THE VISCOELASTIC NATURE OF FILAMENTOUS FERMENTATION BROTHS AND ITS INFLUENCE ON THE DIRECTLY MEASURED YIELD STRESS

Dynamic viscosity measurements and rotating vane tests on filamentous broths of Aspergillus niger and Streptomyces levoris showed that filam entous suspensions are often highly viscoelastic and possess a yield s tress. The storage modulus of filamentous fermentation broths of A. ni ger was greater than the loss (viscous) modulus at very small deformat ions, but the storage modulus decreased more rapidly than the loss mod ulus as the strain increased. The ''static'' vane yield stress was com pared to the ''dynamic'' yield stress measured by extrapolating the sh ear stress-shear rate data using the Casson and Herschel-Bulkley rheol ogical models. For S. levoris broths, there was a close agreement betw een the vane yield stress and those obtained by extrapolation; however , for A. niger broths the vane yield stress was much greater than thos e determined by conventional methods. Larger particles, more heterogen eous characteristics, and therefore highly viscoelastic properties of the A. niger broths were mainly responsible for the higher ''static'' yield stress. The vane technique measures a combination of the viscous and elastic properties and provides a realistic representation of the ''true'' yield stress at zero shear rate that may be significantly hi gher than the yield stress obtained by extrapolating shear stress-shea r rate data to zero shear rate.