Effects of agitation intensity on mycelial morphology and protein production in chemostat cultures of recombinant Aspergillus oryzae

The effects of agitation on fragmentation of a recombinant strain of Asperg illus oryzae and its consequential effects on protein production have been investigated. Constant mass, 5.3-L chemostat cultures at a dilution rate of 0.05 h(-1) and a dissolved oxygen level of 75% air saturation, have been c onducted at 550, 700, and 1000 rpm. These agitation speeds were chosen to c over a range of specific power inputs (2.2 to 12 kW m(-3)) from realistic i ndustrial levels to much higher values. The use of a constant mass chemosta t linked to a gas blender allowed variation of agitation speed and hence ga s holdup without affecting the dilution rate or the concentration of dissol ved oxygen. The morphology of both the freely dispersed mycelia and clumps was characterized using image analysis. Statistical analysis showed that it was possible to obtain steady states with respect to morphology. The mean projected area at each steady state under growing conditions correlated wel l with the "energy dissipation/circulation" function, [P/(kD(3)t(c))], wher e P is the power input, D the impeller diameter, t(c) the mean circulation time, and k is a geometric constant for a given impeller. Rapid transients of morphological parameters in response to a speed change from 1000 to 550 rpm probably resulted from aggregation. Protein production (alpha-amylase a nd amyloglucosidase) was found to be independent of agitation speed in the range 550 to 1000 rpm (P/V = 2.2 and 12.6 kW m(-3), respectively), although significant changes in mycelial morphology could be measured for similar c hanges in agitation conditions. This suggests that mycelial morphology does not directly affect protein production (at a constant dilution rate and, t herefore, specific growth rate). An understanding of how agitation affects mycelial morphology and productivity would be valuable in optimizing the de sign and operation of large-scale fungal fermentations for the production o f recombinant proteins. (C) 1999 John Wiley & Sons, Inc.