Food Microbiology: Factors Influencing Microbial Growth

Overview

• Discussion on food microbiology and factors influencing microbial growth.
• Focus on intrinsic and extrinsic factors affecting microbial growth in food.

Definitions

• Intrinsic Factors: Essential properties inherent to the food itself.
  • Examples: pH, water activity, nutrient content, redox potential, antimicrobial constituents.
• Extrinsic Factors: Environmental conditions where food is stored.
  • Examples: temperature, relative humidity, gaseous atmosphere.

Intrinsic vs. Extrinsic Factors

• Intrinsic Factors:

  • Nutrient content
  • pH
  • Redox potential
  • Water activity
  • Antimicrobial constituents
  • Barriers o Example: Dry fruits have low water activity compared to cooked rice.

• Extrinsic Factors:

  • Temperature of storage
  • Relative humidity
  • Gaseous atmosphere
  • Growth rate

Temperature of Storage

• Microorganisms can grow in a wide range of temperatures.
• Classification of Microorganisms by Temperature:
  • Mesophiles: Optimum temperature around 37°C.
  • Thermophiles: Optimum temperature between 45-70°C (e.g., Bacillus, Clostridium).
  • Extreme Thermophiles: Optimum temperature 80°C or higher, can survive up to 115°C.
  • Psychrophiles: Cold-loving, can grow at 0°C.
  • Psychrotrophs: Optimum growth above 10°C, can grow below 7°C.

Importance of Mesophiles and Psychrotrophs

• Mesophiles are significant for food spoilage at 25-37°C.
• Psychrotrophs can spoil food in refrigerated environments, despite slower growth rates.

Relative Humidity

• Relationship with Water Activity:

  • Low water activity (aw) food stored in high humidity can absorb moisture, increasing water activity and promoting spoilage.
  • Higher temperature typically requires lower relative humidity to prevent spoilage.

• Storage Conditions:

  • Foods prone to spoilage (e.g., molds, yeast) should be stored in low humidity.
  • Proper wrapping can help maintain low humidity conditions.

Best Practices

• Avoid excessive temperature variation during storage to prevent condensation.

Gaseous Atmosphere

• Role of Oxygen: Influences microbial growth and redox potential.
• Carbon Dioxide (CO2): Inhibitory effect on microbial growth; often used in controlled atmosphere packaging.
  • Effects of CO2:
    • Effective against molds and gram-negative bacteria.
    • Gram-positive bacteria (e.g., Lactobacilli) show more resistance.
    • Some organisms, like Brettanomyces, can tolerate high CO2 levels.

Mechanism of CO2

• Bacteriostatic effect: inhibits growth but does not kill microbes.
• Presence of CO2 can lower pH of food, affecting microbial activity.

Conclusion

• Focused on extrinsic factors affecting microbial growth.
• Intrinsic factors to be covered in the next part.