Bioreactor Design and Operation - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 942

18.7.4. Sterilization of Gases
Filtration methods are almost always used when sterilizing process gases. Depth or
surface filters can be used, but surface membrane cartridge filters are predominantly used.
Membrane filters utilize sieving for particle removal. With both depth and surface filters,
pressure drop is critical. Increased operating pressures for utility gas systems significantly
increased the cost of operation. Membrane filters are manufactured with uniformly small
pores to prevent passage of particles with a radius larger than the pore radius. These filters
can be steam sterilized many times. Condensate formed on non-sterile side cannot pass into
the sterile side. Filters used on gas services are generally hydrophobic, not allowing liquids to
pass through. All sterile filters require testing for integrity, usually before and after each use.
At bubble point, diffusion testing is commonly utilized to conduct integrity test sterile filters.
As indicated earlier, there is a significant need to minimize pressure drop.
18.7.5. Ensuring Sterility
Thermal sterilization is the most common techniques applied in the industry. Temperature
and time are to be selected based on the thermal stability of medium, as well as the thermal
stability of the potential undesirable microorganisms present. Usually, tests need to be con-
ducted to verify sterile conditions. For sterilizing known microorganisms, one must take into
account subcultures that aremore thermal stable. Although the populationmight be small, these
more thermal stable subcultures could have much lower activation energies (of deactivation).
One particularly resistant undesired biocontaminant is prions or infectious proteins.
Commonly, infectious particles possessing nucleic acid are dependent upon it to direct their
continued replication. Prions, however, are infectious by their effect on normal versions
(or desired folding forms) of the protein. Prions induce normal proteins to re-fold and to
be added (grow) to prions. Sterilizing prions therefore require the denaturation of the protein
to a state where the molecule is no longer able to induce the abnormal folding of normal
proteins. Prions are generally quite resistant to proteases, heat, radiation, and formalin treat-
ments, although their infectivity can be reduced by such treatments. Effective prion decon-
tamination relies upon protein hydrolysis or reduction or destruction of protein tertiary
structure. Examples include bleach, caustic soda, and strongly acidic detergents. Prion is
very thermal resistant. In a pressurized steam autoclave, 134 C (274 F) for 18 min may
not be enough to deactivate the agent of disease. Ozone sterilization is being studied as
a potential method for prion denaturation and deactivation. Partially denatured prions can
be renatured to an infective status under certain artificial conditions.
The World Health Organization recommends any of the following three procedures for the
sterilization of all heat-resistant surgical instruments to ensure that they are not contami-
nated with prions:
(1) Immerse in a pan containing 1 mol/L NaOH and heat in a gravity-displacement
autoclave at 121 C for 30 min; clean; rinse in water; and then perform routine
sterilization processes.
(2) Immerse in 1 mol/L NaOH or sodium hypochlorite (2% available chlorine) for 1 h;
transfer instruments to water; heat in a gravity-displacement autoclave at 121 C for 1 h;
clean; and then perform routine sterilization processes.