through a combination of biological and processing strategies, or reducing process costs.
A stainless steel, sterilisable bioreactor constitutes a significant portion of the capital
costs for any fermentation facility and a number of groups have begun to question
traditional design philosophies and investigate the feasibility of innovative and less
expensive but functional alternatives.
At laboratory scale, Fukui and Tanaka (1995) successfully employed a stationary,
envelope-shaped culture vessel (“Culture Bag”), with a working volume of 25 mL,
constructed of 12.5μm thick fluorocarbon polymer film, for the cultivation of N. tabacum
and L. erythrorhizon. Biomass levels comparable to or better than those in shake flasks
were achieved. The bag contents were not agitated and oxygen limitation accounted for
reduced growth in thicker-walled bags. Singh (1999) describes a disposable, plastic-bag
bioreactor (Wave Bioreactor®), mounted on a rocking table, which provides low-shear,
wave-motion agitation. Although originally validated with animal cells, a similar reactor
has been used with hairy root and embryogenic callus plant cultures (Eibl et al., 1999).
The system has been successfully scaled to 200 L (100 L wv) with maximum reported
kLa values of 4 h−^1. To date, there are no reports of trials with suspension cultures and
oxygen transfer is likely to be the limiting factor for high-density cultivation. A
promising option for such cultures is a bubble column reactor, comprising a sterilisable,
plastic liner, clamped to a head-plate and supported within an inexpensive, non-
sterilisable cylinder (Figure 14.6), developed by Curtis and co-workers (Hsiao et al.,
1999). Aeration, via an off-set sparger, coupled with contour baffles, ensures adequate
fluid circulation within the vessel. The system has been demonstrated up to a working
volume of 100 L (Curtis et al., 1999), with biomass levels of up to 400 g FW L−^1. And,
although experience with ALRs (Section on Mixing and Mass Transfer) suggests that this
configuration is unsuitable for higher density suspensions, the cost advantages make a
multiple-vessel train commercially realistic.
Regardless of the bioreactor configuration employed, a bioreactor operating strategy
must be selected, which supports high productivity and high product yield. The choice is
Bioreactor design for plant cell suspension cultures 441