Continuous Cultivation - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 650

depends on the substrate and organism used. Most natural substrates (e.g. grains) require
pretreatment to make the physical structure of substrates more susceptible to mycelial pene-
tration and utilization. Solid substrates are usually treated with antimicrobial agents, such as
formaldehyde, and are steamed in an autoclave. Nutrient media addition, pH adjustment,
and the adjustment of moisture level are realized before inoculation of the fermentation
mash. Koji fermentations are usually realized in a controlled humidity air environment
with air ventilation and agitation. Many solid-state mycelial fermentations are shear sensitive
due to disruption of the mycelia at high agitation/rotation speeds. At low agitation rates,
oxygen transfer and CO 2 evolution rates become limiting. Therefore, an optimal range of
agitation rate or rotation speed needs to be determined. Similarly, there is a minimum level
of moisture content ( 30% by weight) below which microbial activity is inhibited. At high

moisture levels ( 60%), solid substrates become sticky and form large aggregates. Moreover,

moisture level affects the metabolic activities of cells. Optimal moisture level needs to be
determined experimentally for each cell-substrate system. For most of the koji processes,
the optimal moisture level is about 40

>

5%. Particle size should be small enough to avoid
any oxygen e CO 2 exchange or other nutrient transport limitations. Porosity of the particles
can be improved by pretreatment to provide a larger intraparticle surface to volume ratio
for microbial action.
Most of the SSF processes are realized using a rotary tray type of reactor in a temperature-
and humidity-controlled chamber where controlled humidity air is circulated through
stacked beds of trays containing fermented solids. Rotary drum fermentors are used less
frequently because of the shear sensitivity of mycelial cells.

12.6. SUMMARY

The primary form of continuous culture is a steady-state CSTR or chemostat . A chemostat
ensures a time-invariant chemical environment for the cell cultivation. The net specific
growth rate is equal to the dilution rate, which is determined by the flow rate to the chemo-
stat. Thus, the growth rate can be manipulated by the investigator. This is a typical method of
controlling the cell growth (and/or production formation rate). A constant dilution rate gives
rise to a constant specific growth rate, which is equivalent to a fixed exponential growth rate.
Another benefit is the control of substrate concentration in the reactor at a given (low) value,
which can promote a desired product formation. A cytostat (or turbidostat ) adjusts flow rate to
maintain a constant cell density (via turbidity). Cell density control via adjusting flow rate is
not effective except at high flow rates. A turbidostat operates well at high flow rates (near the
washout point) and is useful in selecting cellular subpopulations that have adapted to
a particular stress.
Continuous culturing is more productive than batch culturing. The productivity of chemo-
stat increases with increasing dilution rate to a maximum before sharply decreases near the
washout limit. Higher endogenous needs and/or death rate decreases cell biomass concen-
tration and productivity. The cell biomass concentration is maximum at zero dilution rate if
there are no endogenous requirements and the death rate is zero. A finite death rate gives rise
to a zero viable cell biomass concentration at zero dilution rate. Increasing endogenous needs
decrease cell biomass concentration especially at low dilution rates. There is a maximum cell