Liquid Culture Systems for in vitro Plant Propagation

48 Anne Kathrine Hvoslef-Eide et al.
therefore initially correct. After autoclaving, but before inoculation, a sample
of the medium is withdrawn. The pH of this sample is measured with a
laboratory pH meter and we adjust the bioreactor pH zero point to obtain the
same reading. By adding an amount of acid and withdrawing a second
sample, we could recalibrate the slope, but we do not do so because when
the zero point is correct, the errors caused by slope deviations are small
compared with other error sources, at least for moderate pH changes. The pH
can be regulated between pH 3 and 7 in our bioreactor system.
Active regulation of the pH can be achieved by adding small amounts of
acid or alkali through sterile filters into the bioreactors. We use HCl (0.1
mol) and NaOH (0.1 mol) for this purpose. Choosing the right concentration
of acid or alkali is important, low enough to allow a fine control, but
sufficiently concentrated to allow for pH changes without adding too much
liquid and undesired Cl - and/or Na + to the vessel. The controller controls the
pH by comparing the measured pH to the set point and adding either acid or
base accordingly. The alkali and acid actuators are independent peristaltic
pumps running at constant speed when activated. The pumps are of our own
design, a design that causes least wear on the tubing.
pH
4,8
4,6
4,4
4,2
4,0
3,8
3,6
3,4
3,2
3,0
pH in suspensions
1 3 6 8 10 13
Days
Bioreactors
Flasks
Figure 3: The temporary fall in pH after inoculation and the increase in bioreactors and in
Erlenmeyer flasks of Cyclamen persicum Mill. cultures. Standard deviation shown on each
mean measurement.