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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
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