Q.1 Which type of bioreactor operates with continuous input of fresh medium and simultaneous removal of culture broth at the same rate?
Batch reactor
Fed‑batch reactor
Continuous stirred‑tank reactor (CSTR)
Plug‑flow reactor (PFR)
Explanation - A CSTR maintains a constant volume by feeding fresh medium while withdrawing culture broth at the same rate, achieving steady‑state operation.
Correct answer is: Continuous stirred‑tank reactor (CSTR)
Q.2 The volumetric mass transfer coefficient (kLa) in an aerobic bioreactor primarily depends on which of the following variables?
pH of the medium
Agitation speed and aeration rate
Nutrient concentration
Temperature of the cooling jacket
Explanation - kLa is a function of gas–liquid interfacial area (a) and liquid‑phase mass transfer coefficient (k), both of which increase with higher impeller speed and gas flow rate.
Correct answer is: Agitation speed and aeration rate
Q.3 In a stirred‑tank bioreactor, the power number (Np) is used to calculate:
Oxygen solubility
Shear stress on cells
Impeller power consumption
Heat transfer coefficient
Explanation - The power number (Np) relates impeller geometry, speed, and power draw: P = Np·ρ·N³·D⁵, where N is impeller speed and D is impeller diameter.
Correct answer is: Impeller power consumption
Q.4 Which impeller type is most commonly used for low‑shear mixing in mammalian cell cultures?
Rushton turbine
Marine impeller
Axial flow (pitch‑blade) impeller
Paddle impeller
Explanation - Axial flow impellers generate gentle, predominantly axial flow, minimizing shear damage to shear‑sensitive mammalian cells.
Correct answer is: Axial flow (pitch‑blade) impeller
Q.5 The residence time distribution (RTD) curve of an ideal plug‑flow reactor is:
A narrow spike at the mean residence time
A broad exponential decay
A flat line over time
A Gaussian distribution
Explanation - In an ideal PFR, all fluid elements have the same residence time, producing a Dirac delta‑like spike at the mean residence time.
Correct answer is: A narrow spike at the mean residence time
Q.6 When scaling up a bioreactor from laboratory to pilot scale, which criterion is most reliable for maintaining similar mixing conditions?
Constant impeller tip speed
Constant power per unit volume (P/V)
Constant Reynolds number
Constant oxygen transfer rate (OTR)
Explanation - Keeping P/V constant preserves the energy input per volume, leading to comparable mixing intensity across scales.
Correct answer is: Constant power per unit volume (P/V)
Q.7 What is the main purpose of a sparger in an aerobic bioreactor?
To introduce nutrients
To remove heat
To supply sterile air or oxygen
To stir the culture
Explanation - A sparger disperses gas (air or oxygen) into the liquid, providing the dissolved oxygen needed for aerobic metabolism.
Correct answer is: To supply sterile air or oxygen
Q.8 Which of the following best describes the effect of increasing the impeller diameter while keeping speed constant in a stirred‑tank bioreactor?
Decreases shear stress
Increases power consumption
Reduces oxygen transfer
Lowers mixing time
Explanation - Power consumption scales with D⁵; a larger impeller diameter dramatically raises the power draw at the same speed.
Correct answer is: Increases power consumption
Q.9 Which parameter is most directly related to the heat removal capacity of a bioreactor?
kLa
U·A (overall heat transfer coefficient × area)
pH control range
Dissolved oxygen set‑point
Explanation - The rate of heat removal is given by Q = U·A·ΔT, where U is the overall heat transfer coefficient and A is the heat transfer area.
Correct answer is: U·A (overall heat transfer coefficient × area)
Q.10 In a fed‑batch bioreactor, why is the feed usually added in an exponential manner?
To keep the pH constant
To match the exponential growth of the culture
To avoid foaming
To reduce power consumption
Explanation - Exponential feeding supplies substrate at a rate that matches the cells’ exponential growth, preventing substrate limitation or excess.
Correct answer is: To match the exponential growth of the culture
Q.11 Which of the following is a common method for measuring dissolved oxygen in a bioreactor?
pH electrode
Clark‑type polarographic sensor
Thermocouple
Conductivity probe
Explanation - A Clark electrode measures dissolved O₂ by reducing it at a cathode, generating a current proportional to O₂ concentration.
Correct answer is: Clark‑type polarographic sensor
Q.12 What is the typical range of Reynolds number for turbulent flow in a stirred‑tank bioreactor?
Re < 100
100 < Re < 1000
Re > 10,000
10,000 < Re < 100,000
Explanation - Turbulent regime in bioreactors usually begins at Re > 10,000, providing efficient mixing and mass transfer.
Correct answer is: Re > 10,000
Q.13 Which control strategy is most suitable for maintaining a constant dissolved oxygen level in a bioreactor?
On/off control of temperature
PID control of aeration and agitation
Manual adjustment of feed rate
Constant pH buffer addition
Explanation - A PID controller can continuously adjust gas flow and impeller speed to keep dissolved O₂ at a set point.
