Principle and Influencing Factors of Moist Heat Sterilization

News 8 5 月, 2025

Principle of Moist Heat Sterilization:

Moist heat sterilization works by causing denaturation of proteins and nucleic acids in microorganisms, leading to their death. This denaturation begins with the breaking of hydrogen bonds, which disrupts the internal structure of proteins and nucleic acids, rendering them non-functional.

Moist heat sterilization uses saturated steam, boiling water, or flowing steam to achieve sterilization. Thanks to steam’s high latent heat and strong penetrating power, it efficiently denatures and coagulates proteins, leading to microbial death. This method is more effective than dry heat sterilization and is widely used for its reliability, ease of operation, controllability, and cost-effectiveness, making it the most common sterilization technique in pharmaceutical production.

Moist heat achieves sterilization at lower temperatures than dry heat because:

1. Water makes proteins absorb moisture, making them easier to denature.
2. Water molecules penetrate materials better than air, allowing more uniform heat transfer.
3. Steam releases 529 calories of heat per gram when condensing from gas to liquid, rapidly raising the object’s temperature.

A typical moist heat sterilization is performed at 121°C for 20–30 minutes. For spore-forming microbes, an initial sterilization is followed by incubation at an appropriate temperature for several hours to allow spore germination, then sterilized again to kill the germinated spores.

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Types of Moist Heat Sterilization:

1. Flowing Steam Sterilization:
   Conducted at 100°C atmospheric pressure with flowing steam for 30–60 minutes. Suitable for disinfecting heat-sensitive products but doesn’t reliably kill spores.
2. Intermittent Steam Sterilization (Tyndallization):
   Uses repeated steam exposure (3+ times) at 100°C to kill microbes, including spores. Between cycles, items are incubated at 37°C to encourage spore germination. Lower temperatures (75–80°C) and longer times can be used for sensitive materials like sugar- or milk-based media.
3. Autoclave (High-Pressure Steam Sterilization):
   Steam at 103.4 kPa pressure, 121.3°C for 15–20 minutes. The standard, reliable sterilization method.
4. Pasteurization:
   Uses mild heat to kill pathogens without significantly altering product quality. Commonly used for milk and fermented products, it lowers pathogen levels to safe limits but doesn’t achieve full sterilization.

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Factors Influencing Moist Heat Sterilization:

1. Type and Number of Bacteria:
   Different bacteria and growth phases vary in heat resistance. Fewer bacteria = shorter sterilization time.
2. Steam Quality:

• Saturated steam: high heat content and penetration, best sterilization.
• Wet saturated steam: lower heat content, poorer penetration, reduced effectiveness.
• Superheated steam: higher temperature but poor penetration, lower sterilization efficiency.

3. Medium Composition:
   Substances like sugars and proteins protect bacteria from heat. Microbial heat resistance is highest at neutral pH, lower in alkaline, and lowest in acidic environments.
4. Material Properties & Time:
   Higher temperatures shorten sterilization time. Standard: 121°C for 20–30 minutes. For spore-formers, add an incubation phase before re-sterilizing. Adjust temperature/time if material stability is an issue, while ensuring sterilization efficacy.