I. Basic Chemical Properties Lactic acid, IUPAC name 2-hydroxypropanoic acid, is a widely distributed hydroxycarboxylic acid in nature and an important intermediate product of carbohydrate metabolism. Its chemical formula is C₃H₆O₃, and its molecular weight is 90.08 g/mol. It exists in three common CAS numbers: 50-21-5 (DL-lactic acid), 79-33-4 (L-lactic acid), and 10326-41-7 (D-lactic acid). The lactic acid molecule contains both carboxyl (-COOH) and hydroxyl (-OH) functional groups, belonging to the α-hydroxy acid (AHA) family. It has a chiral center and therefore exists in two enantiomers—L-(+)-lactic acid and D-(-)-lactic acid. L-lactic acid is the predominant naturally occurring configuration in organisms and has important physiological activities. DL-lactic acid is a racemic mixture of the two isomers, is optically inactive, and is widely used in industrial production.

Physical Properties

- Appearance: The pure product is a colorless, transparent liquid. Industrial-grade products are mostly colorless to pale yellow transparent liquids. It has strong hygroscopic properties and may become slightly viscous with prolonged storage or at high concentrations.

- Odor and Taste: It has no obvious pungent odor, but a mild sour taste with a refreshing mouthfeel. It is one of the important sources of sourness in fermented foods. After high dilution, the sour taste is mild and without bitterness.

- Melting Point and Boiling Point: The monomer has a melting point of approximately 18℃ and a boiling point of approximately 227.6℃ (760 mmHg). It has low volatility, and purification requires the use of superheated steam (130℃~200℃) to entrain it. When heated above 100℃, an intermolecular dehydration reaction occurs, producing lactic anhydride.

- Density and Solubility: At 25°C, the density is approximately 1.276 g/cm³, slightly heavier than water; extremely soluble in water, ethanol, and glycerol, miscible with these solvents in any proportion; insoluble in chloroform, carbon disulfide, and petroleum ether.

- Other Properties: Flash point is approximately 109.9°C, classifying it as a flammable liquid; dissociation constant is 3.86 pKa; under physiological pH conditions (7.35-7.45), 99% of lactic acid exists as free lactate anions and protons; chemically stable, not easily decomposed under normal storage conditions, but reacts with strong oxidizing agents.

Chemical Properties

- Dual Functional Group Reaction: Due to the presence of both carboxyl and hydroxyl groups, it possesses the chemical properties of both carboxylic acids and alcohols, and can undergo various reactions such as oxidation, reduction, esterification, and condensation, making it an important intermediate in organic synthesis.

- Oxidation and Reduction Reactions: Under the catalysis of lactate dehydrogenase, lactate can be oxidized to pyruvate, an important reaction in biological metabolism; under the action of a reducing agent, it can be reduced to 1,2-propanediol, the latter being an important chemical raw material; in addition, it can undergo decarbonylation/decarboxylation reactions to yield acetaldehyde.

- Esterification Reactions: The hydroxyl group can undergo esterification reactions with organic and inorganic acids to produce lactic acid esters with different uses, widely applied in fragrances, solvents, food additives, etc. Simultaneously, lactic acid can undergo dimerization and further esterification to yield lactide, a key precursor for the synthesis of polylactic acid.

- Chirality: The two enantiomers have significantly different physicochemical properties and physiological activities. L-lactic acid is the main form of metabolism in organisms, can be normally metabolized and absorbed by the human body, and has high safety; D-lactic acid is mainly used in industrial fields, and excessive intake may burden human metabolism.

- Dehydration Reactions: When heated to a certain temperature, intermolecular dehydration reactions occur to produce lactic anhydride, which can be further dehydrated to produce acrylic acid, providing an important raw material for the chemical industry.

II. Natural Sources and Biosynthesis

1. Main Natural Sources
Lactic acid is one of the most widely distributed hydroxycarboxylic acids in nature. Its sources include microorganisms, plants, animal tissues, and various fermented foods. It is a natural product of biological metabolic processes:

- Microbial Sources: Various lactic acid bacteria (such as Lactobacillus bulgaricus and Lactobacillus acidophilus), yeasts, and molds produce lactic acid during fermentation. It is an important product of microbial glucose metabolism and a major source of lactic acid in fermented foods.

- Plant Sources: Widely found in apples, tomatoes, asparagus, wheat, corn, broccoli, and other plants and plant products. The lactic acid content of some plants increases significantly during damage, ripening, or fermentation. Maple syrup and malt also contain small amounts of lactic acid.

