Page 484 - Biosystems Engineering

456 Cha pte r S i x tee n

processes (Gibbs et al. 2000). Koike et al. (2001) reported the effect of
C/N on ARA production and mycelial morphology in the culture of
M. alpina. The optimal C/N ratio of the medium was 15:20 for ARA
production with a balance between carbon and nitrogen sources.
When an enriched medium was used at a fixed C/N ratio of 20, the
cellular and ARA concentration were proportional to the total con-
centration of carbon and nitrogen sources. The whole pellet size did
not change with increasing C/N ratio when the ratio was below 20.
When the C/N ratio was higher than 20, pellet size increased in pro-
portion to the C/N ratio.
Generally, morphology of the fungi will affect the fermentation
(mycelia growth and metabolite formation) by changing the viscosity
(or rheology) of broth. Filamentous growth of fungi results in highly
viscous broth with a non-Newtonian, pseudoplastic flow behavior
(Papagianni 2004). Pelleted growth exhibited low viscosity and
approached Newtonian flow behavior, thus pellet morphology
allowed easier mixing and better mass transfer to culture broth
(Hamanaka et al. 2001). Although pellet-fermentation broths might be
Newtonian and have low viscosity, problems could arise with the
transport of nutrients inside the pellets, thus reducing productivity.
Therefore, dispersed forms of the pellets predominate in most indus-
trial fermentations (Riley et al. 2000). The morphology of fungi was
found to affect the productivity of ARA, with small pellets (1 to 2 mm
in diameter) showing the highest productivity, indicating that the
pellet form was more suitable, although a filamentous morphology
would be more suitable for ARA production from the viewpoint of
oxygen and mass transfer (Higashiyama et al. 1999b). From the cul-
tures in a shaker and jar fermenter, featherlike morphology with high
concentration of small-size pellets was found to be the most suitable
for ARA production (Park et al. 1999).
The change of the rheology of broth will directly or indirectly
affect other factors that synergistically determine the fermentation
process, for example, mass- and energy-transfer properties, biomass
concentration, mixing and aeration, nutrition and oxygen consump-
tion, temperature, pH, cell differentiation and growth, metabolite for-
mation, and specific growth rate. In turn, morphology of fungi will be
affected by the viscosity of broth, mutually together with other factors
mentioned above. Gibbs et al. (2000) reviewed the problems associ-
ated with mycelial morphology of fungal fermentation, including
formation of heterogeneous and stagnant zones, nutrients and oxy-
gen gradient and limitation, and inaccurate temperature control and
locally excessive heat. These problems may result in cellular stress,
causing strain degradation or mutation, especially on a larger scale.
The effect on nutrient and oxygen consumption is not only
affected by rheology, mixing, or aeration of the media but also by the
different morphology of the biomass. Oxygen is probably the most