## Comparison: Autoclaving vs Dry Heat Sterilisation ### Key Mechanism Difference **Key Point:** Autoclaving uses saturated steam under pressure; dry heat uses hot air in an oven. The presence of moisture fundamentally alters penetration and mechanism of microbial kill. ### Penetration & Diffusion **High-Yield:** Steam has superior penetrating power because water molecules are smaller and diffuse rapidly into fabrics, paper wraps, and instrument lumens. Dry heat relies on conduction through air (poor conductor) and cannot penetrate wrapped bundles effectively. ### Comparative Table | Feature | Autoclaving | Dry Heat | | --- | --- | --- | | **Temperature** | 121°C (15 psi) | 160–180°C | | **Time** | 15–30 min | 1–2 hours | | **Penetration** | Excellent (steam diffuses) | Poor (air conduction) | | **Mechanism** | Protein denaturation + moisture | Oxidative damage | | **Best for** | Wrapped instruments, fabrics, culture media | Glassware, powders, oils | ### Why Autoclaving Penetrates Better Steam condenses on cooler surfaces, releasing latent heat and carrying moisture into crevices and wrapped layers. Dry heat must conduct through multiple layers of material, making it ineffective for wrapped bundles. **Clinical Pearl:** Autoclaves are the gold standard for hospital sterilisation of surgical instruments and linens precisely because steam penetration is superior. Dry heat is reserved for items that cannot tolerate moisture (e.g., oils, powders, glassware). [cite:Textbook of Microbiology Ananthanarayan Ch 5]
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