Growth of filamentous fungi in conventional stirred tank fermenters is associated with many problems. The filamentous morphology of the biomass tends to make the system highly viscous and non-Newtonian in behavior resulting in a reduction in mixing and oxygen transfer rates. These have been quantified by Deindorfer and Gaden' for P . chrysogenum and by Steel and Brierly* for A . niger. Furthermore, the capacity of fungal mats to entrap air tends to make them float at the gas-liquid interface. In practice, the vigorous agitation used to overcome some of these problems, in addition to being energy intensive, can have a detrimental effect on shear sensitive biosystems. The tendency of the mycelial biomass to adhere to the internal surfaces and shaft of the fermenter poses additional operational problems, and makes harvesting and subsequent cleaning of the fermenter a cumbersome procedure. Most of these problems could be overcome if the fungus could be induced to grow as discrete spherical pellets. Various factors that affect the formation and growth of pellets have been reviewed by Metz and K~s s e n ,~ who also highlighted the role of surface active agents, both ionic and nonionic, in pelletization of fungal biomass. Gbewonyo and Wang4 have suggested the use of celite as a microporous inert carrier to confine the mycelial growth and form pellets. Martin and Bailey5 proposed a role for some as yet unidentified factors in peat extract, along with some additional nutrients, in pellet formation.In our laboratory, Neurospora crassa was induced to grow as discrete pellets by the addition of trace amounts of Triton N-101 (ethoxylated nonyl phenol) and used for the biosynthesis of tyrosinase enzyme in low-shear, aerated airlift fermenters. In addition to the pelletized growth of biomass, the efficacy of the system was judged by the ability of the fungus to form the product, which is synthesized as an intracellular secondary metabolite in response to the derepressing action of cycloheximide added during the stationary phase of fermentation."
MATERIALS AND METHODSNeurospora crassa (IARI 1702 CCBS 709.71) was obtained from the Indian Agricultural Research Institute. A conidial suspension was prepared in sterile distilled water from stock cultures and used as an inoculum. Initial spore concentrations were (4.56) x 10% medium in the air-lift fermenter batches and (1.5-3) x 10VL for the shake flask cultures. The medium used was similar to that recommended by Horowitz and coworkers" with either sucrose or sugarcane molasses as the sole carbon source. The initial sugar concentration was adjusted to either 10 or 20 g/L. Full strength "Vogel's mediumh" was used as the source of nitrogen and mineral nutrients instead of the recommended halfstrength. For pelletization, ca. 100 ppm Triton N-101 was added to the medium in all the experiments reported here.Shake flask studies were carried out in 5-L conical flasks with 2 L medium on rotary shakers (100 strokedmin). Medium sterilization was achieved in situ as per standard practice. In t...