Principle and Working of Autoclave
Principle and Working of Autoclave
1. Principle
The autoclave works on the principle of moist heat sterilization using saturated steam under pressure.
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When pressure is applied to water, its boiling point increases.
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At 121 °C (15 psi / 1.05 kg/cm² above atmospheric pressure), saturated steam can kill all forms of microorganisms including resistant bacterial spores within a defined exposure time (usually 15–20 minutes).
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Mechanism of action:
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Steam penetrates the material.
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The condensation of steam on a cooler surface releases latent heat.
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This latent heat rapidly coagulates and denatures microbial proteins and enzymes, leading to cell death.
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Compared to dry heat, moist heat is more effective because steam transfers heat much faster and requires lower temperatures and less time.
2. Working of Autoclave
The working cycle of a steam sterilizer (autoclave) typically includes:
a. Loading
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Materials to be sterilized are loaded.
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Items should not be tightly packed to allow free circulation of steam.
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Indicators (biological, chemical, or physical) are placed inside to validate sterilization.
b. Air Removal
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Air inside the chamber must be completely displaced for effective sterilization.
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Methods:
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Gravity displacement: Steam enters from the top, pushing air out through a drain.
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Pre-vacuum: A vacuum pump removes air before steam entry (faster and more reliable).
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c. Sterilization (Holding) Phase
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Steam fills the chamber.
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Temperature rises to 121–134 °C at 15–30 psi pressure.
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Exposure time (commonly 15–30 minutes depending on load type) is maintained.
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At this stage, microorganisms are killed by moist heat protein denaturation.
d. Exhaust / Depressurization
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After holding time, steam is released.
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Pressure gradually returns to atmospheric level.
e. Drying
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Residual moisture is removed either by continued heating or vacuum drying.
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Especially important for surgical instruments and sterile equipment.
f. Unloading
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Once temperature and pressure normalize, the sterilized load is removed.
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Care must be taken to avoid recontamination.
3. Applications in Pharmaceuticals
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Sterilization of culture media, glassware, rubber stoppers, gowns, and instruments.
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Sterilization of certain aqueous preparations.
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Decontamination of biohazardous waste.
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Moist heat stability testing during validation.
4. Types of Autoclaves
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Gravity displacement autoclave (commonly used in labs).
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Pre-vacuum / high-speed autoclave (used in pharmaceutical production).
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Bench-top autoclaves (small-scale, research use).
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Horizontal / vertical autoclaves depending on design.
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