Lactic Acid

Product Overview

Lactic Acid (CAS 50‑21‑5), chemically named 2‑hydroxypropanoic acid, is a naturally‑occurring hydroxycarboxylic acid and a green renewable organic‑acid raw material. Commercially available grades include industrial‑grade, food‑grade and pharmaceutical‑grade lactic acid. In terms of optical configuration, it is divided into L‑lactic acid, D‑lactic acid and racemic DL‑lactic acid. Among them, L‑lactic acid delivers the best biocompatibility and boasts the broadest application scope. The pure substance is a colourless, transparent, viscous liquid, odourless with a faint sour taste. It features high hydrophilicity and biodegradability, and is one of the few organic acids capable of complete natural degradation.

Lactic acid is miscible with water, ethanol and glycerol in any proportion, while barely soluble in non‑polar solvents such as chloroform and aliphatic hydrocarbons. Its molecule contains both hydroxyl and carboxyl groups, endowing it with excellent performance in acidity adjustment, chelation, moisturising‑softening, bacteriostasis & preservation, and polymer film‑forming. It can undergo various chemical reactions including esterification, polycondensation and neutralisation. It is widely applied in food flavouring & preservation, daily‑chemical moisturising, biomedicine, industrial biodegradable materials, environmental‑friendly cleaning and other fields, serving as a core basic raw material for the fine green chemical and bio‑manufacturing industries.

Basic Parameters

Production Processes

1. Microbial Fermentation (for L‑lactic acid)

Using carbohydrates such as glucose as substrates, homo‑fermentative lactic‑acid bacteria carry out fermentation: one molecule of glucose yields two molecules of L‑lactic acid.

Reaction equation:

Accumulated lactic‑acid inhibits bacterial metabolism. In industrial practice, calcium carbonate is added for neutralisation to generate calcium lactate and stabilise the pH of fermentation broth.

Neutralisation reaction:

The final fermentation broth contains calcium lactate, residual sugar, bacterial cells, protein and pigments. Subsequent separation‑and‑purification from broth adopts mainstream technologies: calcium‑salt process, esterification‑distillation, solvent extraction, electrodialysis and molecular distillation.

2. Purification Technologies for Fermentation Broth

(1) Calcium‑salt Process (traditional mature technology)

Five‑step workflow: Pretreatment → Crystallisation → Washing → Acidification → Recrystallisation

Pretreatment: filter fermentation broth to remove bacterial cells and solid impurities.

Crystallisation & washing: cool to crystallise L‑calcium lactate; wash crystals to remove partial soluble impurities.

Acidification: add concentrated sulfuric acid to calcium lactate for double‑decomposition reaction; calcium sulfate (gypsum) precipitates and free lactic acid is released.

Acidification reaction:

Recrystallisation: concentrate crude lactic‑acid solution and perform recrystallisation for further purification.

Process features: Simple and well‑established; calcium ions promote L‑lactic‑acid fermentation.
Drawbacks: High activated‑carbon consumption; limited removal of sugar impurities; moderate product purity.
Improvement: Combine activated carbon with macroporous adsorption resin for desugaring and decolourisation of calcium‑lactate solution; apply two‑stage cooling crystallisation. Crystallisation rate reaches 80 % and L‑lactic‑acid purity reaches 97.2 %.

(2) Esterification‑Distillation Purification (often used for deep polishing after calcium‑salt process)

Crude lactic acid (or crude lactic‑acid obtained from acidified calcium lactate) reacts with low‑molecular‑weight alcohols (methanol, ethanol) under acidic catalyst to form lactate esters (reversible reaction). Taking methanol as an example:

Methyl lactate has much lower boiling‑point than lactic acid. Methyl lactate is separated via vacuum distillation and then hydrolysed to regenerate high‑purity lactic acid; methanol is recovered and recycled.

Hydrolysis reaction:

Features: Efficient separation of lactic acid from sugar impurities for greatly improved purity. Industrially, crude lactic‑acid from calcium‑salt process is further refined by esterification‑distillation. Yield can reach 97 % and purity meets pharmaceutical‑grade requirements; widely adopted overseas.
Limitations: Methanol is toxic, flammable and explosive; excess alcohol is required to shift equilibrium towards ester formation.

(3) Solvent Extraction

Selective transfer of L‑lactic acid from aqueous fermentation phase into organic phase by extractants, followed by back‑extraction to obtain aqueous lactic‑acid solution. Enables fermentation‑separation coupling: in‑situ removal of generated lactic‑acid alleviates product‑inhibition on microorganisms.

