hyphae of A. niger. Furthermore, Mn2+ deficient
hyphae do not take up citrate at pH 3.3 (Netik et al.,
1997). Therefore, it is conceivable that A. niger
possesses an active transport system, but either does
not use it, or does not need to use it when the external
pH is low.
In contrast, Kontopidis (1997) showed that cells
take up radioactive citrate when the addition of a
buffered citrate solution causes a small, local rise in
the pH. Citrate is not taken up under the same
conditions if the citrate solution is buffered to the pH
of the broth. The most prevalent form of citrate at the
broth pH is the undissociated acid. One of the ionic
forms will be most prevalent at higher pH values. This
suggests that the membrane contains a carrier for a
dissociated form of the acid that can be reversed, if the
external concentration of the dissociated ions rises as a
consequence of a rise in pH. This reversibility is not a
feature of active transport and demonstrates that
facilitated diffusion proteins are available for the
transport of at least one specific citrate ion across the
membrane.
5. Modeling citric acid production by A. niger
5.1. Kinetic modeling by Röhr, Zehntgruber and
Kubicek (1981)
Kinetics of citric acid production by A. niger growing
on sucrose in a pilot plant were investigated by Röhr
et al. (1981). A typical growth curve for A. niger under
citric acid producing conditions shows an initial rapid
growth phase followed by a phase of slow growth. Röhr
et al. (1981) subdivided cell growth and product
formation into several phases, each described by a
simple deterministic model (Fig. 5). The best fitting
model equations in the various phases were identified.
The growth phases identified were: the hyphal growth
phase (Bx), pellet growth phase (Cx), restricted growth
phase (Dx), transition period between trophophase and
idiophase (Ex) and idiophase growth (Fx). The logarithmic,
cube root and linear equations described by Trinci
(1970) were used to describe the growth in each phase,
as follows: