Lecture 18 Food Microbiology.ppt
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Lecture 18 Food Microbiology.ppt






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Lecture 18 Food Microbiology.ppt Lecture 18 Food Microbiology.ppt Presentation Transcript

  • Lecture #9 Food Microbiology
  • Conditions for Spoilage
    • Water
    • pH
    • Physical structure
    • Oxygen
    • temperature
  • Microorganism Growth in Foods
  • Intrinsic Factors
    • composition
    • pH
    • presence and availability of water
    • oxidation-reduction potential
      • altered by cooking
    • physical structure
    • presence of antimicrobial substances
  • 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
  • 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
  • Physical structure
    • grinding and mixing increase surface area and distribute microbes
      • promotes microbial growth
    • outer skin of vegetables and fruits slows microbial growth
  • 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
  • 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
  • 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
  • Food Spoilage
    • Approximately 1/3 rd 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)
  • General Principles
    • Minimize contamination by:
      • Good management processes
      • Acceptable sanitary practices
      • Rapid movement of food through processing plant
      • Well-tested preservation procedures
  • Spoilage
    • Meat
      • Cutting board contamination
      • Conveyor belts
      • Temperature
      • Failure to distribute quickly
      • Fecal bacteria from intestines
    • Fish
      • Polluted waters
      • Transportation boxes
  • 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
  • Food-Borne Diseases
    • two primary types
      • food-borne infections
      • food intoxications
  • Preventing Foodborne Disease
    • Food infections (microbes are transferred to consumer)
    • Food poisoning (results from the toxin consumption)
  • 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
  • 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
  • Controlling Food Spoilage
  • Removal of Microorganisms
    • usually achieved by filtration
    • commonly used for water, beer, wine, juices, soft drinks, and other liquids
  • 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
  • High Temperature
    • canning
    • pasteurization
  • 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
  • Spoilage of canned goods
    • spoilage prior to canning
    • underprocessing
    • leakage of contaminated water into cans during cooling process
  • 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
  • Water Availability
  • Chemical-Based Preservation
    • GRAS
      • chemical agents “generally recognized as safe”
    • pH of food impacts effectiveness of chemical preservative
  • 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
  • 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
  • comparison of PCR and growth for detection of Salmonella
  • nucleic acid can be detected even when plaque-forming ability is lost
  • 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
  • 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
  • Helpful Suggestions
    • Refrigerate quickly
    • Wash hands
    • Clean cutting boards
    • Leftovers
    • Avoid home-canned foods
  • Microbiology of Fermented Foods
    • major fermentations used are lactic, propionic, and ethanolic fermentations
  • Fermentation Any partial breakdown of carbohydrates taking place in the absence of oxygen.
  • Meat and Fish
    • sausages
    • hams
    • bologna
    • salami
    • izushi – fish, rice and vegetables
    • katsuobushi – tuna
  • Wine White vs. Red: juice or juice and skin Yeasts: Ferment when no oxygen around. Saccharomyces species Dry Sweet Sparkling Fortified
  • Production of Breads
    • involves growth of Saccharomyces cerevisiae (baker’s yeast) under aerobic conditions
      • maximizes CO 2 production, which leavens bread
    • other microbes used to make special breads (e.g., sourdough bread)
    • can be spoiled by Bacillus species that produce ropiness
  • Other Fermented Foods
    • silages
      • fermented grass, corn, and other fresh animal feeds
  • 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