B.Sc. 1st year Microbiology

Assistant Professor- Shubham Shukla
Department of Microbiology

Moist Heat Sterilization

Moist heat sterilization uses hot water vapor (steam) or moist heat under pressure to destroy microorganisms by denaturation and coagulation of cellular proteins.
It is more efficient than dry heat because water conducts heat better, penetrates materials faster, and causes more rapid destruction of microorganisms and spores.

Principle of Moist Heat Sterilization

Moist heat kills microorganisms mainly by:

• Protein coagulation and denaturation: disrupts the three-dimensional structure of proteins, leading to loss of function and cell death.
• Enzyme inactivation: most cellular enzymes are heat-labile, and moist heat rapidly inactivates them.
• Membrane disruption: steam heat destroys lipid membranes, causing leakage of cell contents.
• Increased penetrability: steam has high latent heat, allowing rapid heat transfer and deeper penetration into materials.

Because of these effects, moist heat kills spores faster than dry heat.

Forms of Moist Heat Sterilization

Boiling (100°C)

• Kills vegetative bacteria, fungi, protozoa, and some viruses within minutes.
• Not reliable for sterilization because bacterial endospores and some viruses can survive.
• Used for disinfection of water, utensils, bedding.

Pasteurization

• Controlled heating process used to reduce microbial load in liquids without affecting flavor or quality.
• Methods:
• Low Temperature Long Time (LTLT): 63°C for 30 minutes
• High Temperature Short Time (HTST): 72°C for 15 seconds
• Ultra High Temperature (UHT): 135–150°C for 2–5 seconds
• Purpose: not sterilization, but disinfection/reduction.
• Used for milk, juices, beer, wine.

Tyndallization (Intermittent Sterilization)

• Heating at 100°C for 30–40 minutes for 3 consecutive days.
• Kills spores that germinate between heating cycles.
• Used for heat-sensitive culture media containing sugars or gelatin.

Autoclaving (Steam Under Pressure)

• Considered the gold standard for moist heat sterilization.
• Standard conditions: 121°C, 15 psi (1.05 kg/cm²), 15–20 minutes exposure time.
• Other conditions for heavy loads:
• 134°C for 3–5 minutes
• 126°C for 10 minutes
• Mechanism: pressurized steam increases the boiling point of water, ensuring superheated steam, allowing rapid penetration and destruction of microbes.
• Applications: culture media, glassware, surgical instruments, dressings, contaminated waste, fluids and solutions.

Advantages

• More efficient than dry heat
• Lower temperatures and shorter times required
• Penetrates porous materials effectively
• Reliable and widely applicable
• Effective against endospores

Disadvantages

• Not suitable for moisture-sensitive materials such as powders and oils
• Can cause corrosion of metal instruments if improperly maintained
• Plastics and heat-labile items may melt or deform
• Requires careful monitoring of pressure and temperature