Microbial Growth Control: Methods and Effectiveness
Microbial Growth Control
Rate of Microbial Death
Bacterial populations decrease at a constant logarithmic rate.
Effectiveness of Antimicrobial Treatments
- Number of Microbes: An increase in the number of microbes requires a longer time to eliminate the entire population.
- Environmental Influences:
- The presence of organic matter can inhibit the action of chemical antimicrobials.
- Microbes in surface biofilms are difficult for biocides to reach effectively.
- Temperature-dependent chemical reactions affect disinfectant efficacy; disinfectants sometimes work better in warm conditions.
- Fats and proteins are protective, therefore microbes have a higher survival rate.
- Heat is more effective in acidic conditions.
- Time of Exposure: Extended exposure is required to affect resistant microbes and endospores.
- Microbial Characteristics: Microbial characteristics affect the choice of chemical or physical method.
Actions of Microbial Control Agents
- Alteration of Membrane Permeability: Damage to proteins or fats of the plasma membrane causes cellular contents to leak into the surrounding medium, interfering with growth.
- Damage to Proteins and Nucleic Acids: Breakage of bonds by heat or chemicals causes protein denaturation. Damage to nucleic acids by heat, radiation, or chemicals prevents cell replication and function.
Physical Methods of Microbial Control
Heat
- Thermal Death Point (TDP): The lowest temperature at which all cells in a culture are killed in 10 minutes.
- Thermal Death Time (TDT): The time required to kill all cells in a culture.
Atmospheric Pressure and Altitude
- Atmospheric pressure drops at higher altitudes.
- Therefore, the pressure on autoclaves needs to be set higher than at sea level.
Pasteurization
- Reduces spoilage organisms and pathogens.
- 63°C for 30 minutes for milk.
- High Temperature, Short Time (HTST): 72°C for 15 seconds.
Sterilization
- Ultra-High Temperature (UHT): 140°C for <1 second.
- Heat-resistant organisms may survive.
Dry Heat Sterilization
- Kills by oxidation.
- Flaming.
- Incineration.
- Hot-air sterilization.
Cold
- Low temperature inhibits microbial growth.
- Refrigeration.
- Deep freezing.
- Dormant state.
- Slow freezing causes ice crystals to form and grow, disrupting bacterial cellular and molecular structures.
- Thawing is even more damaging.
- Freeze-thaw cycles are extremely damaging.
High Pressure
- Denatures proteins and alters carbohydrates.
- Kills endospores.
- Preserves flavor, color, and nutrients.
- Used for treating fruit juices in Japan and the US.
Desiccation
- Removal of water.
- Stops metabolic activities.
- Freeze-drying (lyophilization) used for coffee and food additives for dry cereals.
Osmotic Pressure
- High concentrations of salts and sugars (hypertonic).
- Causes water to leave cells.
- Molds and yeasts are more resistant and can spoil fruits and grains preserved in this way.
Filtration
- Removes microbes.
- Heat-sensitive solutions are filtered through membrane filters (same as those used for enumeration, 0.22 or 0.45 μm pore size).
- Air is filtered through high-efficiency particulate air (HEPA) filters (0.3 μm pore size).
Radiation
- Radiation damages DNA.
- Ionizing radiation (X-rays, gamma rays, electron beams).
- Nonionizing radiation (UV).
- Microwaves kill by heat; not especially antimicrobial.
Ionizing Radiation (X-rays, Gamma Rays, Electron Beams)
- Carry more energy.
- Can pass through solids.
- Low-level ionizing radiation is approved for processing certain spices, meats, and vegetables.
- High-energy electron beams are used for sterilizing disposable medical supplies.
Nonionizing Radiation (UV)
- UV light damages DNA by cross-linking.
- Used for sterilizing surfaces in hospitals.
- Cannot pass through solids, so anything covered is unaffected.
Microwaves
- Kill by heating up water in foods.
- Otherwise, not especially antimicrobial.
- Heat solid foods unevenly.
- Due to uneven distribution of moisture, foodborne parasites may not be affected.