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Chapter 41
Microbiology of Food
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Microorganism Growth in Foods
Figure 41.1
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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
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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
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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
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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
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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
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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
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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
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Figure 41.2
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Toxins• ergotism
– toxic condition caused by growth of a fungus in grains
• aflatoxins– carcinogens produced in fungus-
infected grains and nut products
• fumonisins– carcinogens produced in fungus-
infected corn
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Figure41.4
intercalateinto DNA,causingframeshiftmutations
aflatoxins
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Figure 41.5
disrupt synthesis and metabolismof sphingolipids
fumonisins
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Controlling Food Spoilage
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Removal of Microorganisms
• usually achieved by filtration
• commonly used for water, beer, wine, juices, soft drinks, and other liquids
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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
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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 Figure 41.6
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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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Water Availability
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Chemical-Based Preservation
• GRAS– chemical agents “generally recognized
as safe”
• pH of food impacts effectiveness of chemical preservative
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Radiation• ultraviolet (UV) radiation
– used for surfaces of food-handling equipment
– does not penetrate foods
• radappertization– use of ionizing radiation (gamma radiation)
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
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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– 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
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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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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
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comparison of PCR andgrowth for detection of Salmonella
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Figure 41.7
nucleic acid can be detectedeven when plaque-formingability is lost
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Surveillance for food-bornedisease• 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 Productionmilk
lactic acid bacteria and rennincurd
removal of whey ripening by microbial action
cheese
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Meat and Fish
• sausages
• hams
• bologna
• salami
• izushi – fish, rice, and vegetables
• katsuobushi – tuna
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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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Figure 41.15racking – removes sediments
or
microbial oxidation of ethanol to acetic acid yields wine vinegar
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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
Figure 41.16
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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
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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
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Other Fermented Foods
• silages– fermented grass, corn, and other fresh
animal feeds
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Figure 41.18
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
• 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
• immunodilation• control of diarrhea• anticancer effects• possible modulation of Crohn’s Disease• in beef cattle,
– decrease E. coli
• in poultry,– limit 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
• symbiotic system – combination of prebiotics and probiotics– results in production of certain acids that
may be responsible for possible beneficial effects of probiotics