Transcript Chapter 41

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Chapter 41
Microbiology of Food
1
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Microorganism growth
in foods
2
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Intrinsic factors
•
•
•
•
Composition
pH
Presence and availability of water
Oxidation-reduction potential
– altered by cooking
• Physical structure
• Presence of antimicrobial substances
3
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Composition and pH
• Putrefaction
– proteolysis and anaerobic breakdown of
proteins, yielding foul-smelling amine
compounds
• pH impacts make up of microbial
community and therefore types of
chemical reactions that occur when
microbes grow in food
4
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Water availability
• 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 water activity
5
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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
7
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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
8
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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
• use of shrink wrap and vacuum technologies to
package food in controlled atmospheres
9
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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
• algal toxins may contaminate shellfish and finfish
• fungal toxins contaminate grains
10
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11
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Toxins
• Ergotism
– toxic condition caused by growth of a fungus,
Claviceps purpura, in grains
(← production of hallucinogenic alkaloids)
• Aflatoxins
– carcinogens produced in fungus-infected grains
and nut products (Aspergillus flavus)
• Fumonisins
– carcinogens produced in fungus-infected corn
(Fusarium moniliforme)
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aflatoxins
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Intercalate
into DNA,
causing
frameshift
mutations
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Fumonisins
FB1: R = OH
FB2: R = H
Disrupt synthesis and metabolism of sphingolipids
15
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Algal toxins
16
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Controlling food spoilage
17
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Removal of microorganisms
• Usually achieved by filtration
• Commonly used for water, beer, wine,
juices, soft drinks, and other liquids
18
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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
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High temperature
• Canning
• Pasteurization
20
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Canning
• Food heated in
special containers
to 115° C for 25 to
100 minutes
• Kills spoilage
microbes, but not
necessarily all
microbes in food
21
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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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Chemical-based preservation
• GRAS
– chemical agents “generally recognized
as safe”
• pH of food impacts effectiveness of
chemical preservative
24
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25
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Radiation
• Ultraviolet (UV) radiation
– used for surfaces of food-handling
equipment
– does not penetrate foods
• Radappertization
– use of ionizing radiation (e.g. gamma ray) 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
26
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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
- produced by some strains of Streptococcus lactis
- used in low-acid foods to inactivate Clostridium
botulinum during canning process
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Food-borne diseases
• Two primary types
– food-borne infections
– food intoxications
28
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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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30
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Nucleic acid can be detected
even when plaque-forming
ability is lost
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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
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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
33
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Comparison of PCR sensitivity and growth
for detection of Salmonella
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Surveillance for food-borne
disease
• 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
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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
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Microbiology of fermented
foods
• Major fermentations used are lactic,
propionic, and ethanolic fermentations
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Fermented milks
• Mesophilic – Lactobacillus and Lactococcus
• Thermophilic – Lactobacillus and
Streptococcus
• Therapeutic – Lactobacillus and
Bifidobacterium
• Yeast lactic – yeasts, lactic acid bacteria, and
acetic acid bacteria
• Mold lactic – filamentous fungi and lactic
acid bacteria
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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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41
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Meat and fish
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Sausages
Hams
Bologna
Salami
Izushi (fermentation of fish, rice, and
vegetables by Lactobacillus spp.)
• Katsuobushi (fermentation of tuna by
Aspergillus glaucus)
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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 heating mixture
• yields wort – clear liquid containing fermentable
carbohydrates
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or
racking – removes sediments
Microbial oxidation of ethanol to acetic acid yields wine vinegar
44
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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
• Bottom yeasts
– used in production of beers
• Top yeasts
– used in production of ales
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pitched
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Distilled spirits
• Similar to beer-making process
– usually begins with ‘sour mash’
• mash inoculated with homolactic
bacterium: lower the mash pH
– following fermentation, is distilled to
concentrate alcohol
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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
causes ropiness
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Other fermented foods
• Silages
– fermented grass, corn, and other fresh
animal feeds
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Making sauerkraut
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Microorganisms as foods
and food amendments
• Variety of bacteria, yeasts, and other
fungi are used as animal and human food
sources
- mushrooms, Spirulina, etc.
• Probiotics
– microbial dietary adjuvants
– microbes added to diet in order to provide
health benefits beyond basic nutritive value
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Benefits of probiotics
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Immunodilation
Control of diarrhea
Anticancer effects
Possible modulation of Crohn’s Disease
In beef cattle,
– Lactobacillus acidophilus decrease E. coli
• In poultry,
– Bacillus subtilis limits Salmonella colonization
of 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 an increase in production of
butyric acid and propionic acids that may be
responsible for possible beneficial effects of
probiotics
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