Physical Properties
Recommended Separation Techniques
Ranked by effectiveness for citric acid recovery from Aspergillus niger fermentation broths.
The classical industrial method. Adding Ca(OH)₂ to hot broth precipitates insoluble calcium citrate trihydrate. Filter the crystals, wash, then re-acidify with H₂SO₄ to release free citric acid and gypsum (CaSO₄). Citric acid is then crystallized by evaporative cooling. Recovery: 85–95%.
Citric acid carries up to -3 charge above pH 6.4 (fully dissociated). Strong anion exchange resins capture citrate while neutral sugars (glucose, residual sucrose) pass through. Elution with HCl or NaOH. Also effective with weak base resins operating below pKa₁ (3.13) for selective adsorption of undissociated acid.
Reactive extraction with tertiary amines (tri-n-octylamine) in organic diluents. Below pKa₁ (pH < 3.13), protonated citric acid forms ion-pair complexes with the amine. Back-extraction with NaOH or temperature swing. Avoids gypsum waste from calcium citrate route.
At 192 Da, citric acid is near NF membrane MWCO (200–500 Da). Charged NF membranes show high rejection of citrate³− at pH > 6.4, enabling concentration and partial purification. Combined with diafiltration to wash out small neutral impurities.
Common Impurity Separations
| Separate From | Key Difference | Best Technique | Selectivity Basis |
|---|---|---|---|
| Glucose | Charge (citrate -3 vs glucose 0) | Ion Exchange | Charge-based binding |
| Oxalic Acid | Solubility & MW (192 vs 90 Da) | Crystallization | Calcium salt solubility difference |
| A. niger Cells | Size (192 Da vs 3–10 µm hyphae) | Vacuum Filtration / MF | Size exclusion |
| Proteins | MW (192 Da vs >10 kDa) | UF (10 kDa MWCO) | Molecular weight cutoff |
Triprotic Acid Dissociation
Citric acid has three ionizable carboxyl groups, each affecting separation behavior differently.
Dissociation Equilibria
H₃Cit ↔ H₂Cit− + H+ (pKa₁ = 3.13) — H₂Cit− ↔ HCit²− + H+ (pKa₂ = 4.76) — HCit²− ↔ Cit³− + H+ (pKa₃ = 6.40)
Speciation vs. Separation
| pH Range | Dominant Species | Net Charge | Separation Impact |
|---|---|---|---|
| pH < 2.0 | H₃Cit (undissociated) | 0 | Best for solvent extraction (amine complexation) |
| pH 3.0–4.0 | H₂Cit− | -1 | Weak anion exchange binding |
| pH 5.0–6.0 | HCit²− | -2 | Good NF rejection, moderate IEX binding |
| pH > 7.0 | Cit³− | -3 | Maximum NF rejection, strong anion exchange |
Frequently Asked Questions
What is the industrial process for citric acid purification?
The classical route is: (1) vacuum filtration to remove A. niger mycelium, (2) precipitation as calcium citrate with Ca(OH)₂, (3) filtration of calcium citrate crystals, (4) acidification with H₂SO₄ to regenerate citric acid + gypsum, (5) carbon treatment and ion exchange polishing, (6) evaporative crystallization. Simulate this route with untangle.bio.
Why is citric acid produced by Aspergillus niger?
A. niger naturally overproduces citric acid as a metabolic overflow product under manganese-deficient, high-sugar, low-pH conditions. Industrial strains yield 80–200 g/L citric acid from sucrose or molasses in submerged or surface fermentation. Global production exceeds 2 million tonnes/year.
Can citric acid be purified without the calcium citrate step?
Yes. Modern processes use direct crystallization after carbon treatment (for high-purity broths), ion exchange chromatography, or solvent extraction with tertiary amines. These routes avoid gypsum waste. Electrodialysis and membrane-based processes are also emerging alternatives.
How does temperature affect citric acid crystallization?
Citric acid solubility increases sharply with temperature: 592 g/L at 20°C to >800 g/L at 80°C. Cooling crystallization from hot concentrated solutions produces citric acid monohydrate (below 36.6°C) or anhydrous crystals (above 36.6°C). Slow cooling yields larger, purer crystals.
Related Molecules
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