Rheological Studies During Submerged Citric Acid Fermentation by Aspergillus niger in Stirred Fermentor Using Apple Pomace Ultrafiltration Sludge

Rheological profile studies for apple pomace ultrafiltration sludge (APS) broth with filamentous fungus, Aspergillus niger NRRL-567 during citric acid (CA) fermentation are reported for the biomass developed in a 7.5 l bench scale fermentor. CA bioproduction of 23.67 ± 1.0 g/l APS and 40.34 ± 1.98 g/l APS was obtained in control (120 h) and in treatment with inducer, MeOH 3% (v/v), respectively, after 132 h of fermentation by A. niger NRRL 567. The rheological properties of the fermentation broth including pellets were analyzed. The control demonstrated non-Newtonian pseudoplastic behaviour due to more filamentous growth. However, in treatment supplemented with inducer (methanol), pellet form was observed during fermentation with less viscous non-Newtonian broths. The power law model was followed with good confidence of fit (77.8–88.9%) throughout the fermentation in treatment with MeOH as an inducer. However, power law was followed with moderate fit of confidence (65.3–75.9%) at the beginning of fermentation until 48 h and later followed a good confidence of fit (75.9–88.2%) until 144 h of fermentation in control. Important rheological changes occurred in the broth during maximal CA production phase along with increase in biomass in the pelleted mycelium form.