Industrial production of citric acid MCQs With Answer

In the industrial production of citric acid, microbial fermentation—primarily using Aspergillus niger—converts sugars into high-value citric acid for pharmaceutical applications and formulation. B. Pharm students should understand fermentation strategies (submerged and solid-state), media composition, carbon and nitrogen balance, pH control, aeration, temperature, and downstream processing such as neutralization, filtration, ion exchange, and crystallization. Key concepts include strain selection, genetic improvement, bioreactor design, process kinetics, yield optimization, contamination control, analytical assays (titration, HPLC), and regulatory quality standards for excipient-grade citric acid. This concise overview emphasizes both bioprocess engineering and pharmaceutical quality considerations. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. Which microorganism is most commonly used in industrial citric acid production?

• Escherichia coli
• Aspergillus niger
• Lactobacillus plantarum
• Saccharomyces cerevisiae

Correct Answer: Aspergillus niger

Q2. What is the primary carbon source used in most large-scale citric acid fermentations?

• Molasses (sucrose-rich substrates)
• Cellulose
• Fatty acids
• Lignin

Correct Answer: Molasses (sucrose-rich substrates)

Q3. Which fermentation mode is most widely applied industrially for citric acid production?

• Continuous fermentation without cell recycle
• Batch fermentation
• Semi-continuous two-stage fermentation
• Chemostat with constant dilution

Correct Answer: Batch fermentation

Q4. Low concentration of which nutrient promotes citric acid accumulation by Aspergillus niger?

• Carbon
• Nitrogen
• Trace metals
• Vitamins

Correct Answer: Nitrogen

Q5. Which pH range is optimal during the production phase of citric acid fermentation?

• pH 7.0–8.0
• pH 5.5–6.5
• pH 2.0–3.5
• pH 9.0–10.0

Correct Answer: pH 2.0–3.5

Q6. Which downstream step is typically used first after fermentation broth to separate fungal biomass from citric acid solution?

• Crystallization
• Neutralization with CaCO3
• Filtration or centrifugation
• Ion exchange

Correct Answer: Filtration or centrifugation

Q7. Why is calcium carbonate often added during citric acid fermentation?

• To supply calcium as a micronutrient
• To precipitate citric acid as calcium citrate and control pH
• To enhance fungal sporulation
• To act as a carbon source

Correct Answer: To precipitate citric acid as calcium citrate and control pH

Q8. Which impurity is of most concern for pharmaceutical-grade citric acid and must be tightly controlled?

• Residual sugars
• Heavy metals (e.g., lead)
• Protein nitrogen
• Sodium chloride

Correct Answer: Heavy metals (e.g., lead)

Q9. What analytical method is commonly used to quantify citric acid in production samples?

• Atomic absorption spectroscopy (AAS)
• High-performance liquid chromatography (HPLC)
• Northern blotting
• Mass spectrometry without chromatography

Correct Answer: High-performance liquid chromatography (HPLC)

Q10. Which by-product formation competes with citric acid production in Aspergillus niger cultures?

• Oxalic acid
• Lactic acid
• Ethanol
• Acetone

Correct Answer: Oxalic acid

Q11. Solid-state fermentation (SSF) for citric acid offers which advantage over submerged fermentation?

• Better sterility and contamination control
• Lower oxygen transfer limitations and simpler equipment
• Higher yields on low-moisture agro-residues
• Faster product recovery by direct crystallization

Correct Answer: Higher yields on low-moisture agro-residues

Q12. Which trace metal must be carefully limited to avoid diverting carbon away from citric acid synthesis?

• Manganese
• Iron
• Zinc
• Copper

Correct Answer: Manganese

Q13. What is the central metabolic reason for citric acid accumulation in A. niger under optimal production conditions?

• High activity of citrate synthase and inhibited aconitase leading to citrate overflow
• Activation of the glyoxylate cycle diverting isocitrate
• Complete blockage of glycolysis
• Excessive beta-oxidation of fatty acids

Correct Answer: High activity of citrate synthase and inhibited aconitase leading to citrate overflow

Q14. After neutralization of fermentation broth with lime, what compound must be formed and then decomposed to recover citric acid?

• Calcium citrate
• Magnesium citrate
• Sodium citrate
• Potassium citrate

Correct Answer: Calcium citrate

Q15. Which process is commonly used to convert calcium citrate back to free citric acid?

• Treatment with sulfuric acid to form insoluble gypsum and free citric acid
• Evaporation under reduced pressure
• Solvent extraction with ethyl acetate
• Ion exchange on a cation resin

Correct Answer: Treatment with sulfuric acid to form insoluble gypsum and free citric acid

Q16. What is a critical design consideration for bioreactors producing citric acid at industrial scale?

