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Operating Instructions
Techniques of Sterilization
Saturated steam is a well-controlled, reliable method for processing items
which can withstand the temperatures and pressures associated with steam
sterilization. The requirements for achieving reproducible results are well
known by many users, but are not always understood by all users.
The condition most likely to result in sterilization problems is a failure to remove
all of the air from the items being processed. For example, placing an empty
beaker or bowl in an upright position in a gravity displacement sterilizer may
result in the object not being sterilized, or may require exceptionally long
sterilization times. This problem is due to the fact air has almost twice the
density as does saturated steam under the same conditions. Thus, the air sits
in the bottom of the container, and the steam forms a stable layer over the air.
This effect is similar to oil forming a stable layer over water. As long as there
is no mechanism for actively mixing the two, the bottom of the container will
only see dry heat, which is not an effective sterilization method at the
temperatures typically used in steam processes.
There are two methods for enhancing the sterilization of solid bottom contain-
ers in gravity displacement cycles. These are:
•
Place 1 to 2 mL of water in the bottom of each container. The expansion
of the water into steam as the product is heated will force most of the air
out of the object, thus allowing steam to reach all surfaces and effect
sterilization.
•
The better, more reliable method is to orient all objects in a manner which
would allow water to flow out. When the steam enters the chamber, it will
tend to layer over the air. However, the object is now oriented so the air
can flow out. As the air flows out of the container, it will be replaced by
the steam. The steam can now reach all surfaces and effect sterilization.
The best type of cycle for assuring sterilization of containers, and of objects
which contain lumens or tortuous paths, is the prevacuum cycle. In this
process, several vacuum pulses remove all of the air from the load. The steam
can then immediately contact all surfaces. This immediate contact results in
shorter sterilization times than are required when complete air removal cannot
be assured. Items which take 15 to 30 minutes to sterilize in a gravity
displacement cycle can be sterilized in 4 minutes or less at 132°C (270°F).
Objects which do not allow easy passage of steam or air cannot be effectively
sterilized with any steam process. For example, pipette cans with lids in place
do not allow all the air to flow out, or the steam to flow in, even with prevacuum
cycles. In a gravity cycle, these items have a high probability of being non-
sterile. In a prevacuum cycle, these items may be crushed by the steam
pressure because the chamber pressure changes much faster than does the
pressure inside the canister.
Items which are hermetically sealed (e.g., empty screw cap bottles) cannot
be sterilized by any steam process because the steam cannot get into the
device, and air cannot get out. If you must process these items, make certain
the screw caps are loosened at least one half turn (more would be better).
Verify your process is capable of sterilizing these objects by running biologi-
cal indicators in the bottom of the bottle. If the biological indicators are not
killed, the caps need to be loosened even further, or the bottles need to be
sterilized separately from the caps (cover the bottles with Kraft paper, peel
pouches, or some other steam-permeable material).
3.4 Recommendations
for Enhancing the
Sterilization Process
