dry heat sterilization
Dry heat sterilization is sterilization by hot air in an oven, with no steam involved. Where an autoclave is a pressure cooker, a dry-heat oven is more like baking: microbes are killed not by wet coagulation but by slow oxidation and scorching of their cell components, so it takes much higher temperatures and much longer times to do the job.
Typical cycles run far hotter and slower than steam — for example 160°C for 2 hours, 170°C for 1 hour, or 180°C for 30 minutes — because dry air carries and transfers heat poorly compared with condensing steam. The product or item must be held at temperature long enough for the heat to reach and persist throughout.
Its niche is exactly the materials steam cannot handle: anhydrous oils and fatty bases, dry powders, and glassware. Glass apparatus, vials and metal equipment are routinely processed this way. A special bonus is that at very high temperatures (around 250°C for half an hour or more) dry heat does what steam cannot — it destroys bacterial endotoxins, so it doubles as a depyrogenation step for glass containers.
The drawbacks are heat and time. The long exposure at high temperature rules out anything heat-sensitive — plastics, rubber, aqueous solutions and most drugs — and the slow, uneven heating of large loads must be carefully validated.
Empty glass vials destined for an injectable product are run through a tunnel oven at around 250°C, which both sterilizes them and destroys any endotoxin on the glass before sterile filling.
Hotter and slower than steam — the method for oils, powders and glassware, and for depyrogenating glass.
Dry heat is the standard way to depyrogenate glass vials and ampoules. Sterilizing kills microbes; the much hotter depyrogenation cycle additionally inactivates the heat-stable endotoxins they leave behind.