Intro to Biochemical Engineering

Question 1

Upstream processing

Answer 1

Before end of fermentation - involves expanding cell feed stock + sterilizing medium/bioreactor + performing fermentation

Question 2

Downstream processing

Answer 2

After fermentation - involves product purification + product formulation/packaging

Question 3

Batch operation

Answer 3

Medium + cells are added to bioreactor at start of fermentation - no further addition until fermentation complete

Question 4

Fed-batch operation

Answer 4

Medium added during cell growth but fed batch only harvested at end - liquid volume increases over time as medium is fed

Question 5

Continuous culture operation

Answer 5

Substrate fed continuously at same volumetric rate as product harvesting - maintains constant volume in bioreactor over time

Question 6

Specific rates

Answer 6

ROC in amount/concentration of molecule/cells normalized to amount of cells present in bioreactor

Question 7

How do cells act as autocatalysts while catalyzing substrate to product conversion?

Answer 7

More cells present in bioreactor = higher ROC in cell/substrate/product concentration

Question 8

Net specific cell growth rate

Answer 8

Difference between rate at which cells divide + rate at which cells die

Question 9

5 main phases of cell growth

Answer 9

1. Lag phase - cells adapt to their environment and mew is approx 0
2. Exponential growth phase - cells divide at constant time intervals - mew > 0 and constant
3. Deceleration phase - some nutrients become limiting + cells reorganize their metabolism to respond to this so mew decreases
4. Stationary phase - some cells begin to die but others still divide using molecules released by dead cells as substrate - cells often undergo programmed cell death (apoptosis) or autophagy (cells consume their own components to maintain functions) and mew approx 0
5. Death phase - accumulation of toxic products + lack of nutrients leads to cell death - cells can’t keep their metabolic functions and mew < 0

Question 10

Balanced growth

Answer 10

State in which average cell composition in culture doesn’t change w/time

Question 11

Imbalanced growth

Answer 11

Observed when cell metabolism undergoes transitions (like during deceleration phase)

Question 12

Contamination

Answer 12

Growth of an organism in fermenter other than organism used for production - may grow faster or slower than organism of interest

Question 13

Main sterilisation methods

Answer 13

1. Thermal sterilization for liquids + equipment
2. Filtration of gases + liquids
3. Chemical sterilization of equipment
4. Irradiation of surfaces/liquids

Question 14

Thermal sterilisation for liquids + equipment

Answer 14

• Most common - small-scale sterilization often uses autoclave (closed chamber into which saturated steam is introduced) or “sterilize-in-place bioreactors” for liquids but for large batches sterilization may occur continuously in heat exchanger before medium introduced to bioreactor
• For gases they’re compressed + filtered

Question 15

Filtration of gases + liquids

Answer 15

• Surface filters used for gas as this reduces pressure drop across filter - medium may also be filter-sterilized if it contains heat-labile components
• Less reliable than thermal sterilization so is usually accomplished in-line (as continuous process)

Question 16

Chemical sterilization of equipment

Answer 16

Toxic chem must be fully removed before introducing cell culture medium - chemicals used are toxic liquids/gases

Question 17

Irradiation of surfaces or liquids

Answer 17

UV irradiation used for surfaces + gamma irradiation used for medical equipment + liquids (less reliable than thermal sterilization for that tho) - gamma irradiation most commonly used to sterilize single-use components (small disposable bioreactors)

Question 18

Decimal reduction time

Answer 18

Time required to reduce expected # of organisms by 10-fold

Question 19

Probability of extinction

Answer 19

Mot crucial in sterilization - probability that all contaminating cells will be eliminated during sterilization + decreases exponentially when scaling up volumes being sterilized in batch process which also increases time required for batch sterilization

Question 20

Why can scaling up the heat sterilization of media be problematic?

Answer 20

Medium components may be damaged by heat + scaling up leads to increased ramp up/down times (time to reach sterilization T) as volume sterilized increases

Question 21

Pasteurization

Answer 21

Thermal sterilization process applied at low T + eliminates most pathogenic organisms but not spores