Correct answer: (3) Risk of contamination is lower than batch fermentation.

In continuous culture-based fermentation, fresh medium is constantly added while culture broth is simultaneously removed, keeping cells in a near-steady state for long periods. This has specific consequences for growth phase, nutrient use, contamination risk, and control of growth rate.

Option analysis:

(1) The exponential phase of growth is extended.

• True.

• In continuous culture, especially in a chemostat, cells can be maintained in the exponential (log) phase for a long duration by continuously supplying nutrients and removing waste and excess cells. This is a key advantage over batch culture, where exponential growth is short-lived.

(2) Nutrients are utilized efficiently and faster.

• Generally true.

• Because fresh nutrients are continuously supplied at controlled rates and cells remain metabolically active, substrate can be used efficiently with high productivity per unit volume and time. There is less downtime than in batch systems, improving overall nutrient utilization.

(3) Risk of contamination is lower than batch fermentation.

• NOT true.

• Continuous systems run for extended periods (days to weeks). Any contaminant entering the system has time to grow and spread, and there are more potential entry points (feeds, sampling, etc.). Maintaining sterility is harder than in batch systems, where each run starts with a freshly sterilized vessel. Therefore, contamination risk is typically considered higher, not lower, in continuous culture. This statement is the incorrect one.

(4) A chemostat allows maintenance of growth rate during fermentation.

• True.

• In a chemostat, the dilution rate (rate of fresh medium addition and culture removal) is set by the operator. This fixes the specific growth rate of the culture (up to a maximum), allowing the growth rate and biomass concentration to be maintained at a defined steady state.