Correct answer is: PID control of aeration and agitation
Q.14 In a perfusion bioreactor, the cell retention device (e.g., spin filter) is used to:
Increase oxygen solubility
Separate cells from spent medium
Adjust pH automatically
Provide sterility filtration
Explanation - Cell retention devices allow continuous removal of waste while keeping cells in the bioreactor for high‑density culture.
Correct answer is: Separate cells from spent medium
Q.15 The term 'shear sensitivity' in bioprocesses most often refers to:
Sensitivity to temperature changes
Sensitivity to pH fluctuations
Sensitivity to mechanical forces causing cell damage
Sensitivity to substrate concentration
Explanation - Shear‑sensitive cells, like mammalian or stem cells, can be damaged by high shear stresses generated during mixing or pumping.
Correct answer is: Sensitivity to mechanical forces causing cell damage
Q.16 Which of the following is NOT a typical scale‑up criterion for bioreactors?
Constant volumetric power input (P/V)
Constant tip speed
Constant oxygen transfer rate (OTR)
Constant impeller blade thickness
Explanation - Blade thickness is a design detail, not a scale‑up criterion; the other three are commonly used to preserve mixing, mass, and heat transfer characteristics.
Correct answer is: Constant impeller blade thickness
Q.17 Foam formation in a bioreactor is typically reduced by adding:
Antifoam agents
More air sparge
Higher agitation speed
More nutrient feed
Explanation - Antifoam compounds lower surface tension, preventing foam stability and collapse of gas‑liquid interfaces.
Correct answer is: Antifoam agents
Q.18 The volumetric oxygen transfer rate (OTR) is given by the equation OTR = kLa·(C* – C), where C* is:
The actual dissolved oxygen concentration
The oxygen concentration at saturation
The inlet gas oxygen fraction
The temperature of the medium
Explanation - C* denotes the saturation concentration of dissolved O₂ under the current temperature and pressure.
Correct answer is: The oxygen concentration at saturation
Q.19 A bioreactor operating in batch mode is most appropriate for:
Producing a constant product concentration over time
Studying kinetic parameters without feed interruptions
Continuous removal of product
High‑throughput screening of many strains
Explanation - Batch mode provides a closed system ideal for monitoring growth curves and metabolic rates without external perturbations.
Correct answer is: Studying kinetic parameters without feed interruptions
Q.20 Which material is commonly used for constructing bioreactor vessels to ensure sterility and ease of cleaning?
Aluminum
Stainless steel (SS 316L)
Polypropylene
Glass
Explanation - SS 316L offers corrosion resistance, durability, and can be sterilized by autoclaving or steam-in-place.
Correct answer is: Stainless steel (SS 316L)
Q.21 The term ‘specific growth rate (µ)’ in microbial kinetics is defined as:
Change in biomass concentration per unit time
Biomass concentration divided by substrate concentration
Rate of product formation per unit biomass
Derivative of ln(biomass) with respect to time
Explanation - µ = (1/X)(dX/dt) = d(ln X)/dt, representing the exponential growth rate of the culture.
Correct answer is: Derivative of ln(biomass) with respect to time
Q.22 In a bioreactor equipped with a heat exchanger, the overall heat transfer coefficient (U) is most affected by:
Impeller geometry
Gas sparging rate
Fouling of the heat transfer surface
pH of the medium
Explanation - Fouling adds thermal resistance, reducing U and the reactor’s ability to remove or add heat efficiently.
Correct answer is: Fouling of the heat transfer surface
Q.23 Which of the following best describes the function of a baffle in a stirred‑tank bioreactor?
To increase the liquid volume
To reduce vortex formation and improve mixing
To provide a surface for cell attachment
To introduce gas bubbles
Explanation - Baffles disrupt circular flow, preventing vortexing and enhancing radial mixing efficiency.
Correct answer is: To reduce vortex formation and improve mixing
Q.24 When performing a scale‑up based on constant tip speed, which parameter will change the most between a 5 L and a 500 L reactor?
Impeller diameter
Impeller speed (rpm)
Power per unit volume
Gas sparge rate
Explanation - Keeping tip speed (π · D · N) constant means that as impeller diameter increases, rpm must decrease proportionally.
Correct answer is: Impeller speed (rpm)
Q.25 A high oxygen transfer rate (OTR) is most critical for which type of bioprocess?
Anaerobic fermentation
Mammalian cell culture producing monoclonal antibodies
Yeast batch production of ethanol
Algal phototrophic growth
Explanation - Mammalian cells are aerobic and highly sensitive to oxygen limitation, requiring high OTR for optimal productivity.
Correct answer is: Mammalian cell culture producing monoclonal antibodies
Q.26 Which type of sensor is typically used for online measurement of pH in a bioreactor?
Amperometric sensor
Glass electrode with reference electrode
Infrared spectrometer
Thermistor
Explanation - A glass pH electrode generates a voltage proportional to hydrogen ion activity, providing rapid pH readings.
