Lactic acid is a naturally occurring organic compound found in dairy products and involved in diverse biological and industrial processes. It is a byproduct of metabolic activities and widely utilized in human applications. This article explores its chemical makeup, formation, functions, and safety.
Chemical Identity
Lactic acid is an organic alpha-hydroxy acid (AHA), featuring both a carboxyl group and a hydroxyl group on the same carbon atom. Its chemical formula is C₃H₆O₃. This structure includes a three-carbon backbone with a carboxyl group (-COOH) at one end and a hydroxyl group (-OH) attached to the adjacent carbon.
In its pure, solid form, lactic acid is white, forming a colorless solution when dissolved. It is miscible with water. Its melting point is around 16-18 °C, and its boiling point is approximately 122 °C at 15 mmHg. A 1 mM solution has a pH of about 3.51 to 3.86.
Origin and Production
Lactic acid occurs widely in nature, within living organisms and fermented products. In animals, particularly humans, L-lactic acid forms in muscle cells during intense physical activity when oxygen is limited, as part of anaerobic metabolism. This process converts glucose into cellular energy and lactate, the ionized form of lactic acid.
It is also a natural component of many fermented foods, contributing to their characteristic flavors and preservation. Examples include yogurt, kefir, sourdough bread, and sauerkraut.
Industrial production primarily relies on microbial fermentation, accounting for most global output. Microorganisms, mainly lactic acid bacteria, convert carbohydrates like glucose, sucrose, or lactose into lactic acid. These carbohydrates often come from renewable agricultural materials such as corn, beet sugar, cane sugar, molasses, or whey.
Chemical synthesis also produces lactic acid, typically by reacting acetaldehyde with hydrogen cyanide, followed by hydrolysis. This method usually yields a racemic mixture, an equal blend of both L- and D-lactic acid forms, unlike fermentation which can be tailored to produce a specific isomer.
Functions and Uses
Lactic acid serves diverse functions across biological systems and human applications. In the human body, it aids energy metabolism during intense physical activity when oxygen is scarce. It is produced from glucose to generate cellular energy, and can be transported to the liver and kidneys for conversion back to glucose or used as an energy source by other tissues.
It also acts as a signaling molecule, influencing gene expression and cellular responses, including attracting immune cells to sites of injury or infection.
In the food industry, lactic acid (E270) is an additive, serving as a natural preservative, flavor enhancer, and acidity regulator. It provides the tart taste in dairy products and fermented foods, and inhibits spoilage microorganisms. It also functions as a gelling agent, curing agent, and discoloration inhibitor in processed meats, beverages, and baked goods.
The cosmetic industry uses lactic acid as an AHA for its exfoliating and moisturizing properties. It removes dead skin cells, promoting skin renewal and improving texture, tone, and hydration. Lactic acid is found in skincare products to reduce fine lines, wrinkles, and dark spots, and can strengthen hair shafts.
In pharmaceuticals, lactic acid creates water-soluble forms of insoluble active ingredients and is a component in intravenous solutions like Ringer’s lactate. Its salts, such as calcium and magnesium lactate, deliver essential minerals. Biodegradable polymers from lactic acid are used in drug delivery systems and bioresorbable surgical sutures.
Lactic acid is a building block for polylactic acid (PLA), a biodegradable plastic. PLA is a sustainable alternative to petroleum-based plastics for packaging, medical devices, and 3D printing. It is also incorporated into industrial and household cleaning products as a disinfectant, descaling agent, and antimicrobial. In the textile industry, it treats cotton, enhancing color retention.
Safety Precautions
Lactic acid requires careful handling due to its corrosive and irritating properties. Direct contact with concentrated forms can cause severe skin and eye burns. It is also corrosive to some metals and tissues. Inhalation of vapors or mists can irritate the respiratory tract. Ingestion of large quantities can result in abdominal pain, nausea, and metabolic acidosis.
When handling lactic acid, wear appropriate personal protective equipment (PPE). This includes safety goggles or a face shield, and impermeable gloves. Protective clothing is recommended. In environments with mists or vapors, respiratory protection should be used. Adequate ventilation is essential to minimize inhalation exposure.
In case of accidental exposure, take immediate first aid. If lactic acid contacts the skin, remove contaminated clothing and wash the affected area with soap and water for at least 10-15 minutes. For eye contact, rinse with large amounts of water for several minutes, removing contact lenses if easy, and continue rinsing before seeking medical attention. If inhaled, move the person to fresh air. If ingested, rinse mouth with water (if conscious) but do not induce vomiting; contact a poison center or doctor immediately.
Lactic acid should be stored in tightly closed, corrosive-resistant containers in a cool, dry, and well-ventilated area, away from direct sunlight and heat. As it is hygroscopic, containers must be kept sealed to prevent dilution or degradation. Store away from incompatible materials like strong bases and oxidizing agents to prevent hazardous reactions. Dispose of surplus solutions via a licensed waste disposal company. Contain spills, neutralize with a weak alkaline solution, and absorb with inert material, preventing entry into drains.
In cosmetic products, concentrations of 10% or less are generally safe for the skin. However, it can increase sun sensitivity, necessitating sunscreen use, and patch testing is advisable before widespread application.