Autoclave sterilisation uses moist heat at 121°C for 15 minutes at 15 psi. The fundamental reason moist heat is more effective than dry heat at equivalent temperatures is:

• A Moist heat achieves higher temperatures than dry heat at the same pressure
• B Water molecules disrupt hydrogen bonds in proteins, causing coagulation/denaturation at lower temperatures; dry heat requires oxidative incineration at much higher temperatures ✓
• C Moist heat generates more reactive oxygen species that damage DNA
• D Steam carries more thermal energy per unit volume than hot air

Explanation

Moist heat kills microorganisms primarily by protein coagulation and denaturation. Water molecules act as a destabilising medium, disrupting hydrogen bonds, van der Waals forces, and hydrophobic interactions within protein structures, causing irreversible coagulation at relatively lower temperatures (60–100°C for vegetative organisms, 121°C for spores). Dry heat kills by oxidation, which requires much higher temperatures (160–180°C for equivalent time) and longer exposure. The coagulation point of proteins in the presence of water is approximately 50–80°C lower than their oxidation temperature. This is why a boiled egg (100°C moist) vs baking at 200°C — both denature proteins but at different temperatures. For sterilisation purposes, moist heat is far more energy-efficient.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

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