Correct answer is: Glass electrode with reference electrode
Q.27 In a bioreactor, the term 'dead zone' refers to:
Area with high temperature
Region where mixing and mass transfer are poor
Zone where cells aggregate
Section of the reactor that is not sterilized
Explanation - Dead zones are stagnant regions near the walls or bottom where flow is insufficient, leading to gradients.
Correct answer is: Region where mixing and mass transfer are poor
Q.28 What is the purpose of a cooling jacket in a bioreactor?
To increase oxygen solubility
To remove excess metabolic heat
To supply nutrients
To sterilize the broth
Explanation - Metabolic activity generates heat; the cooling jacket circulates coolant to maintain optimal temperature.
Correct answer is: To remove excess metabolic heat
Q.29 Which of the following is a typical indicator of microbial contamination in a sterile bioprocess?
Stable pH
Unexpected increase in off‑gas CO₂
Consistent dissolved oxygen levels
Constant temperature
Explanation - Contaminants often alter respiration rates, causing detectable spikes in CO₂ in the exhaust gas.
Correct answer is: Unexpected increase in off‑gas CO₂
Q.30 When operating a bioreactor at high cell densities, which phenomenon becomes a major limiting factor?
Insufficient mixing
Excessive foaming
Oxygen limitation
Low pH
Explanation - High cell densities consume oxygen rapidly, making O₂ transfer the primary bottleneck.
Correct answer is: Oxygen limitation
Q.31 In a fed‑batch process, the term 'feed strategy' most commonly refers to:
The timing of sterilization cycles
The schedule and composition of substrate addition
The method of temperature control
The placement of sensors
Explanation - Feed strategy defines how much and when nutrients are supplied to control growth and product formation.
Correct answer is: The schedule and composition of substrate addition
Q.32 Which of the following equations correctly defines the power number (Np) for a Rushton turbine?
Np = P / (ρ N³ D⁵)
Np = kLa / (N D)
Np = (U A) / (ρ c_p ΔT)
Np = V · µ / (R · T)
Explanation - The power number relates impeller power draw to fluid density, speed, and impeller diameter.
Correct answer is: Np = P / (ρ N³ D⁵)
Q.33 Which parameter is most directly affected by changing the gas sparge hole size in a bioreactor?
Impeller tip speed
Bubble size and gas–liquid interfacial area
Heat transfer coefficient
pH control range
Explanation - Sparge hole size determines bubble formation, influencing the interfacial area (a) and thus kLa.
Correct answer is: Bubble size and gas–liquid interfacial area
Q.34 What is the main advantage of using a perfusion bioreactor over a batch system for protein production?
Higher product concentration in the broth
Lower oxygen requirements
Continuous removal of product reduces inhibition
Simpler temperature control
Explanation - Perfusion keeps product away from cells, minimizing product inhibition and allowing prolonged high productivity.
Correct answer is: Continuous removal of product reduces inhibition
Q.35 The dimensionless group 'Damköhler number (Da)' compares:
Convective transport to diffusion
Reaction rate to mass transfer rate
Inertial forces to viscous forces
Thermal conductivity to convection
Explanation - Da = (reaction rate)/(mass transfer rate); high Da indicates reaction‑limited processes.
Correct answer is: Reaction rate to mass transfer rate
Q.36 In a bioreactor, which method is commonly used to sterilize the system before inoculation?
UV irradiation of the broth
Autoclaving of the vessel and steam‑in‑place (SIP)
Filtration of the inlet air only
Addition of chemical disinfectants
Explanation - SIP uses high‑pressure saturated steam to sterilize the reactor, ensuring aseptic conditions.
Correct answer is: Autoclaving of the vessel and steam‑in‑place (SIP)
Q.37 A bioreactor with a high aspect ratio (height/diameter) is typically used to:
Increase surface area for gas exchange
Facilitate better heat removal
Promote plug‑flow behavior
Reduce shear forces
Explanation - Tall, narrow reactors encourage axial flow and reduce back‑mixing, approximating plug‑flow characteristics.
Correct answer is: Promote plug‑flow behavior
Q.38 Which type of bioreactor is most suitable for cultivating photosynthetic microalgae?
Stirred‑tank reactor
Air‑lift reactor
Packed‑bed reactor
Bubble column reactor
Explanation - Air‑lift reactors provide gentle mixing and high gas transfer, ideal for light‑dependent microalgae cultures.
Correct answer is: Air‑lift reactor
Q.39 The term 'specific oxygen uptake rate (OUR)' is expressed in:
mol O₂ · L⁻¹ · h⁻¹
g product · L⁻¹ · h⁻¹
mmol O₂ · g⁻¹ · h⁻¹
kg biomass · m⁻³
Explanation - OUR is the rate at which cells consume oxygen per unit of biomass (mass basis).
