THE EFFECT OF PARTICLE MORPHOLOGY AND CONCENTRATION ON THE DIRECTLY MEASURED YIELD STRESS IN FILAMENTOUS SUSPENSIONS

The influence of concentration (mass and volume fraction) and particle morphology on the yielding properties of filamentous broths of Asperg illus niger and Streptomyces levoris was investigated using the rotati ng vane technique and compared with those of pulp suspensions. Two met hods were applied to determine the volume fraction of the cells growin g in filamentous form: the measurement of interstitial volume using a high molecular weight dye or dextran, and the measurement of dewatered broth filter cake volume by displacement using a pycnometer. The latt er method provided the most reliable results. Cell concentrations rang ing from 3 to 20 g dw/L, with corresponding volume fractions between 0 .005 and 0.05, were obtained with broths generated in stirred tank and shake flask fermentations. The yield stress values obtained using the vane technique (0.1 less than or equal to tau(y) less than or equal t o 37 Pa) correlated with biomass concentration (X, grams dry weight pe r liter) using the equation tau(y) = aX(b) with ''b'' ranging between 2 and 3. Increasing the length of organisms significantly increased th e yield stress at a given cell concentration; this morphology effect w as described using a ''morphology factor'' (range 0.016 to 0.11 Pa/[g dw/L](2.5)), determined by setting the value of ''b'' to 2.5. The vane yield strain, which is the critical strain or deformation at the yiel d point, was independent of cell concentration, but increased with inc reasing length of organisms. Filamentous fermentation broths with high ly branched organisms had a greater yield stress than pulp suspensions . (C) 1995 John Wiley & Sons, Inc.