Food and industrial microbiology

Food and
Khairul Farihan Kasim
Ability to define, describe and utilize
microbial growth in fermentation and
biological process
At the end of the chapter, the
student should be able to:
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
discuss the sources of microorganisms for use in industrial microbiology and
discuss the preservation of microorganisms
describe the design or manipulation of environments in which desired
processes will be carried out
discuss the management of growth characteristics to produce the desired
list the major products or uses of industrial microbiology and biotechnology
discuss the use of microorganisms in manufacturing biosensors,
microarrays, and biopesticides
discuss the manipulation of microorganisms in the environment to control
Microorganism Growth in
Intrinsic and Extrinsic Factors
Intrinsic Factors
 pH
 presence and availability of water
 oxidation-reduction potential
 altered
by cooking
physical structure
 presence of antimicrobial substances
Food composition
 Carbohydrates–do
 Proteins
not result in major odor
and/or fats result in a variety of foul
odors (e.g., putrefactions)
 low
pH allows yeasts and molds to become
dominant; higher pH allows bacteria to
become dominant; higher pH favors
putrefaction (the anaerobic breakdown of
proteins that releases foul-smelling amine
Physical structure affects the course and
extent of spoilage
 Grinding
and mixing (e.g., sausage and
hamburger) increases surface area, alters
cellular structure, and distributes
microorganisms throughout the food
 Vegetables
and fruits have outer skins that
protect against spoilage; spoilage
microorganisms have enzymes that weaken
and penetrate such protective coverings
Presence and availability of water
Drying (removal of water) controls or eliminates food
Addition of salt or sugar decreases water availability
and reduces microbial spoilage
Even under these conditions spoilage can occur by
certain kinds of microorganisms
Osmophilic–prefer high osmotic pressure
Xerophilic–prefer low water availability
Oxidation-reduction potential can be affected
(lowered) by cooking, making foods more susceptible
to anaerobic spoilage
Many foods contain natural antimicrobial
 coumarins
– fruits and vegetables
 lysozyme – cow’s milk and eggs
 aldehydic and phenolic compounds – herbs
and spices
 allicin – garlic
 polyphenols – green and black teas
Extrinsic Factors
 lower
temperatures retard microbial growth
relative humidity
 higher
levels promote microbial growth
 oxygen
promotes growth
 modified atmosphere packaging (MAP)
 use
of shrink wrap and vacuum technologies to
package food in controlled atmospheres
Temperature and relative humidity–at higher relative
humidity, microbial growth is initiated more rapidly, even
at lower temperatures
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; modified atmosphere packaging
(MAP) involves the use of modern shrink wrap materials
and vacuum technology to package foods in a desired
atmosphere (e.g., high CO2 or high O2)
Microbial Growth and Food
Meats and dairy products are ideal
environments for spoilage by
microorganisms because of their high
nutritional value and the presence of
easily utilizable carbohydrates, fats, and
proteins; proteolysis (aerobic) and
putrefaction (anaerobic) decompose
proteins; in spoilage of unpasteurized
milk, a four-step succession of
microorganisms occurs
Fruits and vegetables have much lower
protein and fat content then meats and
dairy products and undergo different
kind of spoilage; the presence of readily
degradable carbohydrates in vegetables
favors spoilage by bacteria; high
oxidation–reduction potential favors
aerobic and facultative bacteria; molds
usually initiate spoilage in whole fruits
Frozen citrus products are minimally
processed and can be spoiled by
lactobacilli and yeasts
Grains, corn, and nuts can spoil when held under moist
conditions; this can lead to production of toxic
Ergotism is caused by hallucinogenic alkaloids produced by fungi
in corn and grains
Aflatoxins—planar molecules that intercalate into DNA and act as
frameshift mutagens and carcinogens; if consumed by dairy
cows, aflatoxins can appear in milk; have also been observed in
beer, cocoa, raisins, and soybean meal; aflatoxin sensitivity can
be influenced by prior disease exposure (e.g., hepatitis B
infection increases sensitivity)
Fumonisins—contaminants of corn; cause disease in animals and
esophageal cancer in humans; disrupt synthesis and metabolism
of sphingolipids
Shellfish and finfish can be contaminated
by algal toxins, which cause a variety of
illnesses in humans
Controlling Food Spoilage
Removal of microorganisms—filtration of
