1475

7 Fluid Flow and Mixing

140

where 1,/Lis the viscosity of the suspending liquid and ~ is the volume fraction of solids. Eq. (7.13) has been found to hold for yeast and spore suspensions up to 14 vol% solids [21]. Many other cell suspensions do not obey Eq. (7.13); cell concentration can have a much stronger influence on rheological properties than is predicted by the Vand equation. As anexample, Figure 7.11 shows how cell concentration affects the apparent viscosity of various pseudoplastic plant-cell suspensions [22]; a doubling in cell concentration causes the apparent viscosity to increase by a factor of up to 90. Similar results have been found for mould pellets in liquid culture [23]. When viscosity is so strongly dependent on cell concentration, a steep drop in viscosity canbe achieved by diluting the broth with water or medium. Periodic removal of part of the culture and refilling with fresh medium reduces the viscosity and improves fluid flow in viscous fermentations.

readily the pellets are deformed during flow. The extent of branching of hyphal cells can also affect rheology; cells with a high branching frequency are generally less flexible than nonbranchingcells and produce higher viscosities. Sample rheological data for pseudoplastic mycelial broths are shown in Figure 7.12 [9]. Pelleted mycelia are more closely Newtonian in behaviour than filamentous cells; the flow behaviour index n for pellets is closer to unity. As indicated in Figure 7.12(b), the consistency index, and therefore the apparent viscosity, can differ by several orders of magnitudedepending on cell morphology.

7.8.3 Osmotic Pressure
Osmotic pressure of the culture medium affects cell turgor pressure. This in turn affects the hyphal flexibility of filamentous cells; increased osmotic pressure gives a lower turgor pressure making the hyphae more flexible. Improved hyphal flexibility reduces broth viscosity, and can also have a marked effect on yield stress.

7.8.2 CellMorphology
Morphological characteristics exert a profound influence on broth rheology. Disperse filamentous growth produces 'structure' in the broth, resulting in pseudoplasticity, yield-stress behaviour, or both. On the other hand, broths containing pelleted cells tends to be more Newtonian, depending on how

7.8.4 Product and Substrate Concentrations
When the product of fermentation is apolymer, continued excretion in batch culture raises the broth viscosity. For example, during production of exopolysaccharide by Aureobasidium pullulans, apparent viscosity measured at a shear rate of 1 s-1 can reach as high as 24 000 cP [18]. Cell concentration usually has a negligible effect on overall viscosity in these fermentations; the rheological properties of the fluid are dominated by thedissolved polymer. Other products having a similar effect on culture rheology include dextran, alginate and xanthan gum. In contrast, when the fermentation medium contains polymeric substrate such as starch, apparent viscosity will decrease as the fermentation progresses and the polymer is broken down. There could also be a progressive change from non-Newtonian to Newtonian behaviour. In mycelialfermentations this change is usually short lived; as the cells grow and develop a structured filamentous network, the broth becomes increasingly pseudoplastic and viscous.

Figure 7.11 Relationship between apparent viscosity and cell concentration for plant-cell suspensions forming aggregates of various size. ( 9 Cudrania tricuspidata 44-149 lain; (O) C. tr/cuspidata 149-297 lam; ([-1) Vincarosea44-149 lam; (ll) V. rosea 149-297 pm; (A) Nicotiana tabacum 150-800 lam. (From H. Tanaka, 1982, Oxygen transfer in broths of plant cells at high density. Biotechnol. Bioeng. 24, 425-442.) 200 ~
100806040(D

O r~ .,..q

g

20o L~

10 8 6I=h <

1

Cell concentration(g l - l )
II/i
I

I

I

I

20 30

7.9 Mixing
Mixing is a physical operation which reduces nonuniformities...