Lactic acid is an important organic acid, ranking second in both production and consumption after citric acid. Data indicates that lactic acid accounts for approximately 15% of the total organic acid market. In recent years, thanks to its growing use in biodegradable plastics, demand for L-lactic acid has been increasing at a rapid pace—so much so that some predict it could soon catch up with or even surpass citric acid within a few years. Lactic acid is widely found in nature and has even been detected in living organisms.

　　Lactic acid, also known as 2-hydroxypropanoic acid or α-hydroxypropanoic acid, features an asymmetric carbon atom in its molecular structure, resulting in two optically active isomers: the d-form and the l-form, as well as their racemic mixture, the dl-form. Together, these form three distinct optical isomers. In 1780, Scheele first isolated this compound from sour milk, hence the name "lactic acid." In human bodies and animal tissues, both the d- and dl-forms are naturally present, while the l-form occurs as a normal metabolite in mammals. Naturally, lactic acid can also be found in various fruits such as poppies, apples, and tomato juice. However, commercially available lactic acid—whether produced via fermentation or synthetic methods—is always obtained as the racemic dl-form. Due to the presence of both a hydroxyl group and a carboxyl group within its molecule, lactic acid exhibits the characteristic properties of these two functional groups. It readily forms carboxylic acid esters when reacted with acid anhydrides, lactate esters when combined with alcohols, and salts when paired with ions like sodium, calcium, or ferrous compounds. Moreover, because lactic acid contains both hydroxyl and carboxyl groups simultaneously, it can undergo self-esterification, leading to the formation of linear polyesters—or even cyclic compounds, such as the ring-shaped dimer called lactide [2]. Under slow oxidation conditions, lactic acid transforms into pyruvic acid. When heated above 250°C, however, it undergoes chain-breaking decomposition, yielding acetaldehyde along with the release of carbon dioxide and water. Additionally, lactic acid can react with ammonia or amines to produce lactamide. Beyond self-esterification, its hydroxyl and carboxyl groups can also participate together in other reactions—for instance, when interacting with aldehydes or ketones, lactic acid can form cyclic acetals.

　　Sodium lactate is used in medicine to prevent and treat acidosis, while iron lactate and calcium lactate serve as raw materials in the pharmaceutical industry. Ethyl lactate is employed as a fragrance ingredient, and butyl lactate acts as an excellent additive for paints, as well as a crucial component in the plastic industry, serving as both a plasticizer and an enhancer. In the electroplating industry, it is utilized as a rust-removal agent.

　　Molecular weight: 90.08. Melting point: 16.8°C; boiling point: 122°C (at 1.8665–1.9998 kPa); pKa = 1.38 × 10⁻⁹ (at 25°C). Industrially available as a liquid with a sour, acidic odor, it is soluble in water and ethanol, slightly soluble in ether, and completely insoluble in chloroform, carbon disulfide, and petroleum ether. Lactic acid readily mixes with water but crystallizes very poorly. When the concentration of lactic acid exceeds 60%, it exhibits strong hygroscopic properties.

　　Lactic acid, lactates, and their derivatives are widely used in industries such as food, pharmaceuticals, animal feed, and chemicals.

　　Due to lactic acid's mild and stable acidity, which enhances food flavor, it is widely used in the food industry as an acidulant, preservative, and reducing agent. It finds applications in the production of soft drinks, candies, and baked goods, as well as in the processing and preservation of fish, meat, and vegetables. Compared to other edible acids like citric acid and malic acid, lactic acid boasts a strong competitive edge. In the United States, lactic acid has largely replaced citric acid and phosphoric acid in the soft drink industry. Moreover, in beer production, the U.S. prohibits the use of inorganic acids such as phosphoric acid for pH adjustment—instead, lactic acid is now the sole choice. Globally, about one-quarter of all lactic acid produced is dedicated to manufacturing stearoyl lactylate esters. The salts derived from these esters—calcium stearoyl lactylate (CSL) and sodium stearoyl lactylate (SSL)—are extensively employed in bread-making. Not only do they contribute to a softer, more delicate crumb texture, but they also significantly extend the shelf life of bread products.