Lactic acid

Production

Lactic acid is produced industrially by bacterial fermentation of carbohydrates, or by chemical synthesis from acetaldehyde. As of 2009, lactic acid was produced predominantly (70–90%) by fermentation. Production of racemic lactic acid consisting of a 1:1 mixture of D and L stereoisomers, or of mixtures with up to 99.9% L-lactic acid, is possible by microbial fermentation. Industrial production of the D-lactic acid enantiomer is technically more challenging because most naturally occurring lactic acid bacteria preferentially produce the L-form; obtaining high optical purity of D-lactic acid therefore requires genetically engineered microorganisms or specific D-lactate dehydrogenases.

Fermentative production

Fermented milk products are obtained industrially by fermentation of milk or whey by Lactobacillus bacteria: Lactobacillus acidophilus, Lacticaseibacillus casei (Lactobacillus casei), Lactobacillus delbrueckii subsp. bulgaricus (Lactobacillus bulgaricus), Lactobacillus helveticus, Lactococcus lactis, Bacillus amyloliquefaciens, and Streptococcus salivarius subsp. thermophilus (Streptococcus thermophilus).

As a starting material for industrial production of lactic acid, almost any carbohydrate source containing C
₅ (pentose sugar) and C
₆ (hexose sugar) can be used. Pure sucrose, glucose from starch, raw sugar, and beet juice are frequently used. Lactic acid producing bacteria can be divided in two classes: homofermentative bacteria like Lactobacillus casei and Lactococcus lactis, producing two moles of lactate from one mole of glucose, and heterofermentative species, producing one mole of lactate from one mole of glucose, as well as carbon dioxide and acetic acid/ethanol.

Chemical production

Racemic lactic acid is synthesized industrially by reacting acetaldehyde with hydrogen cyanide and hydrolysing the resultant lactonitrile. When hydrolysis is performed by hydrochloric acid, ammonium chloride forms as a by-product; the Japanese company Musashino is one of the last big manufacturers of lactic acid by this route. Synthesis of both racemic and enantiopure lactic acids is also possible from other starting materials (vinyl acetate, glycerol, etc.) by application of catalytic procedures.

Lactic acid is hygroscopic. DL-Lactic acid is miscible with water and with ethanol above its melting point, which is 16–18 °C (61–64 °F). D-Lactic acid and L-lactic acid have a higher melting point. Lactic acid produced by fermentation of milk is often racemic, although certain species of bacteria produce solely D-lactic acid.

Biology

Molecular biology

L-Lactic acid is the primary endogenous agonist of hydroxycarboxylic acid receptor 1 (HCA₁), a G_i/o-coupled G protein-coupled receptor (GPCR). Such activation of hydroxycarboxylic acid receptor 1 (HCA₁) in adipocyte causes inhibition of lipolysis. Meanwhile, such activation in brain causes dampening of neuron excitation and may promoting neurogenesis and angiogenesis.

Lactate can be involved in histone lactylation such as with histone Kla.

Metabolism and exercise

During power exercises such as sprinting, when the rate of demand for energy is high, glucose is broken down and oxidized to pyruvate, and lactate is then produced from the pyruvate faster than the body can process it, causing lactate concentrations to rise. The production of lactate is beneficial for NAD⁺ regeneration (pyruvate is reduced to lactate while NADH is oxidized to NAD⁺), which is used up in oxidation of glyceraldehyde 3-phosphate during production of pyruvate from glucose, and this ensures that energy production is maintained and exercise can continue. During intense exercise, the respiratory chain cannot keep up with the amount of hydrogen ions that join to form NADH, and cannot regenerate NAD⁺ quickly enough, so pyruvate is converted to lactate to allow energy production by glycolysis to continue.

