Physical Properties
Recommended Separation Techniques
Ranked by effectiveness for lactic acid recovery from fermentation broths.
Lactic acid carries a -1 charge at fermentation pH. Strong anion exchange resins selectively bind lactate ions, separating them from neutral sugars (glucose, sucrose) and uncharged impurities. Elution with NaOH or NaCl. Typical recovery: 85–95%.
With MW of 90 Da, lactic acid passes through NF membranes (200–500 Da MWCO) while retaining proteins and larger impurities. Charge effects enhance selectivity—lactate anion is partially rejected by negatively charged NF membranes at pH > pKa (3.86).
Calcium lactate crystallization is the classical industrial method. Add Ca(OH)₂ to neutralize and precipitate calcium lactate crystals. Requires subsequent acidification to regenerate free lactic acid. Solubility-dependent; works well when broth is pre-concentrated.
Lactic acid has a boiling point of 200°C—water can be evaporated at reduced pressure to concentrate the product. Often used as a concentration step before crystallization or final purification. Energy-intensive but straightforward.
Common Impurity Separations
| Separate From | Key Difference | Best Technique | Selectivity Basis |
|---|---|---|---|
| Glucose | Charge (lactate -1 vs glucose 0) | Ion Exchange | Charge-based binding |
| Cells / Biomass | Size (90 Da vs micron-scale cells) | Centrifugation / MF | Size exclusion |
| Proteins | MW (90 Da vs >10 kDa) | UF (10 kDa MWCO) | Molecular weight cutoff |
| Acetic Acid | pKa (3.86 vs 4.76), BP (200 vs 118°C) | Distillation / Reactive Extraction | Volatility difference |
pH-Dependent Behavior
Lactic acid is a weak monoprotic acid. Its ionization state affects separation performance.
Henderson-Hasselbalch Equation
At pH < 3.86 (below pKa): Predominantly protonated (HA) — neutral, lower solubility in organic solvents, suitable for reactive extraction with tertiary amines.
At pH > 3.86 (above pKa): Predominantly dissociated (A−) — charged lactate ion, high aqueous solubility, suitable for ion exchange capture.
Practical Implications
| pH Range | Dominant Form | Separation Impact |
|---|---|---|
| pH 2.0 | >99% HA (protonated) | Best for solvent extraction, esterification |
| pH 3.86 | 50/50 HA/A− | pKa = equal mixture |
| pH 5.0 | >93% A− (lactate) | Best for ion exchange, NF rejection |
| pH 7.0 | >99.9% A− | Fully ionic, high NF rejection |
Frequently Asked Questions
What is the best way to purify lactic acid from fermentation broth?
The most common industrial approach is cell removal (centrifugation or microfiltration), followed by ultrafiltration to remove proteins, then ion exchange chromatography or reactive extraction. For food-grade lactic acid, a final polishing step with activated carbon and/or nanofiltration is typical. Design your specific route with untangle.bio.
Can lactic acid be crystallized directly from broth?
Free lactic acid has very high water solubility (1,000 g/L) and does not crystallize easily. The classical approach is to form calcium lactate crystals by adding Ca(OH)₂, filter the crystals, then re-acidify with H₂SO₄ to regenerate free lactic acid. This adds cost and produces gypsum waste.
What membrane MWCO should I use for lactic acid?
For passing lactic acid (90 Da) through a membrane while retaining proteins, use UF with 10–30 kDa MWCO. Lactic acid passes freely (<2% rejection). For concentrating lactic acid, tight NF (150–300 Da) partially rejects lactate anions at pH > 3.86 due to charge effects (Donnan exclusion).
How does pH affect lactic acid separation?
Below pKa 3.86, lactic acid is protonated and neutral—ideal for solvent extraction. Above pKa, it exists as charged lactate—ideal for ion exchange and enhanced NF rejection. Most fermentation broths operate at pH 5–7 where lactate is the dominant species.
Related Molecules
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