Correct answer is: mmol O₂ · g⁻¹ · h⁻¹
Q.40 When a bioreactor experiences excessive temperature rise despite cooling, the most probable cause is:
Insufficient impeller speed
Faulty temperature sensor
Inadequate cooling water flow
Low dissolved oxygen
Explanation - If cooling water cannot remove heat generated by metabolism and agitation, temperature will rise.
Correct answer is: Inadequate cooling water flow
Q.41 Which computational tool is increasingly used to predict flow patterns and mass transfer in bioreactors?
Finite element analysis (FEA)
Computational fluid dynamics (CFD)
Molecular dynamics (MD)
Monte Carlo simulation
Explanation - CFD solves Navier‑Stokes equations to model mixing, shear, and gas–liquid interaction inside bioreactors.
Correct answer is: Computational fluid dynamics (CFD)
Q.42 In a bioprocess, the term 'critical shear rate' refers to:
The minimum agitation speed needed for mixing
The shear rate at which cell viability begins to decline
The rate of oxygen transfer
The rate of temperature change
Explanation - Critical shear rate is the threshold beyond which mechanical stress damages cells.
Correct answer is: The shear rate at which cell viability begins to decline
Q.43 What is the primary function of a peristaltic pump in a bioreactor system?
To regulate temperature
To provide sterile, pulsatile flow of fluids
To generate gas bubbles
To measure dissolved oxygen
Explanation - Peristaltic pumps move fluids through flexible tubing without contaminating the product.
Correct answer is: To provide sterile, pulsatile flow of fluids
Q.44 A bioreactor is operated at a constant pH of 7.0 using a feedback controller that adds NaOH or HCl. This control loop is an example of:
Open‑loop control
Feed‑forward control
Closed‑loop (feedback) control
Manual control
Explanation - The controller measures pH and adjusts acid/base addition accordingly, forming a feedback loop.
Correct answer is: Closed‑loop (feedback) control
Q.45 In a high‑cell‑density perfusion culture, which phenomenon often limits product yield?
Nutrient excess
Product inhibition due to accumulation in the broth
Oxygen limitation
Excessive foaming
Explanation - High cell densities increase oxygen demand, and despite perfusion, O₂ transfer may become limiting.
Correct answer is: Oxygen limitation
Q.46 What does the term 'biomass yield coefficient (Y_X/S)' represent?
Mass of substrate consumed per mass of biomass produced
Mass of biomass produced per mass of substrate consumed
Rate of product formation per unit biomass
Ratio of dissolved oxygen to substrate concentration
Explanation - Y_X/S = ΔX / ΔS, indicating how efficiently cells convert substrate into biomass.
Correct answer is: Mass of biomass produced per mass of substrate consumed
Q.47 During scale‑up, maintaining a constant 'mixing time' often results in:
Increasing impeller speed disproportionately
Decreasing gas flow rate
Increasing vessel height
Reducing temperature set‑point
Explanation - Larger vessels need higher impeller speeds to achieve the same mixing time due to increased volume.
Correct answer is: Increasing impeller speed disproportionately
Q.48 Which of the following statements about 'bubble column reactors' is true?
They require mechanical agitators for mixing
They provide high shear environments
They rely on gas‑induced circulation for mixing
They cannot be used for aerobic processes
Explanation - Rising bubbles create liquid circulation, eliminating the need for impellers.
Correct answer is: They rely on gas‑induced circulation for mixing
Q.49 In a bioprocess, the term 'downstream processing' refers to:
The cultivation phase in the bioreactor
Purification and recovery of the product after fermentation
Control of temperature and pH
Feeding strategies during fermentation
Explanation - Downstream processing includes separation, purification, and formulation of the target molecule.
Correct answer is: Purification and recovery of the product after fermentation
Q.50 The term 'specific productivity (q_P)' is defined as:
Product concentration per unit volume
Product formation rate per unit biomass
Substrate consumption rate per unit volume
Heat removal rate per unit mass
Explanation - q_P = (dP/dt)/X, indicating how much product each cell produces per unit time.
Correct answer is: Product formation rate per unit biomass
Q.51 When a bioreactor is operated under oxygen‑limited conditions, which metabolic pathway is typically up‑regulated?
Aerobic respiration
Fermentation
Photosynthesis
Glycogen synthesis
Explanation - Cells switch to anaerobic pathways like fermentation to regenerate NAD⁺ when O₂ is scarce.
Correct answer is: Fermentation
Q.52 In a bioreactor design, the term 'aspect ratio' is the ratio of:
Impeller diameter to vessel diameter
Vessel height to vessel diameter
Gas flow rate to liquid volume
Power input to temperature
Explanation - Aspect ratio = H/D; influences flow patterns and mixing characteristics.
Correct answer is: Vessel height to vessel diameter
Q.53 Which of the following is a major advantage of using a single‑use (disposable) bioreactor?
Higher temperature tolerance
Elimination of cleaning and sterilization steps
Greater impeller speed range
Improved heat transfer coefficients
Explanation - Disposable bioreactors are pre‑sterilized and discarded after use, reducing cross‑contamination risk and turnaround time.
