Page 79 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioTechnology

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

Bioreactors 261

of oxygen supply are used in a few bioprocesses (e.g., mal mammalian cells need carbon dioxide as a substrate
Fig. 12). for carboxylation of pyruvate to oxaloacetic acid, but es-
Oxygen is sparingly soluble in aqueous broths and even tablished cell line may not require carbon dioxide.
a short interruption in aeration rate in some microbial fer-
mentations may produce anaerobic conditions that may
3. Heat Removal and Temperature Control
potentially damage the cells. The precise oxygen require-
ments of a fermentation process depend on the microor- All fermentations generate heat. In submerged cultures,
ganism, the degree of oxidation of the substrate being 3–15 kW m −3 of the heat output typically comes from
used for growth, and the rate of oxidation. Oxygen be- microbial activity. In addition, mechanical agitation of
−3
comes hard to supply when the demand exceeds 4–5kg the broth produces up to 15 kW m . Consequently, a
−1
O 2 m −3 hr . Many microbial fermentation broths are fermenter must be cooled to prevent temperature rise and
highly viscous and difficult to mix. This further compli- damage to culture. Temperature is controlled by circulat-
cates the transfer of oxygen from the gas phase to the ing cooling water in a jacket that surrounds the bioreactor
broth. Once the concentration of dissolved oxygen falls vessel (Fig. 20a). In addition to the jacket, an internal
below a critical value, the microbial growth becomes lim- cooling coil (Fig. 20b) becomes necessary in large biore-
ited by oxygen. The critical dissolved oxygen concen- actors. In some cases, cooling is achieved by recirculating
tration depends on the conditions of culture and the mi- the broth through an external heat exchanger (Fig. 20c).
crobial species. Under typical culture conditions, fungi In small vessels, a “ringlet” coil (Fig. 20d) that enters
such as Penicillium chrysogenum and Aspergillus oryzae the vessel through a large port on top, may provide suffi-
have a critical dissolved oxygen value of about 3.2 × cient cooling. Heat removal tends to be difficult because,
−3
10 −4 kg m . For baker’s yeast and Escherichia coli, typically, the temperature of the cooling water is only a
the critical dissolved oxygen values are 6.4 × 10 −5 and few degrees lower than that of the fermentation broth. In-
−3
12.8 × 10 −5 kg m , respectively. dustrial fermentations are commonly limited by the heat
Animal and plant cell cultures have lower oxygen de- transfer capability. The ability to remove heat depends on
mands than microbial cells. Oxygen consumption rates for the surface area available for heat exchange, the temper-
9
animal cells are in the range of 0.05–0.5 mmol/10 cells ature difference between the broth and the cooling water,
per hr. In batch suspension culture of animal cells, the the properties of the broth and the coolant, and the tur-
maximumcellconcentrationtypicallydoesnotexceed2 × bulence in those fluids. The geometry of the fermenter
6
−1
7
−1
10 cells ml . Higher concentrations, >10 cells ml , determines the heat exchange area that can be provided.
are attained in perfusion culture without cell retention. Because metabolic heat generation depends on the
Perfused culture with cell retention permits cell densities oxygen consumption rate, heat removal in large vessels
9
−1
of around 10 cells ml . In vitro cultured animal cells
are generally tolerant of high concentrations of dissolved
oxygen, e.g., up to 100% of air saturation; however, the op-
timal concentration is about 50% of air saturation but may
vary with cell type. Oxygen concentrations of >100% of
air saturation have been associated with oxidative damage
to cells whereas concentrations ≤0.5% of air saturation in-
hibit the TCA cycle and lead to an enhanced production of
lactate. Concentrations <10% of air saturation will limit
growth of some cells, but for others oxygen limitation is
encountered around 0.5% of air saturation. Plant cells con-
sume oxygen at a rate of (3–15) × 10 −5 mol kg −1 DW s −1
and the maximum cell density in suspension culture tends
−1
to be 20–30 gDW L .
As with dissolved oxygen, concentration of dissolved
carbon dioxide influences microbial and cell growth. Too
much carbon dioxide is detrimental to most aerobic fer-
mentations but cultures of photosynthesizing microbes
may require carbon dioxide in the aeration gas. Also, car-
FIGURE 20 Heat-exchange methods for bioreactors: (a) jacket,
bon dioxide is added at roughly 5% (by vol) to gas mix-
(b) full internal coil, (c) recirculation of broth through an external
ture used in sparging animal cell cultures when the culture heat exchanger, (d) ringlet coil. Cooling water (CW) enters and
broth is buffered with carbonate-bicarbonate system. Nor- exits the heat exchange devices, as shown.