Batch cultivations
For evaluation of the malic acid production capabilities, batch cultivations in shake flasks and fermentors were carried out.
The cultivations in bioreactors were performed in 2.7-L Applikon bioreactors (Applikon, Schiedam, The Netherlands) with 2 L of working volume.
Reactors were equipped with two Rushton six-blade disc impellers, and the temperature was maintained at 34 °C. The pH was buffered by the calcium carbonate in the medium. The temperature, agitation, gassing, pH, and composition of the off-gas were monitored and/or controlled using the DasGip monitoring and control system (DasGip, Jülich, Germany).
The stirrer speed was set to 950 rpm and the aeration rate to 1 vvm (volume of gas per volume of fermentation broth per minute).
Concentrations of oxygen and carbon dioxide in the exhaust gas were analyzed with the DasGip fedbatch-pro® gas analysis systems with the off-gas analyzer GA4 based on zirconium dioxide and a two-beam infrared sensor.
In case of shake flask cultivations for enzyme assays and RNA extraction, the strains were incubated in 100 mL MAF4AS medium in 500-mL Erlenmeyer flasks with agitation at 250 rpm in an orbital shaker.
For 13C tracer experiments, the strains were cultivated in 250-mL shake flasks containing 50 mL MAF4AS with 1-13C-labelled glucose as the carbon source at a concentration of 25 g L−1