water, wine, beer, juices, soft drinks and
other liquids can keep bacterial
populations low or eliminate them
Low temperature—refrigeration and/or
freezing retards microbial growth but
does not prevent spoilage
High temperature
 Canning
 Canned
food is heated in special containers called
retorts to 115°C for 25-100 minutes to kill spoilage
 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
PasteurizationCkills pathogens and substantially
reduces the number of spoilage organisms
Low-temperature holding (LTH)—62.8°C for 30
 High-temperature short-time (HTST)—71°C for 15
 Ultra-high temperature (UHT)—141°C for 2 seconds
 Shorter times result in improved flavor and extended
shelf life
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
Water availability—dehydration
procedures (e.g., freeze-drying) remove
water and increase solute concentration
Chemical–based preservation
 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
 Effectiveness depends on pH; nitrites protect
against Clostridium botulinum, but are of some
concern because of their potential to form
carcinogenic nitrosamines when meats preserved
with them are cooked
Radiation—nonionizing (ultraviolet or UV)
radiation is used for surfaces of foodhandling 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); ionizing radiation is
used for seafoods, fruits, vegetables, and
Microbial product-based inhibition
 Bacteriocins—bactericidal
proteins produced by
bacteria; active against only closely related
bacteria (e.g., nisin)
 Bacteriocins function by several mechanisms,
including dissipation of proton motive force,
formation of hydrophobic pores in membranes, or
inhibition of protein and RNA synthesis
Food-borne Diseases
Food-borne illnesses impact the entire
are either infections or intoxications;
 are associated with poor hygiene practices
Food-borne infections
Due to ingestion of microorganisms,
followed by growth, tissue invasion
and/or release of toxins
caused by a variety of Salmonella serovars;
commonly transmitted by meats, poultry, and eggs;
can arise from contamination of food by workers in
food-processing plants and restaurants
Campylobacter jejuni
transmitted by uncooked or poorly cooked poultry
 raw milk and red meats;
 thorough cooking prevents transmission
 transmitted
by dairy products
Enteropathogenic, enteroinvasive, and
enterotoxigenic Escherichia coli
 Spread
by fecal-oral route; found in meat
products, in unpasteurized fruit drinks, and on
fruits and vegetables
 Prevention requires prevention of food
contamination throughout all stages of
production, handling, and cooking
Viral pathogens
usually transmitted by water or by direct
contamination by food processors and handlers;
 recently Norwalk-like viruses have been involved in
major outbreaks on several large cruise ships
Variant Creutzfeld-Jakob disease
transmitted by ingestion of beef from infected cattle;
transmission between animals is due to the use of
mammalian tissue in ruminant animal feeds;
prevention and control is difficult
Foods transported and consumed in
uncooked state are increasingly important
sources of food-borne infection, especially
as there is increasingly rapid movement of
people and products around the world
 Sprouts
can be a problem if germinated in
contaminated water
 Shellfish and finfish can be contaminated by
pathogens (e.g., Vibrio and viruses) found in
raw sewage
 Raspberries are often transported by air to
far-away markets; if contaminated, outbreak
occurs far from source of pathogen
Food intoxications
Ingestion of microbial toxins in foods
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
Clostridium botulinum, C. perfringens, and B. subtilis
also cause food intoxication
Botulism, caused by C. botulinum
C. perfringens is a common inhabitant of food, soil, water,
spices and intestinal tract; upon ingestion, endospores
germinate and produce enterotoxins within the intestine; this
causes food poisoning; often occurs when meats are cooked
Bacillus cereus food poisoning is associated with starchy foods
Detection of Food-borne Pathogens
Methods need to be rapid; therefore, traditional
culture methods that might take days to weeks
to complete are too slow
identification is also complicated by low numbers
of pathogens compared to normal microflora
chemical and physical properties of food can
make isolation of food-borne pathogens difficult
Molecular methods are valuable for three
 They
can detect the presence of a single,
specific pathogen
 They can detect viruses that cannot be
conveniently cultured
 They can identify slow-growing or
nonculturable pathogens
Some examples
DNA probes can be linked to enzymatic,
