Rheology of fermentation systems MCQs With Answer
This quiz collection is designed for M.Pharm students studying Bioprocess Engineering and Technology who need a deeper, application-oriented understanding of fermentation broth rheology. The questions cover fundamental concepts (viscosity, shear rate, rheological models), time-dependent behaviours (thixotropy, rheopexy), measurement techniques (rheometer geometries, vane method), and practical implications for fermentation operation and scale-up (mixing, oxygen transfer, impeller selection, high-cell density broths). Each MCQ challenges you to connect rheological theory with fermentation practice — useful for exam preparation and for designing or troubleshooting lab- and pilot-scale fermentations where rheology critically affects productivity and process control.
Q1. What is the definition of apparent viscosity in fermentation broths?
- The intrinsic viscosity obtained from dilute solution extrapolation
- The ratio of shear stress to shear rate measured at a specific shear rate and temperature
- The viscosity of the continuous phase (usually water) in the broth
- The viscosity obtained after correcting for particulate volume fraction
Correct Answer: The ratio of shear stress to shear rate measured at a specific shear rate and temperature
Q2. Which constitutive equation represents the power-law (Ostwald–de Waele) model commonly used for non-Newtonian fermentation broths?
- τ = μ γ̇
- τ = τ0 + η∞ γ̇
- τ = K · γ̇^n
- τ = η(γ̇) + λ dγ̇/dt
Correct Answer: τ = K · γ̇^n
Q3. A flow behaviour index (n) less than 1 in the power-law model indicates which behaviour of the fermentation broth?
- Newtonian (constant viscosity)
- Shear-thinning (pseudoplastic) behaviour
- Shear-thickening (dilatant) behaviour
- Bingham plastic with yield stress
Correct Answer: Shear-thinning (pseudoplastic) behaviour
Q4. The Herschel–Bulkley model used for some fermentation slurries can be expressed as:
- τ = μ γ̇
- τ = τ0 + K · γ̇^n
- η = η0 exp(−α γ̇)
- τ = K ln(γ̇) + τy
Correct Answer: τ = τ0 + K · γ̇^n
Q5. How does an increase in broth viscosity typically affect the volumetric oxygen transfer coefficient (kLa) in stirred fermenters?
- kLa increases because higher viscosity increases bubble breakup
- kLa decreases because diffusion and bubble dispersion become hindered
- kLa remains unchanged—viscosity affects only power draw
- kLa first increases then decreases as viscosity rises
Correct Answer: kLa decreases because diffusion and bubble dispersion become hindered
Q6. For non-Newtonian power-law fluids, what is commonly used to characterize the hydrodynamic similarity in stirred tanks?
- The classical Reynolds number for Newtonian fluids
- The generalized (power-law) Reynolds number that includes K and n
- The Prandtl number
- The Schmidt number only
Correct Answer: The generalized (power-law) Reynolds number that includes K and n
Q7. Which type of fermentation broth typically exhibits the most pronounced shear-thinning and viscoelastic behaviour?
- Low-cell-density bacterial broths with no extracellular polymers
- Filamentous fungal broths and EPS-rich cultures
- Pure yeast suspensions at low biomass
- Mineral salt solutions without biomass
Correct Answer: Filamentous fungal broths and EPS-rich cultures
Q8. What is thixotropy in the context of fermentation broths?
- Permanent shear-thickening under all shear rates
- Time-dependent decrease in viscosity under constant shear with reversible recovery at rest
- Increase in viscosity upon prolonged rest
- Viscosity that depends only on temperature and not on shear history
Correct Answer: Time-dependent decrease in viscosity under constant shear with reversible recovery at rest
Q9. Which rheometric method is preferred to measure yield stress in particulate or structured fermentation broths with minimal disturbance?
- Cannon-Fenske capillary viscometer
- Cone–plate rheometer with polished surfaces
- Vane-in-cup geometry to reduce wall slip and structural disruption
- Falling-sphere viscometry in turbulent flow
Correct Answer: Vane-in-cup geometry to reduce wall slip and structural disruption
Q10. The Metzner–Otto concept relates the mean shear rate in an agitated vessel to which operational parameter?
