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Encyclopedia of Physical Science and Technology EN002G-67 May 25, 2001 20:8

Bioreactors 255

used in typical microbial culture. Similarly, some animal
cell culture vessels are installed with bundles of micro-
porous polymer tubing for bubble-free oxygen supply to
animal cells that are particularly susceptible to damage
by bursting bubbles. Oxygen or a gas mixture containing
oxygen flows through the tubing and oxygen diffuses into
the culture broth through the liquid film held within pores
in the tube wall. In other instances, a water-immiscible
oxygen carrying liquid, or an oxygen vector (e.g., perflu-
orocarbons and silicone oils), is used to supply oxygen,
as shown in Fig. 12. The carrier fluid is oxygenated in
a sparged column by bubbling with air. The bubble-free
carrier then circulates through the culture vessel where
oxygen is transferred to the broth. The oxygen depleted
FIGURE 11 Illumination using optical fibers or waveguides. carrier loaded with carbon dioxide returns to the aeration
column (Fig. 12).
Instead of using tubes, the photoreceiver is sometimes Other bioreactor designs include the rotating drum fer-
made of transparent plastic sheets, as in Fig. 10d. In other menter (Fig. 13) with internal baffles. This device is used
cases, a conventional vessel with a low surface-to-volume to culture some suspended plant cells. The drum is filled to
ratio may be illuminated by using optical fibers to con- less than 40% of its volume and rotated on rollers for mix-
vey light inside from an external source (Fig. 11), but this ing. Another bioreactor configuration that is suitable for
arrangement is not particularly effective. Irrespective of hairy root cultures of plants is the mist, spray, or fog biore-
actor. The static root mass is contained in a chamber that is
the design of the photoreceiver and the source of illumi-
mostly empty. In this design, the nutrients are supplied as
nation (natural or artificial), light is generally the limiting
a mist of fine droplets suspended in circulating air currents
nutrient in phototrophic culture. Except in optically di-
lute cultures, exponential growth does not persist for long that penetrate the spaces between the roots. The spray of
in photosynthetic microbial culture. Because of light ab- nutrient solution is produced by using a compressed gas
sorption and self-shading by cells in dense culture, light atomizer nozzle or a spinning disc spray device.
soon become limiting and growth kinetics change from
exponential to linear. The depth related decline in light B. Solid-State Culture
intensity is governed by the Beer–Lambert relationship,
as follows: Solid-state culture differs markedly from submerged cul-
ture. The substrates of solid-state fermentations are par-
I
= exp(−K a XL) , (1) ticulate solids that contain little or no free water. Steamed
I o
rice is a typical substrate. Beds of solids are difficult to ag-
where I o is the incident light intensity, I is the intensity at itate and solid-state fermentations do not employ intensive
depth L, X is the biomass concentration, K a is the light mixing. Small particles with large surface-to-volume ra-
absorption or extinction coefficient that depends on the tios are preferred substrates because they present a larger
pigmentcontentofthecells,and L istheculturedepth.Ob- surface for microbial action. However, particles that are
viously, culture depth (i.e., tube diameter, channel depth) too small, and shapes that pack together tightly (e.g., flat
must remain quite shallow, or the local light intensity will flakes, cubes), are undesired because close packing re-
become too low to support growth. duces interparticle voids that are essential for aeration.
Similarly, too many fines in a batch of larger particles
will fill up the voids. For fermentation, the substrate is
7. Other Bioreactor Configurations
loosely packed into shallow layers or heaps. Deep beds of
The basic bioreactor configurations discussed above for substrate require forced aeration with moistened air. Aer-
heterotrophic growth (i.e., stirred tanks, bubble columns ation rates may vary widely; a typical range being (0.05–
3
−1
and airlift bioreactors, packed and fluidized beds) are gen- 0.2) × 10 −3 m kg −1 min . Occasional turning and mix-
erally satisfactory for a great majority of bioprocessing ing improve oxygen transfer, and reduce compaction and
needs. In addition, some basic configurations have been mycelial binding of substrate particles.
especially adapted to better suite specific applications. For Unlike many submerged fermentations, solid-sate pro-
example, stirred vessels for animal and plant cell cultures cessescommonlyusemixedcultures.Hygienicprocessing
employ different designs of impeller compared to ones practices are followed in large industrial operations, but