Lecture 18 Food Microbiology.ppt
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Transcript of Lecture 18 Food Microbiology.ppt
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Food Microbiology
Lecture #9
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Conditions for Spoilage
•Water•pH •Physical structure•Oxygen•temperature
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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 (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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Food Spoilage
•Approximately 1/3rd of all food manufactured in world is lost to spoilage•Microbial content of foods (microbial load): qualitative (which bugs) and quantitative (how many bugs)•Shelf life
•Non-perishable foods (pasta)•Semiperishable foods (bread)•Perishable foods (eggs)
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General Principles
•Minimize contamination by:•Good management processes•Acceptable sanitary practices•Rapid movement of food through processing plant•Well-tested preservation procedures
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Spoilage•Meat
•Cutting board contamination•Conveyor belts•Temperature•Failure to distribute quickly•Fecal bacteria from intestines
•Fish•Polluted waters•Transportation boxes
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Spoilage•Poultry and Eggs
•Human contact•Penetration by bacteria
•Milk and Dairy Products•Lactobacillus and Streptococcus species that survive pasturization (sour milk)
•Breads •Spores and fungi that survive baking
•Grains•Fungi produce toxins
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Food-Borne Diseases
• two primary types– food-borne infections– food intoxications
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Preventing Foodborne Disease
•Food infections (microbes are transferred to consumer)•Food poisoning (results from the toxin consumption)
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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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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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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– psychrophiles and psychrotrophs can still cause
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
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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
• Gamma radiation– use of ionizing radiation (gamma radiation) to
extend shelf life or sterilize meat, seafoods, fruits, and vegetables
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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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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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Helpful Suggestions
•Refrigerate quickly•Wash hands•Clean cutting boards•Leftovers•Avoid home-canned foods
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Microbiology of Fermented Foods
• major fermentations used are lactic, propionic, and ethanolic fermentations
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Fermentation
Any partial breakdown of carbohydrates taking place in the absence of oxygen.
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Meat and Fish
• sausages
• hams
• bologna
• salami
• izushi – fish, rice and vegetables
• katsuobushi – tuna
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Wine
White vs. Red: juice or juice and skin
Yeasts: Ferment when no oxygen around.Saccharomyces species
DrySweetSparklingFortified
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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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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