Lecture Notes: Methods to Control Microbial Growth

Heat as a Method of Microbial Control

• Heat Usage: Common method to kill microorganisms by denaturing proteins.
• Terms:
  • Decimal Reduction Time (D-Value): Time required to kill 90% of organisms at a specific temperature (e.g., 150°C for 30 minutes).
  • Thermal Death Point/Time: Temperature and time to kill microorganisms using heat.

Types of Heat

• Dry Heat:

  • Used for metal instruments, glassware, oils, and powders.
  • Dry heat ovens are slower but effective for items that can't get wet.
  • Example: Used in industrial settings like B Lam for equipment cleaning.

• Moist Heat (Autoclaving):

  • Common in labs for sterilizing equipment and media.
  • Uses steam under pressure; effective for items that can get wet.
  • Found in various sizes—from tabletop models in dental offices to large industrial applications.
  • Validation: Regular checks using ampules to ensure proper sterilization.

Pasteurization

• Heats liquids to reduce microbial load without sterilizing (e.g., milk).
• Different methods vary by time and temperature.

Refrigeration and Freezing

• Refrigeration: Slows down microbial growth, extending the life of food and specimens.
• Freezing: Preserves items for longer periods; used in labs for microbe storage.

Drying and Lyophilization

• Drying: Inhibits growth, but some bacteria can survive (e.g., endospores).
• Freeze Drying (Lyophilization): Combination of freezing and drying; used for long-term preservation.

Radiation

• UV Light:

  • Damages proteins and nucleic acids.
  • Used to decontaminate surfaces and air but has limited penetration.
  • Useful in biosafety cabinets and labs.

• Gamma Radiation:

  • Uses Cobalt 60; effective for food, pharmaceuticals, and medical devices.
  • Creates ions that dislodge electrons, sterilizing the product.

• Microwave Radiation: Sterilizes if the material contains water.

Other Methods

• Ethylene Oxide (EtO) Gas: Disrupts cell metabolism; used for sterilizing breathable packaging.
• E-beam (Electronic Beam): Faster than gamma radiation but less penetrative.
• Chlorine Dioxide Gas: Strong oxidizer; used for water supplies and foods.

Filtration

• Used for sterilizing heat-sensitive liquids.
• Utilizes filters with tiny pores to remove infectious agents.
• Applied in both lab and pharmaceutical settings.

Osmotic Pressure

• High concentrations of sugar or salt draw water out of bacteria, inhibiting growth.
• Traditional method for food preservation (e.g., salted meats, jellies, jams).