77. In a fed batch culture the feed rate of concentrated substrate is increased with timewhile the RPM
of the impeller is kept constant. You will most likely observe one of thefollowing:
(a) A decline in the D.O. values of the culture
(b) An increase in the D.O. values of the culture
(c) An oscillation in the D.O. values of the culture
(d) No change in the D.O. values of the culture

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Correct Answer:

(a) A decline in the D.O. values of the culture

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Explanation:

In a fed-batch culture, a concentrated substrate is fed into the culture medium over time to provide nutrients to the microorganisms or cells, helping them grow and produce the desired product. The feed rate, along with the agitation rate (impeller RPM), can significantly influence the dissolved oxygen (D.O.) levels in the culture, which is critical for cellular respiration and growth.

Impact of Increasing Feed Rate with Constant RPM

• Substrate Feeding and Oxygen Demand: As you increase the feed rate of a concentrated substrate, the organisms or cells in the culture consume more nutrients. This leads to an increased metabolic activity and a higher demand for oxygen.

• Dissolved Oxygen Levels: Oxygen is required for cellular respiration, and when metabolic activity increases due to higher nutrient availability, the dissolved oxygen (DO) consumption also increases. If the impeller RPM is held constant, the mixing and oxygen transfer rate remain unchanged. Therefore, with an increasing oxygen demand and a constant supply rate, the DO levels in the culture are likely to decrease.

Why the Answer is (a) A decline in the D.O. values of the culture:

• Higher Substrate Feed = Increased Metabolic Activity = Higher Oxygen Demand.

• Constant RPM = Constant Oxygen Transfer Rate.

• As a result, the oxygen transfer may not be able to keep up with the increased consumption, leading to a decline in DO values.

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Why are the Other Options Incorrect?

• (b) An increase in the D.O. values of the culture:
  Increasing the feed rate without increasing the impeller RPM or oxygen supply does not lead to an increase in DO values. The oxygen demand increases, but without proper mixing or aeration, DO values are likely to decline.

• (c) An oscillation in the D.O. values of the culture:
  Oscillations in DO values can occur under specific conditions such as rapid fluctuations in substrate consumption or aeration, but increasing the feed rate with constant RPM does not typically lead to oscillations in DO values. Instead, it generally leads to a steady decline as the oxygen consumption increases.

• (d) No change in the D.O. values of the culture:
  Given that substrate feeding increases the metabolic activity of the culture, it is highly unlikely that there will be no change in the DO values. A steady-state condition in DO is not typically observed without adjustments in mixing or aeration.

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Conclusion

When increasing the feed rate of concentrated substrate in a fed-batch culture while keeping the impeller RPM constant, the oxygen demand increases due to higher metabolic activity. Since the oxygen transfer rate remains constant (due to the constant RPM), the DO values will most likely decline. Therefore, the correct answer is (a) A decline in the D.O. values of the culture.