FOOD MICROBIOLOGY

1. History
Early Food Preservation 900 AD –
“Food Poisoning” Recognized
1795-Appert Developed Canning
1854-1864- Food Microbiology
Becomes A Science- Louis Pasteur
Why Study Food Microbiology?
• Provide Clean, Safe, Healthful Food to Consumer
•Food Permits and Control Microbial Growth
•Prevent Food Spoilage and Food borne illness

2. Factors Influencing Growth of Microorganisms
in Food
 The factors influence microbial growth and maintain
the food quality in production as well as preservation
 Conditions naturally present in food termed intrinsic
factors
 Environmental conditions are termed extrinsic
factors

3. Intrinsic Factors
 pH
 Moisture Content
 Oxidation-reduction Potential
 Nutrient Content
 Antimicrobial Constituents
• Multiplication of organisms in food greatly influenced
by inherent characteristics of food (moist, nutritionally
rich, pH neutral)

4. Water availability
In general, lower water activity inhibits microbial growth
Foods have different water availability
 Fresh meats & milk have high water content-Supports
microbial growth
 Breads, nuts and dried foods have low water availability
 Defined populations can grow in these specific environments
 Water activity (aw) used to designate the amount of water
available in foods
 Pure water has aw of 1.0
 Most of the bacteria require aw of above 0.90
 Most of the fungi require aw of above 0.80

5.  Remove and/or Bind Moisture
 Humectants and Dehydration
 Most Fresh Foods - aW > 0.99,
 Lowest aW for Pathogen 0.86 (Staphylococcus aureus)
 Halophilic 0.75
 Xerophilic molds 0.61
 Osmophilic yeasts 0.61
0 1 Pure waterNo water
Water activity lowered by: 1. Drying 2. Addition of salt or
sugar
Osmophilic microorganisms- prefer high osmotic pressure
Xerophilic microorganisms - prefer low water activity

6.  pH
 Important in determining which organisms can survive and thrive on
specific foods
 Many microorganisms inhibited by acid conditions- Exception Lactic acid
bacteria
 Lactic acid bacteria are used in fermentation process and other foods
(unpasteurized milk)
 Fungi are able to survive at relatively low pH
 Most acid foods spoil by fungal contamination
 pH can determine the bacteria’s ability to produce toxin
 Toxin production of many organisms is inhibited by acid pH

7. EFFECTS OF pH
 Enzymes
 Nutrients
 Other Environmental Factors
 Temperature
 Salt
 No known pathogen grows below pH of 4.6
Clostridium botulinum

8. Oxidation- Reduction Potential
 O/R Potential – Eh Value
 “…ease with which the substrate loses or gains
electrons.”
 Loss of electrons – oxidized Gain of electrons – reduced
 Aerobic, Anaerobic, Microaerobic and Facultative anaerobes
Physical structure
 grinding and mixing increase surface area and
distribute microbes
 promotes microbial growth
 outer skin of vegetables and fruits slows microbial
growth

10. Nutrients
 Nutrients present in food - determine which
organisms can grow in foods
Biological barriers
 outer coverings help to protect foods from
microbial invasion
 Microorganisms will eventually breakdown
the coverings and cause spoilage in some
times
Antimicrobial chemicals
 Some foods contain natural antimicrobial
chemicals that inhibit growth of organisms
responsible for spoilage

11. Extrinsic factors
 Extent of microbial growth is depend on storage of food
 Microbes multiply rapidly in warm, oxygen-rich environments
 Preventing growth and metabolic activities of organisms that preserves
food quality
Temperature : lower temperatures retard microbial Growth
Relative humidity : higher levels promote microbial growth
Atmosphere :
• oxygen promotes growth
• modified atmosphere packaging (MAP)
• use of vacuum technologies to pack the food

13. Removal of Microorganisms
•Usually achieved by filtration
•Commonly used in water, beer, juices, wine, soft drinks and
other liquid treatment
•Preservation methods include
Canning
Pasteurization
Cooking
Refrigeration
Freezing
drying,/reducing water availability

14. Temperature
 Microorganisms grow over a wide range of
Temperatures
 Psychrotrophs
 Mesophiles
 Thermophiles
 Psychroduric
 Thermoduric

16.  Heating foods under controlled conditions at high
temperatures for short periods (HTST – 71.30 C FOR 10
min), LTHT- 62.30 C for 2 min and UHT 1300C for 2 sec
 Reduces number of spoilage organisms
 Does not alter taste of food significantly
Cooking
• Can destroy non-spore forming organisms
• Alters characteristics of food
• If heat is uneven some organisms may survive in undercooked portion of
food

17. Refrigeration
•Preserves food by slowing growth rate of spoilage
organisms
• Many organisms unable to multiply in low
temperatures

18.  Freezing
 Stops microbial growth
 Water unavailable due to ice formation
 Portion of organisms can grow when food is thawed
 Drying/reducing water availability
 Inhibits microbial growth by decreasing available
moisture
 Molds may grow eventually

20. Chemical-Based Preservation
GRAS- chemical agents “generally recognized as safe” pH of
food impacts effectiveness of chemical preservative