A stovetop pressure canner that holds 15 psi reaches 121 °C, the temperature a lab autoclave uses. Run it the way an autoclave runs and it sterilizes agar, broth and glassware at home.
Water boils at 100 °C at sea level, and some bacterial spores survive that. Getting hotter than boiling takes steam under pressure.
Princeton University's safety office sets the bar for its autoclaves at 121 °C for at least 30 minutes, using saturated steam at 15 psi or more.
A stovetop canner with a 15 psi weight reaches autoclave temperature. Electric pressure cookers usually run at lower pressure.
In a 2018 study, one electric cooker worked at 113 to 115 °C. All four models tested still sterilized media and microbial cultures, within at most 60 minutes at pressure.
An autoclave does the rest of the job for you. It times the cycle, exhausts slowly so liquids don't boil over, and dries the load. With a canner, you are the timer.
Look for a regulator weight with a 15 psi setting, a pot tall enough for media bottles standing upright, and a lid that seals the same way every time.
The All American 921 sets 5, 10 or 15 psi by turning its regulator weight. The lid seals metal to metal with no gasket, held down by screw-down knobs around the rim.
It holds 21.5 quarts and stands 15.375 inches tall, enough for 18 regular pint jars or 7 quart jars, according to All American. It works on gas, electric and flat-top stoves.
The smaller 915 holds 15.5 quarts and stands 12.25 inches tall, which fits 10 pint jars. The 921's extra height is what lets taller media bottles stand upright.
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21.5-quart canner with a metal-to-metal seal and no gasket, a steam gauge and a 5, 10 or 15 psi regulator weight. Holds 18 pint jars.
Liquids go in glass bottles with the caps loosened and the bottles no more than two-thirds full, the same rule Princeton sets for its autoclaves.
Dry glassware goes in empty and without tight caps.
Disposable plastic Petri dishes come sterile and aren't made to be heated. Sterilize the agar in a bottle, then pour the plates once it has cooled enough to handle.
Stand everything on the rack, not on the bottom of the pot, and add water to the level the canner's manual gives.
Vent it first. With the lid on and the weight off, heat until steam streams from the vent pipe, then let it vent for 10 minutes.
The USDA recommends that step for every pressure canner, because air trapped inside lowers the temperature reached at a given pressure.
Bring it to 15 psi. Put the weight on its 15 psi setting and start timing once the canner is at pressure, as the manual describes.
Hold it for at least 30 minutes. That is Princeton's autoclave standard at 121 °C. Princeton adds time for larger volumes, so give full bottles longer.
Let it cool on its own. Turn off the heat and wait for the gauge to fall to zero. The USDA says not to force-cool a canner.
Open it away from you. Remove the weight, then the lid, tilted so the steam rises away from your face. Hot liquids can still bubble up, so let bottles sit before moving them.
A canner's gauge measures pressure above the air around it, so the same 15 psi gives a lower temperature at elevation. The USDA raises canning pressures as elevation increases.
Above a few thousand feet, follow the elevation adjustments in your canner's manual rather than assuming 121 °C.
Autoclave tape changes color once it reaches 121 °C, but Princeton warns it does not prove organisms died. Pour one plate from each batch, add nothing to it, and check it stays clear for a few days.