Correct answer is: Elimination of cleaning and sterilization steps
Q.54 The term 'biomass concentration' in a bioreactor is usually expressed as:
g/L of dry cell weight
mol/L of dissolved oxygen
kPa of pressure
°C of temperature
Explanation - Biomass concentration quantifies the amount of cells per unit volume, often measured as dry weight.
Correct answer is: g/L of dry cell weight
Q.55 Which of the following phenomena is most directly reduced by using a low‑shear impeller?
Oxygen transfer rate
Foam formation
Cell lysis
Heat generation
Explanation - Low‑shear impellers minimize mechanical forces that can rupture delicate cells.
Correct answer is: Cell lysis
Q.56 The Henry's law constant for oxygen in water is needed to calculate:
kLa from measured O₂ transfer
pH buffering capacity
Heat transfer coefficient
Viscosity of the medium
Explanation - Henry’s constant relates gas partial pressure to its dissolved concentration, essential for O₂ mass transfer calculations.
Correct answer is: kLa from measured O₂ transfer
Q.57 In a bioreactor, the term 'autoclavable' means:
Able to withstand high‑temperature steam sterilization
Resistant to chemical corrosion
Capable of self‑cleaning
Compatible with high‑pressure gas
Explanation - Autoclavable components can be sterilized in an autoclave (121 °C, 15 psi) without damage.
Correct answer is: Able to withstand high‑temperature steam sterilization
Q.58 What is the main purpose of using a pH‑stat feeding strategy in a fed‑batch culture?
To maintain constant temperature
To control substrate addition based on pH changes
To adjust agitation speed
To regulate dissolved oxygen
Explanation - Metabolic acid production alters pH; a pH‑stat adds substrate when pH deviates, keeping growth balanced.
Correct answer is: To control substrate addition based on pH changes
Q.59 Which design parameter is most important for minimizing the formation of dead zones in a stirred‑tank bioreactor?
Number and placement of baffles
Diameter of the sparger
Material of the vessel
Temperature set‑point
Explanation - Baffles disrupt vortex formation and improve radial mixing, reducing stagnant zones.
Correct answer is: Number and placement of baffles
Q.60 When the specific growth rate (µ) exceeds the critical dilution rate (D_crit) in a continuous culture, the result is:
Wash‑out of the cells
Higher product concentration
Increased cell aggregation
Lower oxygen transfer
Explanation - If D > µ_max, cells are removed faster than they can reproduce, leading to wash‑out.
Correct answer is: Wash‑out of the cells
Q.61 Which type of sensor is most suitable for real‑time measurement of glucose concentration in a bioreactor?
Ion‑selective electrode
Enzyme‑based amperometric biosensor
Thermistor
pH electrode
Explanation - Glucose oxidase–based amperometric sensors produce a current proportional to glucose concentration.
Correct answer is: Enzyme‑based amperometric biosensor
Q.62 The term 'mass balance' applied to a bioreactor over a small time interval is expressed as:
In = Out + Accumulation
In – Out = Reaction rate
Accumulation = Production – Consumption
In + Out = Generation
Explanation - Mass balance states that input equals output plus the change stored in the system.
Correct answer is: In = Out + Accumulation
Q.63 In bioreactor scale‑up, maintaining a constant 'oxygen transfer coefficient (kLa)' is most critical for:
Temperature control
pH stability
Aerobic organism productivity
Viscosity reduction
Explanation - O₂ availability is crucial for aerobic microbes; keeping kLa constant helps preserve metabolic rates across scales.
Correct answer is: Aerobic organism productivity
Q.64 Which of the following best describes a 'plug‑flow bioreactor'?
A reactor with perfectly mixed contents
A reactor where fluid elements move as distinct plugs with no back‑mixing
A reactor that operates only in batch mode
A reactor that uses a membrane for cell retention
Explanation - Plug‑flow reactors have laminar flow with minimal axial dispersion, leading to uniform residence times.
Correct answer is: A reactor where fluid elements move as distinct plugs with no back‑mixing
Q.65 The primary reason for using a 'glass‑lined' bioreactor is:
Improved heat transfer
Reduced risk of metal ion contamination
Higher mechanical strength
Lower cost
Explanation - Glass lining prevents leaching of metal ions from stainless steel, important for sensitive products.
Correct answer is: Reduced risk of metal ion contamination
Q.66 Which factor most directly influences the formation of micro‑bubbles in a sparged bioreactor?
Impeller diameter
Sparger hole size and gas flow rate
Cooling jacket temperature
pH of the medium
Explanation - Smaller holes and higher flow rates generate finer bubbles, increasing gas‑liquid interfacial area.
Correct answer is: Sparger hole size and gas flow rate
Q.67 During a fed‑batch run, a sudden drop in dissolved oxygen is most likely caused by:
Increase in agitation speed
Addition of a carbon source feed
Decrease in temperature
Removal of antifoam
Explanation - Feeding substrate stimulates metabolic activity, raising oxygen consumption and causing a dip in dissolved O₂.
