Biomedical Engineering Reference
In-Depth Information
Typically, 3 e 5% of fermentations in an industrial plant are lost due to failure in steriliza-
tion. However, the frequency and causes of contamination vary considerably from process to
process. For example, the nature of the product in antibiotic fermentations presents some
protection from contamination; fewer than 2% of production-scale antibiotic fermentations
are lost through contamination by microorganisms or phage. In contrast, a contamination
rate of 17% was reported for industrial-scale production of b -interferon from human fibro-
blasts cultured in 50-L bioreactors (Morandi M and Valeri A 1988 Industrial Scale Production
of b -interferon. Adv. Biochem. Eng./Biotechnol. 37, 57 e 72).
Industrial fermentors and associated pipings are designed for in situ steam sterilization
under pressure or SIP. The use of saturated steam is common and advantageous over other
hot fluids. As steam is applied to a vessel or pipe, the hot steam transfers its energy (heat) to
the wall of the pipe or vessel, raising its temperature. The transfer of heat from the steam
decreases the enthalpy of the saturated water vapor, causing it to condense at constant
temperature. This condensation decreases the volume of the water by a factor of 815 for steam
at 121 C. This results in a pumping action (pressure drop when water vapor condenses),
drawing more steam to the areas of greatest consumption, i.e. the areas that are the coldest.
This provides a more even temperature profile and makes the heating process less likely to
result in cold spots and dead legs. As a result, the sterilization is uniform.
The vessel should have a minimum number of internal structures, ports, nozzles, connec-
tions, and other attachments to ensure that steam reaches all parts of the equipment. For
effective sterilization, all air in the vessel and pipe connections must be displaced by steam.
The reactor should be free of crevices and potential stagnant areas where liquid or solids can
accumulate; polished welded joints could use in preference to other coupling methods. Small
cracks or gaps in joints and fine fissures in welds are prone to harboring microbial contam-
inants and are avoided in bioreactor system construction whenever possible. After steriliza-
tion, all nutrient medium and air entering the fermentor must be sterile. As soon as flow of
steam entering the fermentor is stopped, sterile air is introduced to maintain a slight positive
pressure in the vessel. The positive pressure in the vessel prevents entry of air-borne contam-
inants. Filters preventing passage of microorganisms are fitted to exhaust gas lines; this
serves to contain the culture inside the fermentor and ensures against contamination should
there be a drop in operating pressure.
Flow of liquids to and from the fermentor is controlled using valves. Because valves are
a potential entry point for contaminants, their construction must be suitable for aseptic oper-
ation. Common designs such as simple gate and globe valves have a tendency to leak around
the valve stem and accumulate broth solids in the closing mechanism. Although used in the
fermentation industry, they are unsuitable if a high level of sterility is required. Values should
be constructed such that no openings out of the pipe even if the valve stem is removed by
accident. This can be achieved by constructing an inner wall with expandable materials to
seal the entire valve section. Fig. 18.12 shows a sketch of a pinch valve where flexible material
is used to build an inner tubing. Flexible sleeves and/or diaphragms are the common choices
for the closing mechanism to be isolated from the contents of the pipe and there are no dead
spaces in the valve structure. Rubber or neoprene capable of withstanding repeated steriliza-
tion cycles is used to construct the valve closure; the main drawback is that these components
must be checked regularly for wear to avoid valve failure. To minimize costs, ball and plug
valves are also used in fermentor construction.
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