- The bulk gas holdup only
- The impeller rotational speed via a proportionality constant (γ̇ = ks · N)
- The medium osmotic pressure
- The ratio of impeller diameter to tank diameter only
Correct Answer: The impeller rotational speed via a proportionality constant (γ̇ = ks · N)
Q11. Which rheological parameter most directly increases power consumption for a stirred fermentation at constant speed?
- Low flow behaviour index (n) below 0.5
- High temperature
- High consistency index (K)
- Low gas flowrate
Correct Answer: High consistency index (K)
Q12. How does increasing broth viscosity affect mixing time in a stirred tank fermenter, all else equal?
- Mixing time decreases because momentum diffusion is faster
- Mixing time increases because viscous damping reduces bulk circulation
- Mixing time is independent of viscosity
- Mixing time oscillates unpredictably with viscosity
Correct Answer: Mixing time increases because viscous damping reduces bulk circulation
Q13. What is the expected effect of higher broth viscosity on cell sedimentation?
- Higher viscosity increases settling velocity of cells
- Higher viscosity reduces settling velocity, improving suspension stability
- Viscosity has no effect; only cell density matters
- Higher viscosity causes cells to aggregate and settle faster
Correct Answer: Higher viscosity reduces settling velocity, improving suspension stability
Q14. Extracellular polysaccharide (EPS) production during fermentation typically causes which rheological change?
- Decrease in broth viscosity and loss of structure
- No change in rheology because EPS are inert
- Increase in viscosity and often enhanced shear-thinning and viscoelasticity
- Instantaneous gelation to a brittle solid
Correct Answer: Increase in viscosity and often enhanced shear-thinning and viscoelasticity
Q15. For a highly viscous, non-Newtonian fermentation broth (e.g., fungal culture), which impeller type is typically most effective for bulk mixing?
- High-speed Rushton turbine with small blades
- Helical ribbon or anchor impeller designed for viscous fluids
- Propellers optimized for low-viscosity fluid only
- Sparger-only mixing without mechanical agitation
Correct Answer: Helical ribbon or anchor impeller designed for viscous fluids
Q16. To protect shear-sensitive cells (e.g., mammalian cultures) during fermentation, which design/operation choice is most appropriate?
- Use very high impeller tip speed to enhance mixing
- Operate an airlift reactor or use low-shear impellers and low tip speeds
- Add abrasive beads to reduce cell clumping
- Maximize rpm to reduce mixing time irrespective of shear
Correct Answer: Operate an airlift reactor or use low-shear impellers and low tip speeds
Q17. Which rheometer geometry is preferred when measuring the rheology of suspensions with large particles or flocs in fermentation broths to avoid wall slip?
- Cone–plate geometry with smooth surfaces
- Capillary viscometer with small diameter capillary
- Vane or concentric cylinder (Couette) geometries that minimize slip
- Ubbelohde viscometer designed for Newtonian liquids
Correct Answer: Vane or concentric cylinder (Couette) geometries that minimize slip
Q18. How is thixotropy commonly quantified in rheological testing of fermentation broths?
- By measuring the instantaneous viscosity at a single shear rate only
- By calculating the area of the hysteresis loop between the increasing and decreasing shear-rate flow curves
- By measuring pH and correlating to viscosity
- By counting colony forming units after shearing
Correct Answer: By calculating the area of the hysteresis loop between the increasing and decreasing shear-rate flow curves
Q19. What is rheopexy and how does it differ from thixotropy in fermentation broths?
- Rheopexy is time-dependent decrease in viscosity; thixotropy is time-independent
- Rheopexy is time-dependent increase in viscosity under shear; thixotropy is time-dependent decrease under shear
- Rheopexy is the same as shear-thinning; thixotropy is shear-thickening
- Rheopexy refers to temperature dependence only
Correct Answer: Rheopexy is time-dependent increase in viscosity under shear; thixotropy is time-dependent decrease under shear
Q20. When scaling up a shear-thinning fermentation process, which scale-up rule helps maintain similar mixing and mass transfer performance?
- Keep geometric similarity but ignore power requirements
- Maintain constant impeller tip speed regardless of viscosity changes
- Scale-up based on maintaining constant power input per unit volume (P/V) or matching kLa when feasible
- Always scale-up by maintaining the same impeller diameter only
Correct Answer: Scale-up based on maintaining constant power input per unit volume (P/V) or matching kLa when feasible