Correct answer is: Addition of a carbon source feed
Q.68 In a bioprocess, the term 'titer' refers to:
The concentration of cells per volume
The amount of product per unit volume of culture broth
The temperature set‑point of the reactor
The flow rate of the feed
Explanation - Titer is a measure of product concentration, typically expressed as g/L or mg/L.
Correct answer is: The amount of product per unit volume of culture broth
Q.69 Which of the following is a typical sign of insufficient mixing in a large‑scale bioreactor?
Uniform temperature throughout the vessel
Consistent pH readings
Gradient in dissolved oxygen levels
Steady gas flow rate
Explanation - Poor mixing leads to local O₂ depletion or accumulation, causing spatial variations in DO.
Correct answer is: Gradient in dissolved oxygen levels
Q.70 A 'mass transfer coefficient' (k) in a liquid phase is defined as:
Rate of heat removal per unit area
Rate of solute transfer per unit interfacial area and concentration difference
Impeller speed divided by vessel diameter
Viscosity of the medium
Explanation - k (m·s⁻¹) quantifies how quickly a solute moves across a phase boundary per area and driving force.
Correct answer is: Rate of solute transfer per unit interfacial area and concentration difference
Q.71 Which bioreactor configuration allows for simultaneous cell growth and product removal without cell loss?
Batch reactor
Fed‑batch reactor
Perfusion reactor with cell retention
Air‑lift reactor
Explanation - Cell retention devices keep cells inside while allowing spent medium and product to be removed continuously.
Correct answer is: Perfusion reactor with cell retention
Q.72 In a bioreactor, the term 'critical agitation speed' generally refers to the speed at which:
The temperature reaches the set‑point
Foam formation begins
The vortex reaches the top of the vessel
Gas bubbles become fully dispersed
Explanation - Beyond the critical speed, a vortex can draw liquid up the impeller shaft, potentially causing air ingestion or overflow.
Correct answer is: The vortex reaches the top of the vessel
Q.73 When scaling up a bioreactor based on constant 'mixing time', the relationship between impeller speed (N) and vessel diameter (D) is approximated by:
N ∝ D⁻³
N ∝ D⁻¹
N ∝ D²
N ∝ D⁰ (independent of D)
Explanation - Mixing time scales inversely with impeller speed and directly with vessel size; to keep mixing time constant, N must increase as D decreases (or vice‑versa).
Correct answer is: N ∝ D⁻¹
Q.74 Which parameter is typically monitored to assess the risk of substrate inhibition in a fed‑batch process?
Dissolved oxygen level
Substrate concentration in the feed line
pH of the culture broth
Agitation speed
Explanation - High substrate levels can inhibit enzyme activity; monitoring feed concentration helps prevent overload.
Correct answer is: Substrate concentration in the feed line
Q.75 In a bioprocess, 'autoclave' and 'steam‑in‑place (SIP)' are both used for:
Cooling the reactor
Sterilizing equipment and media
Measuring dissolved oxygen
Controlling pH
Explanation - Both methods employ high‑pressure saturated steam to achieve sterilization.
Correct answer is: Sterilizing equipment and media
Q.76 The 'mass transfer zone' in a bioreactor refers to:
The region where heat is removed
The area near the impeller where mixing is strongest
The region where gas transfer from bubbles to liquid occurs
The zone where cells adhere to surfaces
Explanation - Mass transfer zone is where interfacial area exists for O₂ (or other gases) to dissolve into the liquid.
Correct answer is: The region where gas transfer from bubbles to liquid occurs
Q.77 Which of the following is an advantage of using a 'dual‑axis impeller' in a bioreactor?
Reduced power consumption
Enhanced axial and radial flow for better mixing
Simplified cleaning process
Lower shear stress compared to a single‑axis impeller
Explanation - Dual‑axis impellers generate both axial and radial components, improving overall fluid circulation.
Correct answer is: Enhanced axial and radial flow for better mixing
Q.78 When a bioreactor is operated at a temperature above the optimal growth temperature for the organism, the most likely outcome is:
Increased cell viability
Reduced product formation
Higher oxygen solubility
Lower viscosity of the broth
Explanation - Temperatures above optimum stress cells, leading to slower growth and lower product yields.
Correct answer is: Reduced product formation
Q.79 The term 'gas holdup' in a bubble column reactor refers to:
The pressure drop across the column
The volume fraction of gas in the liquid phase
The amount of gas dissolved in the liquid
The speed of rising bubbles
Explanation - Gas holdup is the proportion of reactor volume occupied by gas bubbles, influencing mixing and mass transfer.
Correct answer is: The volume fraction of gas in the liquid phase
Q.80 Which of the following best describes the function of a 'check valve' in the feed line of a bioreactor?
To regulate temperature
To prevent backflow of culture broth into the feed reservoir
To control pH
To measure flow rate
Explanation - Check valves allow flow in one direction only, protecting the feed media from contamination.