- Animal and Human Sources: The main type of lactic acid in humans and animals is L-lactic acid, primarily produced by muscles, skin, brain, red blood cells, and intestines. Muscles and skin each account for 25%, the brain 25%, red blood cells 20%, and the intestines 10%. During strenuous exercise or tissue hypoxia, muscle cells produce large amounts of lactic acid, leading to temporary muscle soreness. - Sources of Fermented Foods: Fermented foods such as yogurt, kimchi, sauerkraut, beer, and fruit wine contain a large amount of lactic acid. Lactic acid bacteria can convert lactose and glucose in these foods into lactic acid, giving them a unique sour taste and flavor, while also extending their shelf life.

2. Biosynthesis Process: The biosynthesis of lactic acid mainly involves two pathways: microbial fermentation synthesis and metabolic synthesis in animals. Microbial fermentation synthesis is the primary way lactic acid is produced in nature:
Microbial Fermentation Synthesis: The core pathway is glycolysis. Microorganisms use glucose, sucrose, starch, etc., as carbon sources. Under anaerobic or microaerobic conditions, they break down glucose into pyruvate through glycolysis. Subsequently, under the catalysis of lactate dehydrogenase, pyruvate is reduced to lactic acid. This process is an important way for microorganisms to obtain energy and also helps maintain the acid-base balance of the system, preventing excessive accumulation of acidic metabolites. Different microorganisms synthesize different lactic acid configurations. Lactic acid bacteria mainly synthesize L-lactic acid, while some bacteria can synthesize D-lactic acid or DL-lactic acid.

Metabolic Synthesis in Animals: Lactic acid in humans and animals is primarily produced through anaerobic glycolysis. When the body is under conditions of strenuous exercise or hypoxia, cells cannot meet their energy needs through aerobic respiration and initiate anaerobic glycolysis. Glucose is broken down into pyruvate in the cytoplasm, which is then converted into lactic acid, releasing a small amount of energy for the body's emergency use. Most of the produced lactic acid is transported to the liver and kidneys via the bloodstream, where it is converted back into glucose through gluconeogenesis and recycled for use by muscles and other tissues. This cycle is called the Cori cycle, which prevents acidosis caused by lactic acid buildup and ensures the recycling of energy. Furthermore, tumor cells also produce large amounts of lactic acid under normoxic conditions; this phenomenon is known as the "Warburg effect."

III. Industrial Preparation Methods With the increasing market demand for lactic acid, industrial production mainly employs three methods: microbial fermentation, chemical synthesis, and enzymatic synthesis. Among these, microbial fermentation has become the mainstream method due to its environmental friendliness and high product purity. Each method has its advantages and is suitable for different production needs:

- Microbial Fermentation: Using starch-rich materials such as corn, wheat, and cassava as raw materials, starch is first converted into monosaccharides such as glucose by saccharifying enzymes. Then, high-lactic-acid-producing microorganisms (screened or modified lactic acid bacteria, Bacillus, etc.) are selected for fermentation. After fermentation, the lactic acid in the fermentation broth is neutralized with calcium carbonate to form calcium lactate precipitate, which is then acidified with dilute sulfuric acid to obtain crude lactic acid. Subsequently, impurities are removed through activated carbon decolorization, ion exchange resin treatment, and precision filtration. Finally, high-purity lactic acid is obtained through vacuum concentration and crystallization purification. This method uses readily available raw materials, is environmentally friendly, and the main product is L-lactic acid, aligning with the trend of green chemical development. Yield can be improved and production costs reduced by optimizing fermentation conditions.

- Chemical Synthesis: This method primarily uses acetaldehyde, pyruvate, or acrylonitrile as raw materials to prepare lactic acid through different reaction pathways. The acetaldehyde method is the most commonly used, using acetaldehyde and hydrogen cyanide as raw materials. An addition reaction first produces lactic nitrile, which is then hydrolyzed to obtain lactic acid. The pyruvate method involves the catalytic reduction of pyruvate to produce lactic acid, yielding a high-purity product, but with relatively high raw material costs, making it suitable for small-scale, high-purity lactic acid production. This method can prepare DL-lactic acid; the process is simple and the reaction conditions are mild, but the product may contain small amounts of byproducts requiring further purification.

