Food Microbiology

1
Lecture 9
Food Microbiology
2
Conditions for Spoilage

• Water
• pH
• Physical structure
• Oxygen
• temperature

3
Microorganism Growth in Foods
4
Intrinsic Factors

• composition
• pH
• presence and availability of water
• oxidation-reduction potential
• altered by cooking
• physical structure
• presence of antimicrobial substances

5
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

6
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

7
Physical structure

• grinding and mixing increase surface area and
  distribute microbes
• promotes microbial growth
• outer skin of vegetables and fruits slows
  microbial growth

8
Antimicrobial substances

• coumarins fruits and vegetables
• lysozyme cows milk and eggs
• aldehydic and phenolic compounds herbs and
  spices
• allicin garlic
• polyphenols green and black teas

9
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

10
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

11
(No Transcript)
12
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)

13
General Principles

• Minimize contamination by
• Good management processes
• Acceptable sanitary practices
• Rapid movement of food through processing plant
• Well-tested preservation procedures

14
Spoilage

• Meat
• Cutting board contamination
• Conveyor belts
• Temperature
• Failure to distribute quickly
• Fecal bacteria from intestines
• Fish
• Polluted waters
• Transportation boxes

15
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

16
Food-Borne Diseases

• two primary types
• food-borne infections
• food intoxications

17
Preventing Foodborne Disease

• Food infections (microbes are transferred to
  consumer)
• Food poisoning (results from the toxin
  consumption)

18
(No Transcript)
19
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

20
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

21
(No Transcript)
22
Controlling Food Spoilage
23
Removal of Microorganisms

• usually achieved by filtration
• commonly used for water, beer, wine, juices, soft
  drinks, and other liquids

24
Low Temperature

• refrigeration at 5C retards but does not stop
  microbial growth
• psychrophiles and psychrotrophs can still cause
  spoilage
• growth at temperatures below -10C has been
  observed

25
High Temperature

• canning
• pasteurization

26
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

27
Spoilage of canned goods

• spoilage prior to canning
• underprocessing
• leakage of contaminated water into cans during
  cooling process

28
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

29
Water Availability
30
Chemical-Based Preservation

• GRAS
• chemical agents generally recognized as safe
• pH of food impacts effectiveness of chemical
  preservative

31
(No Transcript)
32
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

33
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

34
comparison of PCR and growth for detection of
Salmonella
35
nucleic acid can be detected even when
plaque-forming ability is lost
36
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

37
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

38
Helpful Suggestions

• Refrigerate quickly
• Wash hands
• Clean cutting boards
• Leftovers
• Avoid home-canned foods

39
Microbiology of Fermented Foods

• major fermentations used are lactic, propionic,
  and ethanolic fermentations

40
Fermentation
Any partial breakdown of carbohydrates taking
place in the absence of oxygen.
41
(No Transcript)
42
(No Transcript)
43
Meat and Fish

• sausages
• hams
• bologna
• salami
• izushi fish, rice and vegetables
• katsuobushi tuna

44
Wine
White vs. Red juice or juice and skin Yeasts
Ferment when no oxygen around. Saccharomyces
species Dry Sweet Sparkling Fortified
45
Production of Breads

• involves growth of Saccharomyces cerevisiae
  (bakers 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

46
Other Fermented Foods

• silages
• fermented grass, corn, and other fresh animal
  feeds

47
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