Paper I — 2014 September (Supplementary) (2010 Scheme) — Q5
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Paper I
2014 September (Supplementary) (2010 Scheme) · 40 marks · 120 min
Question
Sterilization using dry heat
Q52 marksShort Notes
Answer
Dry heat sterilizes by causing oxidative destruction/denaturation of cell constituents — since no moisture is involved, higher temperatures and longer exposure times are required than for moist heat, but dry heat can penetrate materials moist heat cannot (powders, oils, sharp metal instruments not damaged by dry heat’s absence of corrosion/blunting).
Methods:
Red heat — direct flaming of an inoculating wire/loop in a Bunsen flame until it becomes red hot; instantaneous sterilization.
Flaming — passing an object (e.g., mouth of a culture tube, forceps, scalpel blade) briefly through a flame; kills vegetative organisms on the surface but is not a validated sterilization method for spores.
Incineration — complete combustion to ashes at very high temperature; used for destruction of biomedical/infectious waste, animal carcasses, and non-reusable contaminated material.
Hot air oven — the standard laboratory dry-heat sterilizer; uses electrically heated air circulated by a fan for uniform distribution. Standard cycle: 160°C for 1 hour (alternative validated combinations include 170°C for 40 min, or 180°C for 20 min — higher temperature permits shorter time). Used for glassware (pipettes, Petri dishes, flasks), swabs, oils, powders, and metal instruments that would corrode/blunt under moist heat.
Comparative note: dry heat requires much higher temperature and longer exposure than moist heat (autoclaving at 121°C/15–20 min) to achieve equivalent sterilization, because dry heat kills by slow oxidation rather than the faster protein coagulation/denaturation produced by moist heat.
