Lactic Acid

Lactic acid is an organic compound of significant biochemical and industrial importance, known chemically as 2-hydroxypropanoic acid. With the formula C₃H₆O₃, it is a simple carboxylic acid containing a hydroxyl group adjacent to the carboxyl group. Lactic acid occurs naturally in the human body, plants, and fermented foods, and it serves as an intermediate in various metabolic pathways. It has wide applications in medicine, food preservation, polymer production, and cosmetics. Its dual role as both a metabolic product and a valuable industrial chemical makes it an essential subject in biological and chemical sciences.

Chemical Composition and Structure

Lactic acid is composed of three carbon atoms, six hydrogen atoms, and three oxygen atoms. Structurally, it contains a carboxylic acid group (–COOH) and a hydroxyl group (–OH) attached to the second carbon atom, giving it the configuration CH₃–CH(OH)–COOH.
A key feature of lactic acid is its chirality. The carbon atom bearing the hydroxyl group is asymmetric, giving rise to two optical isomers:

• L-(+)-lactic acid (S-enantiomer): The biologically active form produced in human metabolism.
• D-(–)-lactic acid (R-enantiomer): Produced by certain bacterial species.

A racemic mixture containing equal proportions of both forms is termed DL-lactic acid. These enantiomers exhibit distinct properties and biological roles, making chirality an important consideration in pharmaceutical and biochemical applications.
Lactic acid is a weak acid with a dissociation constant (Ka) of approximately 1.4 × 10⁻⁴ at 25°C. It is miscible with water and alcohols, has a pKa value around 3.86, and appears as a colourless, hygroscopic liquid or white solid depending on temperature and concentration.

Historical Background

The discovery of lactic acid dates back to 1780 when Swedish chemist Carl Wilhelm Scheele first isolated it from sour milk, which led to its name derived from the Latin lac, meaning milk. In 1856, Louis Pasteur identified lactic acid as a product of bacterial fermentation, linking it to microbial activity. Later, in the late 19th and early 20th centuries, scientists elucidated its role in muscle metabolism and anaerobic respiration.
Industrial production of lactic acid began in the early 20th century through fermentation processes using lactic acid bacteria. The development of synthetic routes and modern biotechnological techniques has since diversified its sources and applications.

Natural Occurrence and Biological Role

Lactic acid is produced naturally in both microorganisms and higher organisms. In humans and animals, it forms during anaerobic glycolysis, when oxygen supply to muscles is insufficient for complete glucose oxidation. The enzyme lactate dehydrogenase (LDH) catalyses the conversion of pyruvate to lactate, regenerating NAD⁺, which sustains glycolysis and energy production under low oxygen conditions.
In plants and microorganisms, lactic acid results from fermentation pathways, which vary between homofermentative (producing mainly lactic acid) and heterofermentative (producing lactic acid along with ethanol, carbon dioxide, and other compounds) processes.

Production Methods

Lactic acid is produced commercially by two principal methods: fermentation and chemical synthesis.
1. Fermentation Process: Biotechnological fermentation is the predominant method due to its renewable feedstocks and environmental sustainability. It involves the conversion of carbohydrates—such as glucose, sucrose, or lactose—into lactic acid using microorganisms such as Lactobacillus, Lactococcus, or Bacillus species.The general reaction is:C₆H₁₂O₆ → 2CH₃CH(OH)COOH
Fermentation parameters such as pH, temperature (typically 30–45°C), and nutrient supply are carefully controlled. The product is purified through filtration, neutralisation, and crystallisation or distillation.
2. Chemical Synthesis: Synthetic production involves the reaction of acetaldehyde with hydrogen cyanide to form lactonitrile, which is then hydrolysed to lactic acid. Although this method yields racemic mixtures (DL-lactic acid), it is less environmentally friendly compared to fermentation.
Recent advances in biocatalysis and genetic engineering have enabled selective production of optical isomers and improved yields from renewable raw materials such as starch, molasses, and agricultural residues.

Physical and Chemical Properties

Lactic acid exhibits several properties characteristic of hydroxy acids:

• Molecular formula: C₃H₆O₃
• Molecular weight: 90.08 g/mol
• Boiling point: 122°C (at reduced pressure)
• Melting point: 16–18°C
• Solubility: Fully miscible with water, ethanol, and glycerol.
• Acidic nature: Monoprotic acid, partially dissociating in aqueous solution.
• Reactivity: Reacts with alcohols to form lactates (esters) and with bases to form lactate salts such as calcium lactate and sodium lactate.

When heated strongly, lactic acid undergoes dehydration to yield lactide, a cyclic dimer that serves as the precursor for polylactic acid (PLA), a biodegradable polymer.