Correct answer is: To prevent backflow of culture broth into the feed reservoir
Q.81 In a bioreactor, the term 'lag phase' refers to:
The period of rapid cell growth
The initial period where cells adapt to the environment before exponential growth
The phase of product accumulation
The time required to sterilize the vessel
Explanation - During lag phase, cells adjust to new conditions, repairing and synthesizing enzymes before dividing.
Correct answer is: The initial period where cells adapt to the environment before exponential growth
Q.82 Which of the following strategies can be used to increase the oxygen transfer rate (OTR) without changing agitation speed?
Increasing the vessel diameter
Adding antifoam
Increasing sparger gas flow rate
Reducing temperature
Explanation - Higher gas flow creates more bubbles, raising interfacial area and OTR without altering mechanical agitation.
Correct answer is: Increasing sparger gas flow rate
Q.83 The term 'process analytical technology (PAT)' in bioprocessing primarily aims to:
Reduce the size of the bioreactor
Implement real‑time monitoring and control to ensure product quality
Increase the temperature of the culture
Eliminate the need for sterile filtration
Explanation - PAT uses sensors and data analytics to monitor critical process parameters in real time.
Correct answer is: Implement real‑time monitoring and control to ensure product quality
Q.84 In a bioreactor, the term 'dead time' generally refers to:
The time required for the reactor to reach operating temperature
The period between sampling and analysis
The delay between a control signal and the actuator response
The time taken for cleaning the vessel
Explanation - Dead time can cause instability in control loops if not compensated.
Correct answer is: The delay between a control signal and the actuator response
Q.85 Which of the following is a typical advantage of using a 'membrane bioreactor' for wastewater treatment?
Higher shear stress on microbes
Simultaneous biomass retention and permeate clarification
Reduced oxygen transfer
Lower temperature control precision
Explanation - Membrane bioreactors retain high biomass concentrations while allowing clarified water to pass through.
Correct answer is: Simultaneous biomass retention and permeate clarification
Q.86 The term 'biocatalyst' in the context of bioreactors most often refers to:
A mechanical stirrer
A living cell or enzyme that drives a biochemical reaction
A temperature sensor
A pH buffer
Explanation - Biocatalysts are the biological agents (cells or isolated enzymes) that perform the desired conversion.
Correct answer is: A living cell or enzyme that drives a biochemical reaction
Q.87 When operating a bioreactor at a high cell density, the broth viscosity typically:
Decreases
Remains unchanged
Increases, leading to higher power requirements
Becomes zero
Explanation - Higher cell concentrations raise broth viscosity, demanding more power to maintain mixing.
Correct answer is: Increases, leading to higher power requirements
Q.88 In the context of bioreactor design, the term 'scale‑down model' is used to:
Increase the size of the production vessel
Create a smaller, representative system for troubleshooting and optimization
Reduce the amount of substrate used
Eliminate the need for sensors
Explanation - Scale‑down models mimic the larger process at reduced scale to study phenomena and test modifications.
Correct answer is: Create a smaller, representative system for troubleshooting and optimization
Q.89 Which of the following is a common method for controlling foam in a bioreactor?
Increasing temperature
Adding a base to raise pH
Injecting antifoam agents or using mechanical foam breakers
Reducing agitation speed
Explanation - Antifoams lower surface tension; foam breakers physically disrupt foam bubbles.
Correct answer is: Injecting antifoam agents or using mechanical foam breakers
Q.90 In a fed‑batch process, a 'linear feed' strategy means:
Feeding substrate at a constant rate over time
Feeding substrate exponentially
Feeding substrate only at the start of the run
Feeding substrate based on dissolved oxygen levels
Explanation - Linear feed adds a fixed amount of substrate per unit time, leading to a steady increase in concentration.
Correct answer is: Feeding substrate at a constant rate over time
Q.91 Which equation best relates the power number (Np) to Reynolds number (Re) for a Rushton turbine at low Re (laminar flow)?
Np = 16 / Re
Np = 0.5
Np = 0.2·Re
Np = 10·Re⁰·⁵
Explanation - In laminar flow, Np varies inversely with Re; for a Rushton turbine, Np ≈ 16/Re.
Correct answer is: Np = 16 / Re
Q.92 During a bioprocess, the term 'off‑gas analysis' typically monitors:
Concentrations of CO₂ and O₂ in the exhaust gas
pH and temperature of the broth
Viscosity of the culture medium
Cell density
Explanation - Off‑gas analysis provides real‑time information on metabolic activity and respiration rates.
Correct answer is: Concentrations of CO₂ and O₂ in the exhaust gas
Q.93 In a bioreactor equipped with a jacket for cooling, the overall heat transfer coefficient (U) is most affected by:
Impeller speed
Fouling of the jacket surface
Gas flow rate
pH of the culture
Explanation - Deposits on the heat transfer surface increase thermal resistance, lowering U.
