Rheological Characterization and Biological and Mass Transfer Kinetics of Cellulase-Producing T. reesei Viscous Suspensions

Abstract

Steady and dynamic shear measurements are utilized to characterize the rheological behavior of Trichoderma reesi RUT-C30 fungal suspensions during batch growth on xylose (soluble substrate) or cellulose (particulate solid substrate) at three different fermentor impeller speeds (250, 400 and 550 rpm). Biomass concentrations versus time were unimodal on xylose and bimodal on cellulose. This behavior is consistent with relatively rapid, early growth on easily metabolized growth medium components (yeast extract), followed by a second, slower growth phase due to hydrolysis of recalcitrant cellulose by increasing cellulase concentrations. Critical dissolved oxygen (DO) concentration for T. ressei growth on cellulose was found to be 0.073 mmol/L. The DO was kept above this level by supplementing air feed with pure oxygen, implying that mass transfer limitations were not the cause of bimodal cell growth. Steady shear rheological data showed shear thinning behavior and a yield stress for all broth samples regardless of substrate. Casson and Herschel-Bulkley constitutive equations fit steady shear data well. Dynamic shear measurements on broth suspensions indicated a "gel-like" behavior at low strains, with micro structural breakdown at larger displacements. Time variations of the Casson model parameters (yield stress and Casson viscosity) and dynamic moduli (elastic and vscous modulus) strongly followed biomass profiles: a single maximum in all rheological variables resulted when cells were grown on xylose, or on cellulose at impeller speeds of 400 or 550 rpm, and dual maxima were observed for cellulose - grown cells at 250 rpm.