In fed-batch (batch-fed) operation, how is the growth-limiting substrate supplied to the bioreactor?

Introduction / Context:
Fed-batch processes are widely used to avoid substrate inhibition, control specific growth rate, and extend production phases. Unlike simple batch, fed-batch introduces fresh substrate after startup to manage cell physiology and productivity.

Given Data / Assumptions:

• A growth-limiting substrate is selected (e.g., glucose, ammonia).
• Objective is to regulate growth and product formation.
• Operation does not include continuous outflow (unlike chemostat).

Concept / Approach:
In fed-batch, substrate is supplied during fermentation—either in pulses (periodically) or as a controlled continuous feed. This keeps the bulk concentration low while sustaining metabolism, preventing overflow or repression, and enabling high cell densities.

Step-by-Step Solution:

Verification / Alternative check:
Industrial processes (e.g., recombinant protein expression, high-gravity ethanol) demonstrate improved titers by fed-batch feeding compared to one-shot batch additions.

Why Other Options Are Wrong:

• Only at the beginning: describes batch, not fed-batch.
• Only at the end: does not support controlled growth.
• Never added: contradicts the definition of fed-batch.
• Only if washout occurs: washout is a chemostat concern; fed-batch has no continuous outflow.

Common Pitfalls:
Confusing fed-batch with continuous culture; fed-batch has inflow but no outflow (except sampling), so volume increases over time.

Final Answer:
Added during the process, typically periodically or continuously as a feed