Agricultural/Bioengineering Bioprocess Engineering 2 — Questions and Answers
Question 1: In fed-batch fermentation, substrate is added incrementally rather than all at once. What is the primary reason for this feeding strategy?
- To reduce the cost of sterilization cycles
- To prevent substrate inhibition and maintain optimal growth conditions (Correct answer)
- To simplify downstream product recovery
- To minimize oxygen demand in the bioreactor
Correct answer: To prevent substrate inhibition and maintain optimal growth conditions
Controlled substrate feeding in fed-batch fermentation prevents substrate inhibition by keeping the substrate concentration below inhibitory levels while sustaining cell growth and product formation.
Question 2: What are the standard conditions for steam sterilization (autoclaving) of fermentation media?
- 100°C for 30 minutes at atmospheric pressure
- 121°C for 15–20 minutes at 15 psi (Correct answer)
- 135°C for 5 minutes at 20 psi
- 115°C for 45 minutes at 10 psi
Correct answer: 121°C for 15–20 minutes at 15 psi
The standard autoclave cycle at 121°C (250°F) and 15 psi for 15–20 minutes provides sufficient thermal energy to destroy all vegetative cells and spores.
Question 3: The Reynolds number (Re) in a stirred bioreactor is primarily used to determine:
- Dissolved oxygen uptake rate by cells
- Nutrient diffusion rate through cell membranes
- Flow regime (laminar vs. turbulent) within the vessel (Correct answer)
- Cell growth rate under shear stress
Correct answer: Flow regime (laminar vs. turbulent) within the vessel
Re = ρND²/μ for a stirred tank; low Re indicates laminar flow with poor mixing while high Re indicates turbulent flow with efficient bulk mixing.
Question 4: Which microorganism is the primary industrial producer of citric acid through submerged fermentation?
- Saccharomyces cerevisiae
- Aspergillus niger (Correct answer)
- Escherichia coli
- Bacillus subtilis
Correct answer: Aspergillus niger
Aspergillus niger accounts for over 90% of global citric acid production by fermenting sucrose or glucose under nitrogen-limited, low-pH conditions.
Question 5: During bioprocess scale-up from laboratory to industrial scale, which parameter is most commonly kept constant to preserve similar mixing and oxygen transfer characteristics?
- Aeration rate expressed as vvm (volume of air per volume of liquid per minute)
- Power input per unit volume (P/V) (Correct answer)
- Impeller rotational speed (rpm)
- Temperature difference across the vessel wall
Correct answer: Power input per unit volume (P/V)
Maintaining constant P/V (power per unit volume) preserves similar turbulence intensity, mixing time, and oxygen transfer rate, making it the most reliable scale-up criterion for stirred bioreactors.
Question 6: What is the primary purpose of a dissolved oxygen (DO) probe in a bioreactor control system?
- To measure pH changes caused by CO₂ accumulation
- To monitor oxygen levels and trigger adjustments in aeration or agitation (Correct answer)
- To detect microbial contamination by foreign organisms
- To measure residual substrate concentration online
Correct answer: To monitor oxygen levels and trigger adjustments in aeration or agitation
DO probes provide real-time feedback to control systems that adjust airflow rate, agitation speed, or back-pressure to maintain adequate oxygen supply for aerobic fermentation.
Question 7: In bioprocess engineering, 'aseptic technique' refers to the set of practices designed to:
- Maximize dissolved oxygen transfer efficiency in bioreactors
- Prevent contamination of pure microbial cultures by extraneous organisms (Correct answer)
- Increase the yield coefficient of the fermentation product
- Immobilize enzymes on solid support matrices
Correct answer: Prevent contamination of pure microbial cultures by extraneous organisms
Aseptic technique encompasses all operational procedures — including sterilization, sterile connections, and laminar flow — that prevent contamination of the process culture by unwanted microorganisms.
Agricultural/Bioengineering Bioprocess Engineering 2
In fed-batch fermentation, substrate is added incrementally rather than all at once.
What is the primary reason for this feeding strategy?