Multiphase Bioreactor Design

batch cycle (Kieran et al., 1993). However, the osmotic potential of the medium, which is
predominantly determined by salt and sugar levels, has been identified as the most
influential factor (e.g. Park and Kim, 1993; Zhang et al., 1997). As the sugar supply in
the medium is depleted, the osmotic pressure is reduced, and the cells expand, resulting
in high FW/DW ratios towards the end of the growth cycle. For high-density perfusion
cultures of Anchusa officinalis (Su et al., 1993), reductions in average cell size were
clearly associated with increased medium osmolarity.
Manipulation of the cell size via osmotic pressure regulation has been investigated as a
strategy for reducing broth viscosity in cell suspensions of Panax notoginseng (Zhang et
al., 1997). In a 200 g FW L−^1 suspension subjected to a glucose shock (0.15 M glucose),
aggregate shrinkage resulted in a reduction in broth apparent viscosity from 80 cP to 10
cP.

Figure 14.3 Fresh weight (FW), dry

weight (DW) and FW/DW profiles for

M. citrifolia shake flask suspension

cultures (Cusack, 1998).

This approach offers the possibility of increasing the sustainable biomass levels in a
bioreactor. While some data are available to suggest that cells may be effectively adapted
to high salt concentrations (Binzel et al., 1985), studies must be performed to determine
the effects of elevated osmotic potential on cell growth and secondary metabolism.

Oxygen Requirements

By comparison with microbial systems, the oxygen requirements of plant cells are
modest. Specific oxygen consumption rates (go) depend on cell line, cultivation

Multiphase bioreactor design 426