- Enzymatic Method: This is a novel preparation method developed in recent years. It utilizes specific enzymes (such as lactate dehydrogenase and aldolase) to catalyze the conversion of substrates (such as pyruvate and glycerol) into lactic acid. It offers advantages such as high reaction efficiency, high product purity, and environmental friendliness, eliminating the need for complex separation and purification steps. It is gradually being applied in industrial production and is expected to become an important direction for future lactic acid preparation.

IV. Diverse Application Areas Lactic acid, with its mild sour taste, good biocompatibility, and diverse chemical activities, is widely used in various fields such as food, medicine, chemicals, agriculture, and daily chemicals. The food sector accounts for the largest share of demand, approximately 70%, and it is closely related to people's production and daily life. Specific applications are as follows:

1. Food Sector (Core Application)
Lactic acid is an internationally recognized edible acidulant and preservative. It can be used in various foods as needed, including infant formula. Its applications are very broad:

- Food Flavoring and Acidity Adjustment: Used in beverages, juices, jams, candies, biscuits, bread, etc., it provides a mild and refreshing sour taste, enriches flavor layers, and adjusts the pH value of food, improving its texture and color. In fermented foods (yogurt, kimchi, sauerkraut), it can supplement lactic acid content, enhance fermentation flavor, and improve product quality.

- **Preservation and Preservation:** Lactic acid has excellent antibacterial properties, inhibiting the growth and reproduction of harmful microorganisms (such as E. coli and mold) in food, extending shelf life. It is particularly suitable for pickling fruits and vegetables, canning, grain processing, and fruit storage, reducing spoilage and maintaining freshness.

- **Nutritional Fortification:** Lactic acid promotes the absorption of minerals such as calcium, iron, zinc, and selenium, and is commonly used in dairy products and health supplements to enhance their nutritional value. Furthermore, as an intermediate product of human metabolism, lactic acid is highly safe and suitable as an additive in infant formula and special dietary foods.

2. **Pharmaceutical Field:**

- **Disinfection and Sterilization:** Lactic acid has strong disinfecting properties and can be used to produce preparations for disinfecting small-area, mild external wound infections (such as ethacridine lactate solution). It effectively kills harmful bacteria on the wound surface and promotes wound healing.

- Pharmaceutical Intermediates and Pharmaceutical Media: As an important pharmaceutical intermediate, lactic acid can be used to synthesize various drugs, such as ferrous lactate oral solution, ferrous lactate tablets, and vitamin D2 calcium lactate tablets, to supplement the body's need for iron, calcium, and other minerals. Simultaneously, it can serve as a pharmaceutical mediator, improving drug solubility and stability, and enhancing drug efficacy.

- Clinical Diagnostic Indicators: Blood lactate concentration is an important biochemical indicator for clinical diagnosis. The normal range for fasting whole blood lactate is 0.5–1.7 mmol/L (50–150 mg/L). A blood lactate concentration exceeding 5 mmol/L and a pH value < 7.25 indicates significant lactic acidosis, which can aid in the diagnosis of diseases such as circulatory failure, diabetic ketoacidosis, and respiratory failure.

3. Chemical Industry

- Preparation of Biodegradable Materials: Lactic acid is the core raw material for the synthesis of polylactic acid (PLA), a fully biodegradable green polymer material. It can be used to produce disposable tableware, packaging materials, fibers, etc., replacing traditional plastics, reducing white pollution, and aligning with environmental protection trends.

- Organic Synthesis and Solvents: As an intermediate in organic synthesis, lactic acid can be used to synthesize chemical products such as lactic acid esters, acrylic acid, and 1,2-propanediol, and is widely used in the fragrance, coating, plastics, and detergent industries. Simultaneously, lactic acid can be used as a solvent to dissolve certain organic and inorganic compounds, and is suitable for various chemical reactions.

4. Agriculture and Animal Husbandry

- Plant Growth Regulators: Lactic acid can be used in the production of slow-release pesticides and plant growth promoters to regulate plant growth, improve crop yield and quality, and improve soil pH, promoting the growth of beneficial microorganisms in the soil.

- Feed Additives: Lactic acid and its polymers can regulate the pH of animal intestines, increase beneficial intestinal flora, improve animal immunity, promote animal growth and reproduction, and reduce mortality. It can also serve as an animal health indicator, monitoring the metabolic state of animals and preventing diseases caused by excessively high lactic acid concentrations.

5. Daily Chemical Industry

As an important member of the alpha-hydroxy acid (AHA) family, lactic acid has gentle exfoliating, moisturizing, and whitening effects. It is widely used in skincare products (toners, lotions, masks, serums) to promote skin cell turnover, improve rough and dull skin, and enhance skin hydration, making it suitable for sensitive skin. It can also be used in oral care products such as toothpaste and mouthwash to regulate oral pH, inhibit oral bacteria, and freshen breath.