isotopic, chromogenic, or luminescent/
fluorescent markers; are very rapid
PCR can detect small numbers of
pathogens (e.g., as few as 10 toxinproducing E. coli cells in a population of
100,000 cells isolated from soft cheese
samples; as few as two colony- forming
units of Salmonella); PCR systems are
being developed for Campylobacter jejuni
and Arcobacter butzleri
Food-borne pathogen fingerprinting is an
integral part of an initiative by the Centers
for Disease Control (CDC) to control foodborne pathogens; The CDC has
established a procedure (PulseNet) in
which pulse-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 in an surveillance network
Microbiology of Fermented
Fermented milks
 at
least 400 different fermented milks are
produced throughout the world;
 fermentations are carried out by mesophilic,
thermophilic, and therapeutic lactic acid
 as well as by yeasts and molds
 acid
produced from microbial activity at
temperatures lower than 45°C causes protein
denaturation (e.g., cultured buttermilk and
sour cream)
 fermentations
carried out at about 45°C (e.g.,
 fermented
milks may have beneficial
therapeutic effects
Acidophilus milk contains L. acidophilus; improves
general health by altering intestinal microflora; may help
control colon cancer
Bifid-amended fermented milk products (containing
Bifidobacterium spp.) 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
Yeast lactic
 these
fermentations include kefir, which is
made by the action of yeasts, lactic acid
bacteria, and acetic acid bacteria
Mold lactic
 this
fermentation is used to make viili, a
Finnish beverage;
 carried out by the mold Geotrichium
candidum and lactic acid bacteria
CheesesCproduced by coagulation of curd,
expression of whey, and ripening by
microbial fermentation; cheese can be
internally inoculated or surface ripened
Meat and Fish
 Meat
products include sausages, country-cured
hams, bologna, and salami; these fermentations
frequently involve Pediococcus cerevisiae and
Lactobacillus plantarum
 Fish
products include izushi (fresh fish, rice, and
vegetables incubated with Lactobacillus spp.) and
katsuobushi (tuna incubated with Aspergillus
Production of Alcoholic
Wines and champagnes
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)
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”
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
RackingCremoval of sediments accumulated
during the fermentation process
Brandy (burned wine) is made by distilling wine
to increase alcohol concentration; wine vinegar
is made by controlled microbial oxidation (by
Acetobacter or Gluconobacter) to produce acetic
acid from ethanol
For champagnes, fermentation is continued in
bottles to produce a naturally sparkling wine
Beers and ales
Malt is produced by germination of the
barley grains and the activation of their
 mash is produced from malt by enzymatic
starch hydrolysis to accumulate utilizable
 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
Beer is produced with a bottom yeast,
such as Saccharomyces carlsbergensis 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
Distilled spiritsCbeerlike fermented liquid
is distilled to concentrate alcohol;
type of liquor depends on composition of
starting mash;
flavorings can also be added;
a sour mash involving Lactobacillus
delbrueckii mediated fermentation is
often used
Production of breads
Aerobic yeast fermentation is used to
increase carbon dioxide production and
decrease alcohol production; other
metabolic products add flavors
Other microorganisms make special
breads, such as sourdough
Bread products can be spoiled by Bacillus
species that produce ropiness
Other fermented foods
Sufu, fermented tofu (a chemically
coagulated soybean milk product) and
tempeh, made from soybean mash, are
made by the action of molds
SauerkrautCfermented cabbage; involves
a microbial succession mediated by
Leuconostoc mesenteroides, Lactobacillus
plantarum, and Lactobacillus brevis
Pickles are cucumbers fermented in brine
by a variety of bacteria; fermentation
process involves a complex microbial
Silages–animal feeds produced by
anaerobic, lactic-type mixed
fermentation of grass, corn, and other
fresh animal feeds
Microorganisms as Foods
and Food Amendments
Microbes that are eaten include a variety
of bacteria, yeasts, and other fungi (e.g.,
mushrooms, Spirulina)
 the
addition of microorganisms to the diet in
order to provide health benefits beyond basic
nutritive value
also called microbial dietary adjuvants
 oligosaccharide
polymers that are not
processed until reaching the large intestine;
 often combined with probiotics to create a
synbiotic system
 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