EffectofPulsingMixingInterruptionsontheAspergillusnigerMorphology andCitricAcidProduction

The effect of short pulsing mixing interruptions on the Aspergillus niger R3 morphology and fermentation performance was studied in 5 l agitated fermenters with double Rushton turbine impellers. A stable increase in the mycelia concentration and higher biomass concentrations at the end of the process were achieved in two continuous mixing fermentations, while in two fermentations with mixing interruptions biomass accumulation level was significantly lower. The highest citric acid concentration, in comparison with continuous mixing fermentations, was obtained in the pulsed on/off mixing fermentation with gas blending, and the pulsed mixing fermentation without gas blending. Small pellets (with about 0.6 – 0.8 mm mean equivalent diameter) developed in continuous mixing fermentations. The largest mean equivalent pellet diameter (approximately 0.9 – 1.1 mm) and the longest hyphae in the outer zone of a pellet were observed in the most productive fermentation with mixing oscillations accompanied by gas blending.