CHAPTER 7:
STERILIZATION
Introduction
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Sterilization is the process to eliminate
microorganisms from a medium or equipment.
It is used to:
 Presterilize
equipments
 Sterilization of feedstocks
 Maintenance of sterile operating conditions
Batch Sterilization
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Uses steam to eliminate living organisms.
Heat losses, heating and cooling are major steps and is a time consuming
process.
Air is evacuated and replaced with steam.
It wastes energy and overcooks the medium.
There is no conservation of energy, therefore not economical for
implementation in large scale.
Done in an autoclave.
When the pressure from the steam has been built up, a venting valve for the
outlet of air chamber is closed and the inlet valve allows the steam to enter
the chamber. The pressure of the chamber is increased to 105 kPa (15
psig).
At this point, the sterilization time begins to count down. Sterilization of 1 L
media at 121°C requires 20 min. Larger volumes may require longer
retention time.
Autoclave
Continuous sterilization
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Direct use of live steam by injection into the medium. Heat
exchanger is eliminated. The medium stays in a loop for a
predetermined holding time until the entire medium is sterile.
The problem with directly injecting steam is dilution of media
since it is initially cold.
However it has better heat economy since it substitutes heat
exchangers. It has a holding coil for detention long enough
to kill all the microorganisms.
The medium from a make-up vessel flowing through the
exchanger is held in the coil, and then goes through the heat
exchanger, heating more unsterile medium while becoming
cool itself, as it is collected in a sterile bioreactor.
Continuous sterilization
Dry heat sterilization
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Used for equipments that can withstand high
temperature and dry heat but cannot withstand wet
or steam autoclave.
Often used for glasswares as it dries and sterilizes
in one operation. Pipettes must be wrapped in dust
proof aluminium foil or placed in metal pipette
cans.
Disposable items are not recommended for dry heat
sterilization.
Sterilization with Filtration
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Certain media components are susceptible to heat.
It denatures if heated. Therefore they must be
added to the media after autoclaving.
To do so, filtration is necessary using a 0.22 μm
pore size filter that is appropriate to the solvent
used.
Chemical Sterilization
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Chemical agents are used to sterilize heat-sensitive
equipments.
Application of an oxidizing agent agent such as
10% chlorox for 20 min or longer proves the system
free from contamination.
Excess amounts of chemical agents have to be
removed, otherwise organisms won’t be able to
grow under toxic environment.