Chemostat with Saccharomyces cerevisiae — At steady state, how do the concentrations of glucose, biomass, ethanol, and glycerol change with time?

Introduction:
Chemostats are designed to achieve steady state, where state variables such as biomass, substrate, and product concentrations are time-invariant. This allows reproducible physiology and simplifies data interpretation for organisms like Saccharomyces cerevisiae producing ethanol and glycerol.

Given Data / Assumptions:

• Constant dilution rate and feed composition.
• Well-mixed reactor with constant volume.
• Stable operating conditions without disturbances.

Concept / Approach:

At steady state, accumulation terms in the dynamic mass balances are zero: dC/dt = 0 for each measured component. The actual constant values depend on kinetics and dilution rate, but once steady state is reached, they do not change with time until conditions are altered.

Step-by-Step Solution:

Verification / Alternative check:

Plotting online measurements (for example, off-gas, OD, residual glucose) versus time at steady conditions should yield flat lines within noise after transients decay.

Why Other Options Are Wrong:

A and B: Monotonic drift contradicts steady state. D: Random changes imply instability, not steady operation. E: All tracked components stabilize when the chemostat is truly at steady state.

Common Pitfalls:

Confusing set-point chasing during start-up with steady state; wait for at least 3–5 residence times for stabilization and confirm with replicate samples.

Final Answer:

They remain constant with time (time derivatives are zero).