Correct answer is: Fouling of the jacket surface
Q.94 The main reason for installing baffles in a cylindrical bioreactor is to:
Increase the surface area for gas transfer
Prevent formation of a central vortex and improve radial mixing
Facilitate cleaning
Reduce the need for temperature control
Explanation - Baffles break the circular flow, suppress vortexing, and enhance mixing efficiency.
Correct answer is: Prevent formation of a central vortex and improve radial mixing
Q.95 In a continuous bioreactor, the dilution rate (D) is defined as:
Flow rate of feed divided by reactor volume
Impeller speed per minute
Heat removal rate per unit volume
Oxygen transfer rate per unit biomass
Explanation - D = F/V, where F is the volumetric feed rate and V is the reactor volume.
Correct answer is: Flow rate of feed divided by reactor volume
Q.96 Which of the following is a typical indicator that a bioreactor is operating under substrate‑limited conditions?
Rapid increase in biomass concentration
Accumulation of by‑product metabolites
Stable dissolved oxygen levels
Decrease in CO₂ evolution rate
Explanation - Limited substrate reduces metabolic activity, leading to lower CO₂ production.
Correct answer is: Decrease in CO₂ evolution rate
Q.97 When designing a bioreactor for a temperature‑sensitive protein, which feature is most critical?
High shear impeller
Precise temperature control with rapid response
Large gas sparger
Wide pH range capability
Explanation - Temperature‑sensitive proteins require tight regulation to maintain stability and activity.
Correct answer is: Precise temperature control with rapid response
Q.98 The term 'specific heat capacity' (c_p) is used in bioreactor calculations to:
Determine the amount of heat required to raise the temperature of the broth
Calculate the oxygen solubility
Estimate the viscosity of the medium
Measure the cell growth rate
Explanation - c_p (J·kg⁻¹·K⁻¹) quantifies the heat needed to raise the temperature of a unit mass by one kelvin.
Correct answer is: Determine the amount of heat required to raise the temperature of the broth
Q.99 In a bioreactor, which of the following would most directly increase the Reynolds number (Re)?
Increasing the temperature of the broth
Increasing impeller diameter while keeping speed constant
Decreasing impeller speed
Adding antifoam
Explanation - Re = (ρ·N·D²)/μ; larger D raises Re, promoting turbulent flow.
Correct answer is: Increasing impeller diameter while keeping speed constant
Q.100 Which of the following is a common cause of pH drift during a high‑cell‑density fermentation?
Accumulation of CO₂ leading to carbonic acid formation
Increase in dissolved oxygen
Higher temperature
Excess antifoam addition
Explanation - CO₂ dissolves to form carbonic acid, lowering pH unless neutralized.
Correct answer is: Accumulation of CO₂ leading to carbonic acid formation
Q.101 The primary function of a 'heat exchanger' in a bioreactor system is to:
Increase oxygen solubility
Remove metabolic heat generated by the culture
Sterilize the broth
Supply nutrients
Explanation - Heat exchangers transfer excess heat from the broth to a cooling medium to maintain set temperature.
Correct answer is: Remove metabolic heat generated by the culture
Q.102 In a bioprocess, the term 'turbidity' is most often measured to estimate:
Cell density or biomass concentration
Oxygen solubility
pH stability
Temperature uniformity
Explanation - Turbidity (optical density) correlates with the concentration of suspended particles, such as cells.
Correct answer is: Cell density or biomass concentration
Q.103 Which of the following statements about 'fed‑batch' processes is true?
They always operate at constant volume
They allow higher cell densities than batch by feeding substrate gradually
They require continuous removal of product
They are identical to continuous cultures
Explanation - Fed‑batch feeds substrate over time, preventing substrate inhibition and enabling higher biomass.
Correct answer is: They allow higher cell densities than batch by feeding substrate gradually
Q.104 When a bioreactor is operated at a very low impeller speed, which problem is most likely to occur?
Excessive shear damage to cells
Insufficient mixing leading to concentration gradients
Overheating of the broth
High foam formation
Explanation - Low agitation reduces turbulence, causing zones of low nutrient or oxygen concentration.
Correct answer is: Insufficient mixing leading to concentration gradients
Q.105 Which of the following is an example of a 'non‑sterile' bioprocess?
Industrial ethanol production using Saccharomyces cerevisiae in a non‑sterile fed‑batch
Monoclonal antibody production in a GMP‑grade CSTR
Vaccine production in a sterile bioreactor
Cultivation of CHO cells in a closed system
Explanation - Ethanol fermentation can tolerate contaminating microbes, allowing non‑sterile operation.
Correct answer is: Industrial ethanol production using Saccharomyces cerevisiae in a non‑sterile fed‑batch
Q.106 The term 'heat of combustion' is most relevant when:
Designing a bioreactor for aerobic cultures
Calculating the energy released from substrate oxidation
Choosing a sensor for pH measurement
Determining the size of the sparger
Explanation - Heat of combustion quantifies the energy released during substrate metabolism, influencing heat removal requirements.
Correct answer is: Calculating the energy released from substrate oxidation