Section 4: Bioprocess Engineering and Process Biotechnology
Mass transfer in bioprocesses (kLa)
Subtopic: Fundamentals of Mass transfer in bioprocesses (kLa)
Question Statement:
Which one of the following can NOT be a limiting substrate if Monod's growth kinetics is
applicable?
Options:
(A) Extracellular carbon source
(B) Extracellular nitrogen source
(C) Dissolved oxygen
(D) Intracellular carbon source
Official Answer Key:
D
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2019 Q.25)**
**1. Governing Concept**:
- **Topic**: Bioprocess Engineering & Microbial Nutrition
- A limiting substrate in microbial fermentation is a dissolved chemical nutrient whose availability constrains the specific growth rate ($\mu$) according to Monod kinetics:
$\mu = \frac{\mu_{\max} S}{K_s + S}$
- Nutrients that dissolve in the aqueous growth medium and can act as limiting substrates include:
- Carbon/energy sources (e.g., glucose, sucrose, methanol, ethanol).
- Nitrogen sources (e.g., ammonium salts, nitrates, urea).
- Phosphates, sulfates, and trace minerals ($\text{Fe}^{2+}, \text{Mg}^{2+}$).
- Dissolved oxygen in aerobic cultures.
- Insoluble, volatile, or non-metabolizable inert materials cannot serve as a limiting substrate.
**2. Option Analysis**:
- **Option (A) is INCORRECT**: Glucose is the classic growth-limiting carbon substrate.
- **Option (B) is INCORRECT**: Dissolved oxygen frequently acts as the growth-limiting nutrient.
- **Option (C) is INCORRECT**: Ammonium is a common limiting nitrogen source.
- **Option (D) is CORRECT**: Formally validated by bioprocess substrate definition.
**3. Final Answer**:
- **Correct Choice**: **D**
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