Industrial and Commercial Applications

Lactic acid’s versatility has led to its extensive use in multiple industries:
1. Food and Beverage Industry:

• Acts as a preservative, acidulant, and flavouring agent in processed foods, dairy products, and beverages.
• Regulates acidity in pickles, yoghurt, and fermented vegetables.
• Used in baking to enhance dough texture and flavour.
• Its salts, such as calcium lactate and sodium lactate, serve as firming agents and moisture retainers.

2. Pharmaceutical and Medical Applications:

• Used in intravenous solutions to maintain electrolyte balance.
• Functions as an antiseptic and pH regulator in topical preparations.
• Plays a role in controlled drug delivery systems using PLA polymers.
• L-lactic acid is preferred for biocompatible and resorbable medical materials such as sutures, tissue scaffolds, and implants.

3. Polymer Industry: Lactic acid is the building block for polylactic acid (PLA), a biodegradable thermoplastic. PLA is synthesised through polymerisation of lactide and used in:

• Packaging materials and disposable containers.
• Biodegradable plastics as alternatives to petroleum-based polymers.
• Medical implants, owing to its biocompatibility and controlled degradation.

4. Cosmetic and Personal Care Products:

• Functions as an exfoliant and skin-conditioning agent in creams and lotions.
• Helps reduce wrinkles and improve skin hydration by promoting cell turnover.
• Widely used in chemical peels and anti-ageing formulations.

5. Chemical and Environmental Applications:

• Used as an intermediate in manufacturing solvents, plasticisers, and green cleaning agents.
• Plays a role in biodegradable polymer production and as a pH control agent in industrial processes.

Physiological Significance

Lactic acid plays a pivotal role in human metabolism and exercise physiology. During strenuous activity, when oxygen supply to muscles is inadequate, glucose is converted to lactic acid via anaerobic glycolysis, leading to its accumulation in muscle tissues. Contrary to early beliefs, lactic acid itself is not the direct cause of muscle fatigue; rather, it serves as a temporary energy substrate.
Once oxygen becomes available, lactic acid is reconverted to pyruvate and utilised in the Krebs cycle for aerobic energy production. Additionally, excess lactate is transported to the liver, where it undergoes gluconeogenesis to form glucose in the Cori cycle, thereby maintaining blood glucose levels.
Normal blood lactate concentration ranges between 0.5–2.0 mmol/L. Elevated levels, termed lactic acidosis, can occur due to hypoxia, sepsis, liver dysfunction, or intense exercise.

Environmental and Economic Importance

The global lactic acid market has grown rapidly due to its renewable origin and applications in sustainable materials. Annual production exceeds 2 million tonnes, with Asia, North America, and Europe as leading producers. Key manufacturers include Corbion, NatureWorks LLC, and Galactic.
The compound’s role in bioplastics is particularly significant in reducing plastic pollution. PLA-based materials decompose naturally under composting conditions, aligning with circular economy and environmental sustainability goals.
Fermentation-based lactic acid production also supports agricultural and food waste valorisation, promoting a bio-based economy.

Types of Lactic Acid Fermentation

Lactic acid is produced biologically through two principal types of fermentation:

1. Homolactic Fermentation:
   • Yields mainly lactic acid as the sole product.
   • Carried out by Lactobacillus and Streptococcus species.
   • Reaction: Glucose → 2 Lactic Acid + Energy
2. Heterolactic Fermentation:
   • Produces lactic acid along with ethanol, acetic acid, and CO₂.
   • Performed by Leuconostoc and Weissella species.

These fermentation processes underpin the production of yoghurt, cheese, kimchi, and other fermented foods worldwide.

Safety, Handling, and Toxicology

Lactic acid is generally recognised as safe (GRAS) by food and drug authorities. However, high concentrations can cause skin and eye irritation due to its acidity. Industrial workers handling concentrated solutions must use protective equipment.
Physiologically, excessive accumulation of lactic acid leads to acidosis, a condition characterised by reduced blood pH. In such cases, symptoms include rapid breathing, confusion, and weakness, necessitating medical intervention.

Modern Research and Developments

Recent research on lactic acid focuses on sustainable production and innovative applications:

• Biotechnological advancements have improved microbial strains for higher yield and optical purity.
• Waste-to-lactic acid conversion utilises agricultural residues, food waste, and lignocellulosic biomass.
• Nanocomposite materials incorporating PLA are being developed for packaging and biomedical uses.
• Studies on lactic acid’s role in cancer metabolism have highlighted its involvement in tumour microenvironment regulation and energy transfer.

In cosmetics and dermatology, ongoing studies examine lactic acid’s efficacy in promoting collagen synthesis and improving skin elasticity.

Global Outlook and Significance

Lactic acid’s dual nature—biological metabolite and industrial raw material—illustrates its multifaceted significance. It bridges life sciences and material technology, symbolising the integration of biology and industry. The compound’s renewable production pathways, biocompatibility, and role in sustainable materials make it a cornerstone of the emerging bio-based economy.