78. In an anaerobic fermentation producing ethanol, the ethanol yield (Yp/s) –
(a) Decreases with increasing biomass yield (Yx/s)
(b) Increases with increasing biomass yield (Yx/s)
(c) Initially increases & then decreases with increasing biomass yield (Yx/s)
(d) Is independent of biomass yield (Yx/s)

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Correct Answer:

(a) Decreases with increasing biomass yield (Yx/s)

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Explanation:

In an anaerobic fermentation process, especially for ethanol production, the microbial cells convert sugars (such as glucose) into ethanol and biomass. The relationship between ethanol yield and biomass yield plays a crucial role in optimizing fermentation processes. The ethanol yield is often measured as Yp/s (ethanol produced per substrate consumed), while Yx/s represents the biomass yield (biomass produced per substrate consumed).

Understanding the Relationship Between Ethanol Yield and Biomass Yield:

1. Ethanol Production: In anaerobic conditions, yeast or other microorganisms convert sugar into ethanol and carbon dioxide. The main objective in an ethanol production process is to maximize ethanol production while minimizing biomass growth.

2. Biomass Formation: As biomass yield increases (i.e., the microorganisms are growing more), more of the available substrate is used for cell growth rather than for ethanol production. This results in less substrate being available for ethanol synthesis, thus reducing the overall ethanol yield.

3. The Trade-Off:

   • When the biomass yield Yx/s increases, the microorganism uses more of the available substrate to produce new cells, leading to a lower ethanol yield.

   • Conversely, when the biomass yield is lower, more substrate is directed toward ethanol production, resulting in a higher ethanol yield.

4. The Ideal Scenario: In an efficient ethanol production process, it’s desirable to minimize biomass formation and maximize ethanol production. This is why ethanol yield typically decreases with increasing biomass yield.

Why the Other Options are Incorrect:

• (b) Increases with increasing biomass yield (Yx/s):
  This is incorrect because, as biomass yield increases, more substrate is used for growth, leaving less substrate available for ethanol production. Therefore, ethanol yield typically decreases with increasing biomass yield.

• (c) Initially increases & then decreases with increasing biomass yield (Yx/s):
  While this could be true in some complex metabolic pathways, in simple ethanol fermentation, increasing biomass typically leads to a decrease in ethanol yield. There is no clear phase where it initially increases, so this is not the best answer.

• (d) Is independent of biomass yield (Yx/s):
  This statement is incorrect because, in fermentation, there is an inverse relationship between ethanol yield and biomass yield. As biomass yield increases, ethanol yield tends to decrease.

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Conclusion:

In anaerobic fermentation for ethanol production, the ethanol yield (Yp/s) tends to decrease with increasing biomass yield (Yx/s) because more substrate is used for the growth of microbial cells rather than for ethanol production. Therefore, the correct answer is (a) Decreases with increasing biomass yield (Yx/s).