Multiphase Bioreactor Design

Figure 14.2 Rheological profiles for

batch cultures of (a) M. citrifolia

(Cusack and Kieran, 1998) and (b) P.

somniferum (Curtis and Emery, 1993)

suspension cultures. (Figure 2(b)

reproduced with permission. © 1993,

Wiley-Liss, Inc., a subsidiary of John

Wiley & Sons, Inc.)

concentration (data not shown), but are negligible compared to the corresponding
increases in the whole broth viscosity. The poppy suspension filtrates display Newtonian
behaviour at all stages. At a biomass level of 200 g FW L−^1 , the data in Figures 14.2(a)
and (b) indicate approximately a 25-fold variation in the broth apparent viscosity between
the M. citrifolia and poppy suspensions, which is most easily explained by the
morphological differences between the two cultures. The influence of cell morphology
and aggregation patterns on broth rheology has long been recognised (Wagner and
Vogelmann, 1977; Tanaka, 1982). The M. citrifolia (Figure 14.1) cells are elongated and
occur in unbranched, multi-cellular chains, while poppy cells are broadly spherical in
shape. Curtis and Emery (1993) recorded low, Newtonian viscosities for a range of cell
lines, all with roughly spherical cells. In N. tabacum batch cultures, cell elongation
corresponded to increased viscosity and power law behaviour, while semi-continuous
cultivation of the same cell line yielded spherical cells and low viscosity broths.
Cultivation conditions, as defined by reactor configuration and mode of operation may
also influence rheology. Rodriguez-Monroy and Galindo (1999) observed Theological
differences between Beta vulgaris suspensions cultivated in shake flasks and those from a
2 L STR. The higher viscosity of the bioreactor-cultivated suspensions was associated
with a stress-induced increase in the level of extra-cellular compounds, possibly
polysaccharides. There was no significant variation in cell/aggregate morphology.
The available data identify biomass level, cell line and morphological characteristics
as determining factors for broth rheology. At the biomass levels necessary for
commercially viable production, heightened viscosity can be expected in high ECP-
yielding lines and, in particular, in suspensions of elongated cells. However, the data also
highlight the potential for reducing broth viscosity by manipulating cell morphology.

Fresh Weight/Dry Weight Ratio

In suspension culture, the ratio of fresh weight (FW) to dry cell weight (DW) varies
significantly over the course of a batch cycle. A typical profile for M. citrifolia is
presented in Figure 14.3. Broadly similar trends have been reported for a wide range of
other cell lines (e.g. Park and Kim, 1993; Singh and Curtis, 1994a). However, absolute
values of the FW/DW ratio are both line-specific and dependent on cultivation
conditions. Curtis and Emery (1993) quoted values of between 7 and 25 for a range of
late log/early stationary phase cell suspensions. These values reflect higher cellular water
content and larger cell sizes than in bacterial systems. Some variation in cell size is
associated with cell growth and division and is apparent during the early stages of the

Bioreactor design for plant cell suspension cultures 425