The resulting lactate can be used in two ways:

• Oxidation back to pyruvate by well-oxygenated muscle cells, heart cells, and brain cells
  • Pyruvate is then directly used to fuel the Krebs cycle
• Conversion to glucose via gluconeogenesis in the liver and release back into circulation by means of the Cori cycle
  • If blood glucose concentrations are high, the glucose can be used to build up the liver's glycogen stores.

Lactate is continually formed at rest and during all exercise intensities. Lactate serves as a metabolic fuel being produced and oxidatively disposed in resting and exercising muscle and other tissues. Some sources of excess lactate production are metabolism in red blood cells, which lack mitochondria that perform aerobic respiration, and limitations in the rates of enzyme activity in muscle fibers during intense exertion. Lactic acidosis is a physiological condition characterized by accumulation of lactate (especially L-lactate), with formation of an excessively high proton concentration [H⁺] and correspondingly low pH in the tissues, a form of metabolic acidosis.

The first stage in metabolizing glucose is glycolysis, the conversion of glucose to pyruvate⁻ and H⁺:

C₆H₁₂O₆ + 2 NAD⁺ + 2 ADP³⁻ + 2 HPO₄^2– → 2 CH₃COCO₂^– + 2 H⁺ + 2 NADH + 2 ATP⁴⁻ + 2 H₂O

When sufficient oxygen is present for aerobic respiration, the pyruvate is oxidized to CO₂ and water by the Krebs cycle, in which oxidative phosphorylation generates ATP for use in powering the cell. When insufficient oxygen is present, or when there is insufficient capacity for pyruvate oxidation to keep up with rapid pyruvate production during intense exertion, the pyruvate is converted to lactate⁻ by lactate dehydrogenase), a process that absorbs these protons:

2 CH₃COCO₂^– + 2 H⁺ + 2 NADH → 2 CH₃CH(OH)CO₂^– + 2 NAD⁺

The combined effect is:

C₆H₁₂O₆ + 2 ADP³⁻ + 2 HPO₄^2– → 2 CH₃CH(OH)CO₂^– + 2 ATP⁴⁻ + 2 H₂O

The production of lactate from glucose (glucose → 2 lactate⁻ + 2 H⁺), when viewed in isolation, releases two H⁺. The H⁺ are absorbed in the production of ATP, but H⁺ is subsequently released during hydrolysis of ATP:

ATP⁴⁻ + H₂O → ADP³⁻ + H⁺ + HPO₄^2–

Once the production and use of ATP is included, the overall reaction is

C₆H₁₂O₆ → 2 H⁺ + 2 CH₃CH(OH)CO₂^–

The resulting increase in acidity persists until the excess lactate and protons are converted back to pyruvate, and then to glucose for later use, or to CO₂ and water for the production of ATP.

Information regarding exogenous lactate can be found on a separate page.

pH Regulation

Lactate production and export contribute significantly to intracellular pH regulation in metabolically active tissues. In skeletal muscle, accumulation of lactic acid lowers intracellular pH, and monocarboxylate transporters facilitate the efflux of both lactate and H⁺, thereby helping maintain acid-base homeostasis and delaying fatigue.

Neural tissue energy source

Although glucose is usually assumed to be the main energy source for living tissues, there is evidence that lactate, in preference to glucose, is preferentially metabolized by neurons in the brains of several mammals. According to the lactate-shuttle hypothesis, glial cells are responsible for transforming glucose into lactate, and for providing lactate to the neurons. Because of this local metabolic activity of glial cells, the extracellular fluid immediately surrounding neurons strongly differs in composition from the blood or cerebrospinal fluid, being much richer with lactate, as was found in microdialysis studies.

Uses

In 2023, lactate was the 289th most commonly prescribed medication in the United States, with more than 500,000 prescriptions.

Polymer precursor

Two molecules of lactic acid can be dehydrated to the lactone lactide. In the presence of catalysts lactide polymerize to either atactic or syndiotactic polylactide (PLA), which are biodegradable polyesters. PLA is an example of a plastic that is not derived from petrochemicals.

