Factors affecting Microbial Growth - Food Microbiology - Lecture Slides, Slides of Microbiology

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Food: what sorts of microbes live

there, and how?

How do we know? How do we find out? What are the factors affecting microbial growth?

See Fig. 2.1 in textbook

Detection and enumeration methods are

artificial

• Climate control
• Microbial growth media (liquid or solid)
• Isolation/pure culture techniques
• Enumeration methods may require dilution or enrichment

Intrinsic factors: pH

• What is pH?
• Hydrogen ion concentration in a solution
• Pure water is “neutral” (pH7)
• pH of acid lower than 7, base higher than 7
• Most cells have internal pH around 7

Significance of food (environmental) pH

• Most foodborne pathogens cannot grow at pH of less than 4 (very acidic environment). They either die or cannot grow
• Fungi and lactic acid bacteria can grow at low pH
• “Successful” organisms have specific processes to adapt to acid pH (genetic)
• Effect of “wrong” pH on microbes?

Intrinsic factor: water activity (aw )

A few words about water

• All living things require water
• Cohesion
• High specific heat
• Density of ice
• Solvent

Aw ranges for different foods

• 0.10-0.
  • Cereals, crackers, salt, sugar
• <0.
  • Noodles, honey, chocolate
• 0.60-0.
  • Jams and jellies, nuts, dried fruits, dry cheeses
• 0.85-0.
  • Fermented sausages, dried cured meats, maple syrup
• 0.93-0.
  • Evaporated milk, bread, fresh sausage, processed cheese, fruit juice
• 0.98-0.
  • Fresh meat, fruits, vegetables, milk, eggs

Can you change the aw? Sure!

• Hydrating
• Freezing; drying
• Adding solutes
• Treatments to control microbial growth might be needed if aw is very high - Adding preservatives - Antimicrobial treatment, e.g., Pasteurization - Adjusting pH

Aw requirements for foodborne microbes

• Most Gram-negative microbes require a high aw (more than 0.93)
• Staphylococcus can grow at a low aw but won’t produce toxin
• Spore formers are less sensitive
• Molds (especially) and yeast can grow at a lower aw than bacteria
• See Table 2.6 for actual values

Cigna.com

Extrinsic factors: Temperature

A few words about microbial metabolism

• Living organisms generate ATP to do work (convert one form of energy to another)
• Series of chemical reactions that move electrons around
• Some organisms use oxygen as a terminal electron acceptor (aerobes)
• Some cannot use oxygen but use something else (anaerobes, fermenters)
• Some can use either (facultative anaerobes)

Comparison of different types of

respiratory pathways