Features: Low energy consumption; realises integrated fermentation‑separation.
Limitations: Most available extractants are high‑cost and some are toxic. This technology remains at laboratory stage without large‑scale industrialisation. It is a physical separation without characteristic chemical reaction.

(4) Electrodialysis (membrane‑separation process)

Cation‑exchange membranes are applied in dual‑feed mode with sodium L‑lactate and sulfuric acid to realise simultaneous ion‑exchange and electrodialysis without lactic‑acid loss.

Features: Effective removal of sugar impurities from lactic acid.
Drawbacks: Cannot separate lactic‑acid from other small‑molecule organic acids in system; membrane fouling and flux decay are major bottlenecks for industrialisation. Physical membrane separation without chemical reaction.

(5) Molecular Distillation

Molecular distillation is a special distillation operated under high vacuum: feed evaporates upon heating. The mean free path of vapour molecules exceeds the distance between heating surface and condensation surface, so molecules travel directly to condenser and condense. It breaks conventional vapour‑liquid‑equilibrium constraints and operates below atmospheric boiling‑point, suitable for heat‑sensitive substances such as lactic acid to avoid thermal decomposition.

Features: Laboratory validates purification feasibility for synthetic lactic‑acid mixtures under high‑vacuum low‑temperature conditions.
Drawbacks: Real fermentation broth contains residual sugars and multiple organic acids; extensive optimisation of pretreatment and process parameters is still needed. High vacuum requires mechanical pump plus diffusion pump, leading to high energy‑consumption and conflicting with energy‑saving targets. Every additional downstream unit increases operating cost. It is a hot research topic and not yet deployed in large‑scale industry.

Product Grades

1. Food‑Grade Lactic Acid

Applicable Standard: GB 1886.173‑2016 National Food‑Safety Standard - Food Additive: Lactic Acid. Old standard GB 2023‑2003 has been repealed. GB 1886.173‑2016 is currently effective.

Main Applications: Food‑processing industries including beverages, pastries, preserved fruits, meat products and compound seasoners. Functions: acidity adjustment, flavour modification and moderate bacteriostatic preservation.

Core Quality Indicators

Mass fraction of L‑lactic acid in total acid: ≥95 % (key food‑grade index for optical‑configuration control).

Total lactic‑acid content: multiple concentration specifications; common commercial concentrations: 80 %, 85 %.

Heavy metals (as Pb): ≤10 mg/kg.

Arsenic (As): ≤1 mg/kg.

Additional controlled items: chloride, sulfate, iron, ignition residue, reducing sugar; cyanide shall not be detected.

2. Pharmaceutical‑Grade (Pharmacopoeia‑Grade) Lactic Acid

Applicable Standards: Refer to Pharmacopoeia of the People's Republic of China domestically; USP (United States Pharmacopoeia), BP (British Pharmacopoeia) for overseas markets. Classified as pharmaceutical excipient.

Main Applications: API synthesis, pharmaceutical solvent, topical preparations, partial injection excipients; subject to the strictest safety control among all grades.

Core Quality Indicators

Lactic‑acid content: typical pharmacopoeia specification 85 %‑90 % (mass fraction).

Heavy metals (as Pb): ≤10 ppm.

Strict control over arsenic, chloride, sulfate, readily oxidisable substances, reducing sugar, ignition residue; some pharmacopoeias specify optical purity and microbial limits.

3. Industrial‑Grade Lactic Acid

Applicable Standards: No unified national standard in China; implemented per industrial or enterprise standards.

Main Applications: Electroplating & descaling, industrial cleaners, leather tanning, synthesis of lactate esters and lactate salts, coating additives and other chemical‑industry scenarios. Must NOT be used in food, pharmaceutical or feed applications.

Core Quality Indicators

Lactic‑acid content: typical specification 80 %; custom grades ranging 60 %‑90 % are available in market.

Looser impurity‑control; key items: colour, total acid, ignition residue. Limits for heavy‑metals and arsenic are far less stringent than food‑/pharmaceutical‑grade. No mandatory requirement for L‑lactic‑acid proportion in total acid.

4. Feed‑Grade Lactic Acid

Applicable Standards: Enterprise / industrial standards for feed additives; classified as feed acidity‑regulator additive.

Main Applications: Additive for livestock and aquafeed. It adjusts feed acidity, suppresses harmful microorganisms and optimises animal intestinal health. Often compounded with carriers such as silica to produce solid formulations.

Core Quality Indicators

Lactic‑acid content: liquid feed‑grade normally 50 %‑55 %; compound solid products follow labelled declaration.