Pharmaceutical and cosmetic applications

Lactic acid is also employed in pharmaceutical technology to produce water-soluble lactates from otherwise-insoluble active ingredients. It finds further use in topical preparations and cosmetics to adjust acidity and for its disinfectant and keratolytic properties.

Lactic acid containing bacteria have shown promise in reducing oxaluria with its descaling properties on calcium compounds.

Foods

Fermented food

Lactic acid is found in many fermented foods.

• Sour milk products, such as kumis, laban, yogurt, kefir, and some cottage cheeses, derive their flavor from lactic acid. The casein in fermented milk is coagulated (curdled) by lactic acid.
• Lactic acid is also responsible for the sour flavor of sourdough bread.
• Some beers (sour beer) purposely contain lactic acid, one such type being Belgian lambics. Most commonly, this is produced naturally by various strains of bacteria. These bacteria ferment sugars into acids, unlike the yeast that ferment sugar into ethanol. After cooling the wort, yeast and bacteria are allowed to "fall" into the open fermenters. Brewers of more common beer styles would ensure that no such bacteria are allowed to enter the fermenter. Other sour styles of beer include Berliner weisse, Flanders red and American wild ale.
• In winemaking, a bacterial process, natural or controlled, is often used to convert the naturally present malic acid to lactic acid, to reduce the sharpness and for other flavor-related reasons. This malolactic fermentation is undertaken by lactic acid bacteria.
• Pickling vegetables in brine creates a sour flavor as bacteria convert sugars into lactic acid.
• Fermented sausages

In lists of nutritional information lactic acid might be included under the term "carbohydrate" (or "carbohydrate by difference") because this often includes everything other than water, protein, fat, ash, and ethanol. If this is the case then the calculated food energy may use the standard 4 kcal/g (17 kJ/g) that is often used for all carbohydrates. But in some cases lactic acid is ignored in the calculation. The actual energy density of lactic acid is 3.62 kcal/g (15.1 kJ/g).

While not normally found in significant quantities in fruit, lactic acid is the primary organic acid in akebia fruit, making up 2.12% of the juice.

Separately added

As a food additive it is approved for use in the EU, United States and Australia and New Zealand; it is listed by its INS number 270 or as E number E270. Lactic acid is used as a food preservative, curing agent, and flavoring agent. It is an ingredient in processed foods and is used as a decontaminant during meat processing. Lactic acid is produced commercially by fermentation of carbohydrates such as glucose, sucrose, or lactose, or by chemical synthesis. Carbohydrate sources include corn, beets, and cane sugar.

Forgery

Lactic acid has historically been used to assist with the erasure of inks from official papers to be modified during forgery.

Cleaning products

Lactic acid is used in some liquid cleaners as a descaling agent for removing hard water deposits such as calcium carbonate. It is also used as an antibacterial agent in some hard surface liquid cleaners.

Clinical diagnostic use

Lactic acid levels are a blood test in medicine. Elevated lactate is not universally defined but usually has a cut-off between 2.0 and 2.5 mmol/L, whereas highly elevated lactate is usually defined as greater than 4 mmol/L. Lactic acidosis is often used clinically to describe elevated lactate but should be reserved for instances in which there is also acidosis with pH of less than 7.35. Elevated lactate is most commonly due to tissue hypoxia or hypoperfusion, such as shock, the post-cardiac arrest state, or regional ischemia. However, other causes and contributing factors to elevated lactate also exist. Elevated lactate can additionally occur due to heavy exercise and hence can occur without pathology.

Blood testing

Blood tests for lactate are performed to determine the status of the acid base homeostasis in the body. Blood sampling for this purpose is often arterial (even if it is more difficult than venipuncture), because lactate levels differ substantially between arterial and venous, and the arterial level is more representative for this purpose.

During childbirth, lactate levels in the fetus can be quantified by fetal scalp blood testing.