This document discusses factors that influence microbial growth in foods and food spoilage. It covers intrinsic factors like composition, pH and water activity, and extrinsic factors like temperature and atmosphere. Various methods for controlling microbial growth and food spoilage are described, including removal of microorganisms, low temperature, high temperature processes like canning and pasteurization, controlling water availability, use of chemical preservatives, radiation, and surveillance programs. Fermented foods produced via lactic, propionic, and ethanolic fermentations are also summarized.

1. Food Microbiology
Lecture #9

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 – cow’s 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

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)

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

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 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

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

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-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

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.

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
(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

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