C₃H₆O₃ — Lactic Acid

Lactic acid, also known as 2-hydroxypropanoic acid, is a naturally occurring organic acid belonging to the class of alpha-hydroxy acids (AHAs). It is widely known for its role in muscle metabolism, where it is produced during intense exercise under anaerobic conditions. The molecular formula of lactic acid is \(C_3H_6O_3\). It is a chiral molecule existing in two optical isomers: L-(+)-lactic acid and D-(-)-lactic acid, with the L-form being biologically active and naturally found in humans and other organisms.

Lactic acid was first isolated in 1780 by the Swedish chemist Carl Wilhelm Scheele from sour milk. Today, it is industrially produced by both fermentation and synthetic methods. The compound is used in various industries including food, pharmaceuticals, cosmetics, and biodegradable polymer production due to its non-toxic and biodegradable nature.

Lactic acid consists of three carbon atoms, six hydrogen atoms, and three oxygen atoms. Its structure can be represented as:

The molecule contains both a hydroxyl (–OH) and a carboxylic acid (–COOH) functional group, making it both an alcohol and a carboxylic acid. The presence of these groups allows lactic acid to engage in extensive hydrogen bonding, contributing to its relatively high boiling point and solubility in water.

Due to the asymmetric carbon atom (the central carbon attached to –OH, –COOH, –CH3, and –H), lactic acid exhibits optical isomerism. The L-(+)-isomer is naturally occurring and metabolically active, while the D-(–)-isomer is synthetic or found in certain bacterial processes. A racemic mixture (DL-lactic acid) contains equal parts of both enantiomers.

Lactic acid is naturally produced in animal muscles and bacterial systems. During anaerobic respiration (when oxygen supply is insufficient), glucose breaks down into lactic acid through glycolysis. The overall process can be summarized as:

This buildup of lactic acid in muscles causes temporary fatigue or soreness during vigorous exercise. It is later converted back to glucose in the liver via the Cori cycle once oxygen becomes available.

In nature, lactic acid is also produced by various bacteria such as Lactobacillus, Streptococcus, and Bacillus species during the fermentation of carbohydrates. This process is fundamental to dairy and fermented food industries, producing yogurt, cheese, sauerkraut, and pickles.

1. Fermentation Process:

The primary industrial method for producing lactic acid is by fermenting carbohydrates such as glucose, sucrose, or lactose using lactic acid bacteria.

This biological route is environmentally friendly and produces high-purity L-lactic acid.

2. Synthetic Method:

Chemical synthesis involves the hydration of acrylonitrile followed by oxidation or hydrolysis of intermediates to yield racemic lactic acid:

The synthetic route is less commonly used today due to the sustainability and cost-effectiveness of fermentation methods.

Physical Properties:

• State: Colorless, syrupy liquid or crystalline solid depending on temperature.
• Odor: Slightly sour.
• Solubility: Miscible with water, alcohol, and ether due to hydrogen bonding.
• Acidity: Moderately strong organic acid with pKa ≈ 3.86.

Chemical Properties:

• Acidic nature: Lactic acid behaves as a weak acid, forming lactate salts with bases.

\(CH_3CH(OH)COOH + NaOH \rightarrow CH_3CH(OH)COONa + H_2O\)

• Ester formation: Reacts with alcohols to form esters used in flavorings and solvents.

\(CH_3CH(OH)COOH + C_2H_5OH \rightarrow CH_3CH(OH)COOC_2H_5 + H_2O\)

• Oxidation: Oxidized to pyruvic acid or acetic acid depending on the conditions.
• Polymerization: Lactic acid can polymerize to form polylactic acid (PLA), a biodegradable plastic widely used in packaging.

• Food Industry: Used as a preservative, acidity regulator, and flavoring agent in dairy, bakery, and beverage products.
• Pharmaceuticals: Found in intravenous fluids, anti-acne formulations, and oral hygiene products.
• Cosmetics: Acts as an exfoliant and moisturizer in skin-care products, promoting cell renewal and smoother texture.
• Polymer Industry: Used in manufacturing polylactic acid (PLA), an eco-friendly bioplastic used in packaging and medical implants.
• Leather and Textile Industry: Serves as a mordant and pH regulator.
• Chemical Intermediate: Used to produce lactate salts, esters, and solvents.

Lactic acid is vital in human metabolism, especially during anaerobic glycolysis. When oxygen is limited, pyruvate is reduced to lactic acid by the enzyme lactate dehydrogenase:

This allows continued ATP production in muscle cells. The lactate produced is later transported to the liver and converted back to glucose via the Cori cycle. Lactic acid bacteria also maintain gut health by lowering intestinal pH and inhibiting the growth of harmful microorganisms.

Lactic acid is generally recognized as safe (GRAS) for food and pharmaceutical uses. In skincare, it gently removes dead skin cells and improves hydration. Excess lactic acid in the bloodstream, however, can lead to lactic acidosis, a rare but serious condition in which blood pH drops dangerously low.

Environmentally, lactic acid and its polymers are biodegradable and renewable, making them an excellent alternative to petroleum-based chemicals and plastics. Polylactic acid decomposes naturally without releasing harmful residues, contributing to sustainable industrial practices.