the addition of
microorganisms to the diet in order to
provide health benefits beyond basic nutritive
value?
a. probiotics
b. microbial dietary adjuvants
c. prebiotics
d. synbiotics
7.
Which of the following determines whether
or not a wine is considered to be dry?
a. the percentage of alcohol after
fermentation
b. the amount of free sugar after
fermentation
c. the amount of carbon dioxide after
fermentation
d. the type of grape used for starting
material
8. Why are molecular methods replacing
traditional culture methods to detect diseasecausing organisms in foods?
a. Culture methods are too slow.
b. Culture methods are not sensitive
enough to detect low levels of
pathogens against a high background of
normal microflora.
c. Both (a) and (b) are correct.
d. Neither (a) nor (b) is correct.
9. Bacteriocins are important for controlling
which microorganism or disease?
a. Streptococcus lactis
b. Listeria monocytogenes
c. variant Creutzfeld-Jakob disease
d. All of the above are correct.
10. Which of the following is true about
molecular methods of detecting
microorganisms in foods?
a. They can detect the presence of a single
specific pathogen.
b. They can be used to detect viruses that
are not easily cultured.
c. They can be used to detect slowgrowing pathogens.
d. All of the above are true about the use
of molecular detection methods.
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11. Which of the following represents ways
in which canned foods may undergo
spoilage?
a. spoilage before canning
b. underprocessing during canning
c. leakage of contaminated water through
can seams during cooling
d. All of the above are ways in which
canned foods may undergo spoilage.
12. At which step of food handling and
processing can food spoilage occur?
a. during harvesting
b. during transport
c. during final preparation
d. during storage
e. Spoilage can occur at all of the above
steps.
13. Which organism causes a food-borne
infection?
a. Staphylococcus
b. Bacillus
c. Salmonella
d. all of the above
14. In which food products has E. coli O157:H7
been found?
a. in meat products such as hamburger
b. in unpasteurized fruit drinks
c. on fruits and vegetables
d. all of the above
15. What factor is contributing to increases in
food-borne illness?
a. the popularity of raw foods
b. international shipment of raw foods
c. Both (a) and (b) are contributing.
d. Neither (a) nor (b) is contributing.
TRUE/FALSE
____ 1.
Food grinding and mixing tend to reduce spoilage by causing mechanical damage to the
contaminating microorganisms.
____ 2. Even at freezer temperatures, some microorganisms may grow and cause food spoilage, but the
process is greatly slowed.
____ 3. Microorganisms can be removed from some foods by filtration, resulting in sterilized food
preparations.
____ 4. Cooking can lead to increased anaerobic spoilage by lowering the food’s oxidation-reduction
potential.
____ 5. Nitrites are used to protect against microorganisms but their use is of some concern because of the
formation of carcinogenic nitrosamines upon cooking preserved foods.
____ 6. The same microbial fermentation processes produce sour cream and cultured buttermilk, but the
starting material is different.
____ 7. Some fermented dairy products have been suggested to have beneficial effects, including a
reduction in the incidence of colon cancer and the minimization of lactose intolerance.
____ 8. White wines are produced from white (or green) grapes. If a red grape is used, the result will always
be a red wine.
____ 9. In bread making, yeast growth is usually carried out under aerobic conditions, which results in more
carbon dioxide production and less alcohol accumulation.
____ 10. The cause of most food-borne illnesses is known.
____ 11. Prior illness with hepatitis B decreases susceptibility to aflatoxin-induced liver cancer.
____ 12. Norwalk-type viruses, Campylobacter, and Salmonella are thought to be the most important causes
of food-borne illness.
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CRITICAL THINKING
1.
Go through your refrigerator and kitchen shelves. List all the food products made by the activities of
microorganisms.
2.
Discuss the making of wines, champagnes, and other fermented grape products. In your discussion give
particular consideration to the actions of the microorganisms involved. What steps are normally taken to
control the quality of the final product?
ANSWER KEY
Terms and Definitions
1. i, 2. f, 3. p, 4. a, 5. g, 6. c, 7. k, 8. h, 9. e, 10. d, 11. o, 12. b, 13. j, 14. n, 15. m, 16. l
Fill in the Blank
1. intrinsic; extrinsic 2. PulseNet; FoodNet 3. infection; intoxication 4. ergotism; aflatoxins; fumonisins; algal
5. must; mashing; wort 6. malt; mash; wort; pitching; bottom; lagered 7. probiotic; competitive exclusion 8.
yogurt; kefir 9. osmophilic; xerophilic 10. modified atmosphere
Multiple Choice
1. d, 2. d, 3. c, 4. a, 5. c, 6. a. 7. b, 8. c, 9. b, 10. d, 11. d, 12. e, 13. c, 14. d, 15. c
True/False
1. F, 2. T, 3. T, 4. T, 5. T, 6. T, 7. T, 8. F, 9. T, 10. F, 11. T, 12. T
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