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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

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Lecture #9

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lecture 9
Lecture #9

Food Microbiology

conditions for spoilage
Conditions for Spoilage
  • Water
  • pH
  • Physical structure
  • Oxygen
  • temperature
intrinsic factors
Intrinsic Factors
  • composition
  • pH
  • presence and availability of water
  • oxidation-reduction potential
    • altered by cooking
  • physical structure
  • presence of antimicrobial substances
composition and ph
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
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
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
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
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
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
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)
general principles
General Principles
  • Minimize contamination by:
    • Good management processes
    • Acceptable sanitary practices
    • Rapid movement of food through processing plant
    • Well-tested preservation procedures
  • Meat
    • Cutting board contamination
    • Conveyor belts
    • Temperature
    • Failure to distribute quickly
    • Fecal bacteria from intestines
  • Fish
    • Polluted waters
    • Transportation boxes
  • 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
Food-Borne Diseases
  • two primary types
    • food-borne infections
    • food intoxications
preventing foodborne disease
Preventing Foodborne Disease
  • Food infections (microbes are transferred to consumer)
  • Food poisoning (results from the toxin consumption)
food borne intoxications
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
  • 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
removal of microorganisms
Removal of Microorganisms
  • usually achieved by filtration
  • commonly used for water, beer, wine, juices, soft drinks, and other liquids
low temperature
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
High Temperature
  • canning
  • pasteurization
  • 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 of canned goods
  • spoilage prior to canning
  • underprocessing
  • leakage of contaminated water into cans during cooling process
  • 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
chemical based preservation
Chemical-Based Preservation
  • GRAS
    • chemical agents “generally recognized as safe”
  • pH of food impacts effectiveness of chemical preservative
  • 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
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



nucleic acid can be detected

even when plaque-forming

ability is lost

surveillance for food borne disease
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
  • 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
Helpful Suggestions
  • Refrigerate quickly
  • Wash hands
  • Clean cutting boards
  • Leftovers
  • Avoid home-canned foods
microbiology of fermented foods
Microbiology of Fermented Foods
  • major fermentations used are lactic, propionic, and ethanolic fermentations

Any partial breakdown of carbohydrates taking place in the absence of oxygen.

meat and fish
Meat and Fish
  • sausages
  • hams
  • bologna
  • salami
  • izushi – fish, rice and vegetables
  • katsuobushi – tuna

White vs. Red: juice or juice and skin

Yeasts: Ferment when no oxygen around.

Saccharomyces species





production of breads
Production of Breads
  • involves growth of Saccharomycescerevisiae (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
other fermented foods
Other Fermented Foods
  • silages
    • fermented grass, corn, and other fresh animal feeds
microorganisms as foods and food amendments
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