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Chapter 41
Microbiology of Food
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Microorganism growth in foods
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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
• 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• fungal toxins contaminate grains
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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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Intercalateinto DNA,causingframeshiftmutations
aflatoxins
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Disrupt synthesis and metabolism of sphingolipids
Fumonisins
FB1: R = OHFB2: R = H
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Algal toxins
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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 to 115° C for 25 to 100 minutes
• Kills spoilage microbes, but not necessarily all microbes in food
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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
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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
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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
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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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Nucleic acid can be detectedeven when plaque-formingability 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
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Comparison of PCR sensitivity and growth for detection of Salmonella
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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 production
Milk
lactic acid bacteria and rennin Curd
removal of whey ripening by microbial action
Cheese
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Meat and fish
• 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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racking – 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
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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
• 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