V. Safety and Protective Measures
Hazards

- Corrosivity: High concentrations of lactic acid (such as industrial-grade lactic acid) are highly corrosive and strongly irritate the skin, eyes, and respiratory tract. Contact may cause skin itching, burning sensation, skin burns, red and swollen eyes, tearing, respiratory discomfort, coughing, and other symptoms.

- Flammability: It is a flammable liquid with a flash point of approximately 109.9℃. It is easily ignited by heat, sparks, or open flames. Its vapor can form explosive mixtures with air. Keep away from fire sources during storage and use.

- Toxicity and Metabolic Hazards: Acute toxicity is low; the oral LD50 in rats is greater than 5 g/kg. However, long-term or high-concentration exposure may affect the liver. Excessive accumulation of lactic acid in the human bloodstream can cause lactic acidosis, manifested as extreme weakness, muscle pain, difficulty breathing, stomach upset, dizziness, and arrhythmia. In severe cases, it can be life-threatening and is commonly seen in cases of tissue hypoxia, liver disease, and in diabetic patients using biguanide drugs.

- Environmental Hazards: It has potential toxicity to aquatic organisms. Leaks into water bodies may pollute the aquatic environment and damage aquatic ecosystems. Fires will produce irritating and corrosive gases, further polluting the environment.

Safety Operation and Protection Recommendations:

1. Operating Environment: Operations should be carried out in a well-ventilated environment to avoid the accumulation of lactic acid vapors. If necessary, operations should be conducted in a fume hood to prevent vapor inhalation and accumulation that could lead to hazards. The operating area should be kept clean to prevent leaks.

2. Ignition Source Control: Smoking, open flames, and other sources of ignition are prohibited in the operation and storage areas. Keep away from oxidizers, strong acids and alkalis, and other flammable, explosive, or corrosive substances to prevent combustion, explosion, or chemical reactions.

3. Personal Protective Equipment: Wear standard protective goggles, acid-resistant gloves, and a respirator during operation. Wear protective clothing to avoid direct skin and eye contact with high concentrations of lactic acid. Wash hands and skin thoroughly after handling to remove residues.

4. Storage Conditions: Store in a cool, dry, and well-ventilated warehouse, away from heat sources and direct sunlight. Maintain a storage temperature of 2-8℃. Store separately from food ingredients, oxidizers, strong acids and alkalis to avoid cross-contamination. Containers must be tightly sealed to prevent leakage and moisture absorption.

5. Emergency Treatment: For skin contact, immediately flush with plenty of running water for at least 15 minutes; for eye contact, flush continuously with water or saline solution, and seek medical attention if necessary; in case of a leak, evacuate personnel quickly, absorb the leaked material with sand or other inert materials to prevent it from flowing into sewers or water bodies; in case of fire, use dry powder or carbon dioxide fire extinguishers, do not use water to extinguish the fire.

VI. Summary Lactic acid, as a naturally occurring hydroxycarboxylic acid, is both an important intermediate product of biological metabolism and a versatile industrial raw material. Its unique chemical structure and physicochemical properties make it suitable for use in food, medicine, chemicals, and other industries.

Lactic acid plays an irreplaceable role in multiple fields, including agriculture and daily chemicals. Its natural sources are widespread, and its biosynthesis is a crucial component of microbial metabolism and energy cycling in animals. Industrial production utilizes three pathways: microbial fermentation, chemical synthesis, and enzymatic methods, achieving large-scale, high-purity production. Microbial fermentation aligns with the trend of green and sustainable development and has become the mainstream production method.

From flavoring and preservation in the food industry, and nutritional fortification, to disinfection and sterilization in the pharmaceutical industry, and drug preparation, and then to the synthesis of biodegradable materials in the chemical industry, lactic acid is closely related to people's daily lives. Its applications in agriculture, animal husbandry, and daily chemicals are also continuously expanding. Although high-concentration lactic acid has certain corrosive and hazardous properties, its application value can be fully realized by strictly adhering to safe operating procedures. With continuous technological advancements and in-depth research into its biosynthetic mechanisms, purification technologies, and application potential, the application prospects of lactic acid in green chemistry, new pharmaceuticals, and environmentally friendly materials will become even broader, providing important support for the sustainable development of various industries.