• Minimizing dissolved oxygen to near zero
• Providing high aeration and efficient oxygen transfer
• Maintaining very high shear to fragment fungal pellets
• Operating at extremely high temperatures (above 50°C)

Correct Answer: Providing high aeration and efficient oxygen transfer

Q17. Which genetic strategy is commonly applied to improve citric acid producing strains?

• Deletion of citrate synthase gene
• Overexpression of genes increasing glycolytic flux and citrate export
• Insertion of ethanol production pathways
• Knockout of glucose uptake systems

Correct Answer: Overexpression of genes increasing glycolytic flux and citrate export

Q18. What effect does high dissolved manganese concentration have on citric acid fermentation?

• Increases citric acid yield
• Stimulates biomass formation and reduces citric acid yield
• Eliminates oxalic acid formation
• Prevents spore formation without affecting yield

Correct Answer: Stimulates biomass formation and reduces citric acid yield

Q19. For pharmaceutical excipient use, what quality test is essential for citric acid raw material?

• Assay for active pharmaceutical ingredient (API) potency
• Assay for citric acid content and heavy metal limits
• Microbial challenge testing under packaging conditions
• Viscosity measurement

Correct Answer: Assay for citric acid content and heavy metal limits

Q20. Which parameter is used to express productivity in citric acid fermentation processes?

• g citric acid / g dry biomass
• g citric acid / L broth / h
• mol CO2 produced / mol glucose consumed
• kg biomass / m3 reactor

Correct Answer: g citric acid / L broth / h

Q21. Why is strict control of contamination important in industrial citric acid plants?

• Contaminants always increase citric acid yield
• Contamination can consume substrate and reduce product yield or produce unwanted by-products
• Contaminants are easily removed later by crystallization
• Contamination reduces the need for aeration

Correct Answer: Contamination can consume substrate and reduce product yield or produce unwanted by-products

Q22. Which solvent extraction method is occasionally used for citric acid recovery from dilute solutions?

• Extraction with hexane
• Extraction with organic solvents using tertiary amines
• Supercritical CO2 extraction without modifiers
• Extraction with methanol at high pH

Correct Answer: Extraction with organic solvents using tertiary amines

Q23. What is the main purpose of ion-exchange chromatography in citric acid purification?

• To concentrate citric acid and remove ionic impurities
• To sterilize the product
• To precipitate proteins
• To crystallize citric acid directly

Correct Answer: To concentrate citric acid and remove ionic impurities

Q24. How does temperature typically affect citric acid production in Aspergillus niger fermentations?

• Extremely high temperatures (>50°C) increase yield
• Optimal moderate temperatures (around 28–30°C) favor production
• Temperature has no effect on fungal metabolism
• Production is best at refrigeration temperatures (4°C)

Correct Answer: Optimal moderate temperatures (around 28–30°C) favor production

Q25. Which strategy can increase citric acid concentration in fed-batch processes?

• Continuous addition of nitrogen-rich feed
• Controlled feeding of carbon source to avoid substrate inhibition
• Rapid pH swings between 2 and 7
• Elimination of aeration during feed

Correct Answer: Controlled feeding of carbon source to avoid substrate inhibition

Q26. What role does oxygen play in citric acid biosynthesis by A. niger?

• Oxygen is not required; process is anaerobic
• High oxygen tension supports oxidative metabolism and citrate formation
• Oxygen inhibits glycolysis and reduces production
• Oxygen converts citrate to oxalate directly

Correct Answer: High oxygen tension supports oxidative metabolism and citrate formation

Q27. Which regulatory documentation is important when sourcing citric acid for pharmaceutical use?

• GRAS notice only
• Compendial compliance (e.g., USP/Ph.Eur.) and certificate of analysis
• Only vendor promotional literature
• No documentation is required for excipients

Correct Answer: Compendial compliance (e.g., USP/Ph.Eur.) and certificate of analysis

Q28. Which physical property of citric acid affects its handling and storage in pharmaceutical production?

• High volatility
• Hygroscopicity
• Magnetic susceptibility
• High flammability

Correct Answer: Hygroscopicity

Q29. What is the typical industrial approach to minimize oxalate contamination in citric acid product?

• Use of manganese-rich media
• Control of metal ions and pH to suppress oxalic acid pathway
• Heating broth to evaporate oxalate
• Addition of sodium oxalate to the medium

Correct Answer: Control of metal ions and pH to suppress oxalic acid pathway

Q30. Which final purification step is commonly used to obtain crystalline citric acid of pharmaceutical grade?

• Spray drying of fermentation broth
• Recrystallization from aqueous solution
• Direct lyophilization of crude broth
• Adsorption on activated carbon only

Correct Answer: Recrystallization from aqueous solution

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