Heavy metals (as Pb): relaxed limit ≤20 ppm; arsenic is also controlled.

Less stringent optical‑purity requirement (L‑lactic‑acid ratio) compared with food‑grade.

Industry Chain

Lactic acid is a core green raw material in bio‑chemical industry with well‑established closed‑loop industry chain, matching low‑carbon development trends. Upstream key feedstocks: corn starch, glucose, molasses, fermentation strains, purified water. Downstream covers food processing, personal‑care & cosmetics, biomedicine, biodegradable polymers, environmental‑friendly cleaning, textile & leather and many other industries.

Safety, Storage & Transportation

Lactic acid is a mild natural organic acid: non‑toxic, biodegradable and non‑strongly corrosive. It is non‑flammable & non‑explosive and not classified as hazardous chemical. Storage and transportation follow general‑chemical‑cargo specifications. Concentrated stock solution exerts slight irritation to skin and ocular mucosa; risks are low under normal operating conditions.

Storage requirements: Store in dry, cool, ventilated and clean warehouse. Avoid prolonged high‑temperature exposure and direct sunlight to prevent self‑polymerisation, discolouration and degradation. Keep containers tightly sealed to prevent dust, microbial contamination and water evaporation. Segregate storage; do not co‑store with strong alkalis or strong oxidants to avoid neutralisation or oxidative deterioration. Food‑grade and pharmaceutical‑grade products shall be stored separately to eliminate cross‑contamination.

Transportation requirements: Follow general chemical‑cargo rules. Take protective measures against rain, moisture, sunlight and package rupture/leakage. Ensure tight and stable packaging to prevent leakage during transit. Do not co‑transport with toxic, odorous‑corrosive or strong‑alkaline goods. High‑purity grades require clean transport to guarantee quality and compliance.

Safety notes: Stable under ambient temperature and pressure, non‑toxic with good biocompatibility. Dilute solutions are non‑irritant; concentrated stock solution mildly irritates mucous membranes. No fire‑explosion or severe‑corrosion hazard. Standard PPE including work gloves and dust‑mask is sufficient; maintain good ventilation at work site. In case of eye contact with stock solution, flush eyes thoroughly with plenty of running water. Operations are safe and controllable.

FAQ

Q: What are the core differences between L‑lactic acid and DL‑racemic lactic acid? How to select grades?
A: They share similar basic physicochemical properties. Major differences lie in optical activity and biological compatibility. L‑lactic acid is natural‑configuration, fully bio‑metabolisable without adverse effects, suitable for high‑demand scenarios: food, pharmacy, biodegradable polymers and high‑end personal‑care. DL‑racemic lactic acid is mixed‑configuration with weak biological activity and lower cost. It shall only be applied to industrial cleaning, leather‑textile processing and general chemical‑work without optical‑purity requirements. It must not substitute L‑lactic acid in high‑end applications.

Q: Why does stored lactic acid turn darker and become more viscous? How to prevent this phenomenon?
A: Lactic‑acid molecules contain hydroxyl and carboxyl groups. Prolonged high‑temperature, sunlight exposure or open‑container storage triggers partial self‑esterification‑polymerisation, raising viscosity and colour. Open storage also brings risks of microbial contamination and foreign‑ingress. Preventive measures: keep tightly sealed, store away from light, control warehouse temperature, avoid long‑term high‑temperature stacking and prohibit open storage. Proper storage maintains clear appearance and stable properties for long periods.

Q: What are the key differences and application prohibitions for industrial‑, food‑ and pharmaceutical‑grade lactic acid in China?
A: All grades share identical main chemical constituent; distinctions are impurity‑control precision, optical purity and regulatory qualification. Industrial‑grade has loose impurity specifications without biosafety certification and is restricted to industrial‑only usage; contact with food, pharmaceuticals and human skin is forbidden. Food‑grade undergoes sterilisation and decolourisation with strict heavy‑metal limits and holds food‑safety certification for food‑manufacturing. Pharmaceutical‑grade delivers the highest optical purity and cleanliness for high‑end biomedicine, medical‑aesthetics and biodegradable‑material production.

China is one of the world's major lactic‑acid producers and exporters. Abundant biomass feedstock resources and mature bio‑fermentation technologies support complete supporting industry‑chain and large‑scale production capacity. China can stably supply full‑series lactic‑acid products featuring high optical purity and low impurity. It possesses global‑leading advantages in production capacity, quality stability and cost‑competitiveness, satisfying domestic high‑end‑manufacturing demands as well as overseas export requirements.