DEPENDENCE OF MYCELIAL MORPHOLOGY ON IMPELLER TYPE AND AGITATION INTENSITY

The influence of the agitation conditions on the morphology of Penicil lium chrysogenum (freely dispersed and aggregated forms) was examined using radial (Rushton turbines and paddles), axial (pitched blades, pr opeller, and Prochem Maxflow T), and counterflow impellers (Intermig). Culture broth was taken from a continuous fermentation at steady stat e and was agitated for 30 min in an ungassed vessel of 1.4-L working v olume. The power inputs per unit volume of liquid in the tank, P/V-L, ranged from 0.6 to 6 kW/m(3). Image analysis was used to measure mycel ial morphology. To characterize the intensity of the damage caused by different impellers, the mean total hyphal length (freely dispersed fo rm) and the mean projected area (all dispersed types, i.e., also inclu ding aggregates) were used. [In this study, breakage of aggregates was taken into account quantitatively for the first time.] At 1.4-L scale and a given P/V-L, changes in the morphology depended significantly o n the impeller geometry. However, the morphological data (obtained wit h different geometries and various P/V-L) could be correlated on the b asis of equal tip speed and two other, less simple, mixing parameters. One is based on the specific energy dissipation rate in the impeller region, which is simply related to P/V-L and particular impeller geome trical parameters. The other which is developed in this study is based on a combination of the specific energy dissipation rate in the impel ler swept volume and the frequency of mycelial circulation through tha t volume. For convenience, the function arising from this concept is c alled the ''energy dissipation/circulation'' function. To test the bro ader validity of these correlations, scale-up experiments were carried out in mixing tanks of 1.4, 20, and 180 L using a Rushton turbine and broth from a fed-batch fermentation. The energy dissipation/circulati on function was a reasonable correlating parameter for hyphal damage o ver this range of scales, whereas tip speed, P/V-L, and specific energ y dissipation rate in the impeller region were poor. Two forms of the energy dissipation/circulation function were considered, one of which additionally allowed for the numbers of vortices behind the blades of each impeller type. Although both forms were successful at correlating the data for the standard impeller designs considered here, there was preliminary evidence that allowing for the vortices would be valuable . (C) 1996 John Wiley & Sons, Inc.