# Bioprocess Engineering Questions and Answers – Continuous Culture

This set of Bioprocess Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Continuous Culture”.

1. In the following equation, what is the unit of Dilution rate”D”? (Given: F = dm3h-1 and V = dm3)
D = F/V
a) dm3
b) h-1
c) dmh-1
d) dm3h-1

Explanation: The flow of medium into the vessel is related to the volume of the vessel by the term dilution rate, D, defined as:
D = F/V
Where, F is the flow rate (dm3h-1)
V is the volume (dm3)
Thus, D is expressed in units h-1.

2. under continuous culture which of the following condition is applicable?
a) μmax < D
b) μmax > D
c) μmax = D
d) μmax = 0

Explanation: The net change in cell concentration over a time period may be expressed as:
dx/dt = growth – output
dx/dt = μx – Dx
Under steady-state conditions, the cell concentration remains constant, thus dx/dt = 0 and:
μx = Dx
μ = D
Under steady-state conditions the specific growth rate is controlled by the dilution rate. Under batch culture conditions an organism will grow at its maximum specific growth rate and, therefore, it is obvious that a continuous culture may be operated only at dilution rates below the maximum specific growth rate. Thus, within certain limits the dilution rate may be used to control the growth rate of the culture.

3. If one starts with 10,000 (104) cells in a culture that has a generation time of 2 h, how many cells will be in the culture after 4 and 48 h?
a) 4.0×104 cells, 1.7×1011 cells
b) 4.2×104 cells, 1.1×1011 cells
c) 4.6×104 cells, 1.5×1011 cells
d) 4.8×104 cells, 1.3×1011 cells

Explanation: Use the equation X= 2n X0, where X0 is the initial number of cells, n is the number of generations, and X is the number of cells after n generations.
After 4 h, n=4 h/2 h per generation= 2 generations:
X=22 (104) = 4.0×104 cells
After 48 h, n= 24 generations:
X = 224 (104) = 1.7×1011 cells

This represents an increase of less than one order of magnitude for the 4-h culture, and seven orders of magnitude for the 48-h culture.

4. “The growth rate of the cells will be greater than the dilution rate”, Is this statement applicable for decrease in biomass production?
a) True
b) False

Explanation: The growth rate of the cells will be less than the dilution rate and they will be washed out of the vessel at a rate greater than they are being produced, resulting in a decrease in biomass concentration.
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5. An alternative of chemostat is turbidostat?
a) True
b) False

Explanation: An alternative type of continuous culture to the chemostat is the turbidostat, where the concentration of cells in the culture is kept constant by controlling the flow of medium such that the turbidity of the culture is kept within certain, narrow limits.

6. What do you mean by wall growth in bioreactors?
a) Growth of cells in the wall
b) Consumption of paints by the cells from the coated walls
c) Biomass concentration is increased
d) Immobilized cells consumes substrate within reactors

Explanation: Wall growth is commonly encountered practical difficulty in which the organism adheres to the inner surfaces of the reactor resulting an increase in heterogeneity. The immobilized cells are not subject to removal from the vessel but will consume substrate resulting in the suspended biomass concentration being lower than predicted.

7. The yield factor is proportional to the dilution rate?
a) True
b) False

Explanation: This is attributed to the phenomenon of microorganisms utilizing a greater proportion of substrate for maintenance at low dilution rates. Effectively, the yield factor decreases at low dilution rates.

8. Which is the correct meaning of the internal feedback from the following?
a) Effluent stream is more concentrated than in the vessel
b) Effluent stream is less concentrated than in the vessel
c) Effluent stream is equally concentrated with the vessel
d) Effluent stream is not concentrated

Explanation: Internal feedback means that limiting the exit of biomass form the chemostat such that the biomass in the effluent stream is less concentrated than in the vessel.

9. Mixed cultures can be maintained using chemostat cultures in _________
a) Fed-Batch culture
b) Semi-Batch culture
c) Batch culture
d) Continuous culture

Explanation: Important feature of chemostats and other continuous culture systems is that they are well-mixed so that environmental conditions are homogenous or uniform and microorganisms are randomly dispersed and encounter each other randomly. Therefore, competition and other interactions in the chemostat are global, in contrast to biofilms.

10. Which of the following is not an advantage of continuous culture?
a) Can be used for different reactions every day
b) Little risk of infection or strain mutation
c) Long growth periods of subtrates/microbes
d) Eliminating the inherent down time for cleaning and sterilization

Explanation: Long growth periods not only increase the risk of contamination but also dictate that the bioreactor must be extremely reliable and consistent, incurring a potentially larger initial expenditure in higher quality equipment.

11. Which of the following is not a disadvantage of continuous culture?
a) Long growth periods of subtrates/microbes
b) Maintenance of mixed cultures
c) Requires feed-batch culturing
d) Viscosity of mixture for filamentous organisms

Explanation: Continuous culture provides a higher degree of control than a batch culture. Growth rates can be regulated and maintained for extended periods. By varying the dilution rate, biomass concentration can be controlled. Secondary metabolite production can be sustained simultaneously along with growth. In steady state continuous culture, mixed cultures can be maintained using chemostat cultures – unlike in a batch process where one organism usually outgrows another.

12. Continuous addition of sugars in “Fed-batch” fermentation is done to __________
a) Produce methane
b) Purify enzymes
d) Obtain antibiotics

Explanation: A fed-batch is a biotechnological batch process which is based on feeding of growth limiting nutrient substrate to a culture continuous addition of sugars in fed batch fermentation is done to purify enzymes.

13. Name the phase which is a period of adaptation of the cells to the new environment.
a) Lag phase
b) Log phase
c) Exponential phase
d) Stationary phase

Explanation: Lag time is defined as the initial period in the life of a bacterial population when cells are adjusting to a new environment before starting exponential growth. Many factors influence the duration of lag time, including inoculum size, the physiological history of the cells, and the precise physiochemical environment of both the original and the new growth medium.

14. In chemostat, constant cell concentration is maintained?
a) True
b) False

Explanation: In the chemostat, in the steady state adjusting the concentration of one substrate controls cell growth. In the turbidostat, cell growth is kept constant by using turbidity to monitor the biomass concentration and the rate of feed of nutrient solution is appropriately adjusted. In the chemostat, constant chemical environment is maintained, while in a turbidostat constant cell concentration is maintained.

15. Which of the following is used to grow bacterial cultures continuously?
a) Haemostat
b) Chemostat
c) Bacteria cannot be grown in continuous culture
d) Thermostat

Explanation: A chemostat (from chemical environment is static) is a bioreactor to which fresh medium(Bacteria) is continuously added, while culture liquid containing left over nutrients, metabolic end products and microorganisms are continuously removed at the same rate to keep the culture volume constant.

Sanfoundry Global Education & Learning Series – Bioprocess Engineering.

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