Lactic Acid

LACTIC ACID (α-hydroxypropionic acid) [CH₃-CH(OH)COOH], contains an asymmetric carbon atom and therefore exists in three stereoisomeric forms. Lactic acid obtained synthetically is racemic. - Synthetic lactic acid is a colorless, hygroscopic syrup without odor. Lactic acid is volatile with water vapor; boiling point at 12 mm Hg is 119°, at 0.5-3 mm is 82-85°; specific gravity 1.2485 at 15°; soluble in water, alcohol, ether; when evaporated in vacuum, crystals are formed, melting at 18°; upon prolonged standing (faster when heated to 140°) it passes into an anhydride called dilactic or lactyl-lactic acid, CH₃CH(OH)COO·OCH(CH₃)COOH. When distilled at ordinary pressure, lactic acid decomposes into water, lactide, aldehyde, CO, CO₂. When heated with dilute sulfuric acid or oxidizers (potassium permanganate, chromic acid), it gives acetaldehyde and formic acid CH₃CH(OH)COOH = CH₃CHO + HCOOH. Synthetically, lactic acid is obtained by the usual method for obtaining hydroxy acids: 1) by the action of hydrocyanic acid on aldehyde followed by hydrolysis of the nitrile with hydrochloric acid; 2) by the action of nitrous acid on β-alanine; 3) by the action of silver oxide on α-iodopropionic acid. By fractional crystallization of lactate salts of strychnine, quinine, or morphine, racemic lactic acid can be split into d- and l-isomers. Official, inactive lactic acid, also called "lactic acid of fermentation," was isolated from sour milk by Scheele in 1780; it is formed as a result of the fermentative action of lactic acid fermentation bacteria on lactose (Pasteur). Buchner and Meisenheimer proved the existence of "zymase of lactic acid fermentation." Hexoses give lactic acid according to the equation: C₆H₁₂O₆ = 2 C₃H₆O₃ (see Glycolysis, Fermentation).-Disaccharides (cane, milk, and malt sugar) are first fermentatively converted to hexose. Arabinose, raffinose, trehalose, and α-methylglycoside give lactic acid only under the action of a special variety of lactic acid fermentation bacteria, which is used to differentiate these bacteria. Dextrins give traces of lactic acid. The fermentation process is the only industrial method for obtaining lactic acid. In Germany, potato starch is mainly used, less frequently rice and corn starch, previously pasteurized, subjecting it to fermentation at t° 50° by the action of a pure culture of Bac. Delbrückii; the formation of d,l-lactic acid proceeds in a neutral medium, therefore lime or zinc oxide is added to the initial substance to neutralize the formed lactic acid. In America, in addition to starch, glucose is used. In the USSR, whey remaining after butter and cheese making is used. Purification is mainly done by distillation with steam in vacuum. For the detection of lactic acid, the Uffelmann reaction is used: the initial violet color of a mixture of 2% phenol solution with 1-2 drops of ferric chloride changes to a cherry-yellow color upon addition of a liquid containing lactic acid (the reaction is not specific). For biological material, the Hopkins and Fletcher test is most suitable (only a positive result is conclusive): to 5 cm³ of concentrated sulfuric acid, add 1 drop of saturated CuSO₄ solution and a few drops of the test solution (preferably an alcoholic solution), shake, place in a boiling water bath for 1-2 minutes, cool, add 1-2 drops of a very dilute thiophene solution (10-20 drops per 100 cm³ alcohol) and again place in the boiling bath: in the presence of lactic acid, a cherry-red coloring is formed, which persists upon cooling.-Quantitative determination in blood-see Blood. Quantitative determination of lactic acid in urine is performed as follows: to 25 cm³ of urine, add 45 cm³ of 20% phosphotungstic acid and 5 cm³ of 25% sulfuric acid, filter the next day. Into an Erlenmeyer flask with a capacity of 200 cm³, pour 60 cm³ of the filtrate, add 5 g Ca(OH)₂, shake until the red-violet color of the liquid turns blue. Add 15 cm³ of 10% CuSO₄, shake, filter after an hour. To the filtrate, add 1-2 drops of sulfuric acid until neutral reaction and then enough so that its content in the filtrate equals 0.5%. For each determination, take 25 cm³; then perform oxidation with KMnO₄ and determination of the resulting aldehyde (see Blood).-d,l-lactic acid is found along with d-lactic acid in muscles and has important significance in the process of muscle work (see Muscles). Its presence in gastric contents is an important differential diagnostic sign: stagnation of food pap caused by a tumor in the pyloric region is accompanied by lactic acid fermentation of food carbohydrates; in addition, cancer cells themselves secrete lactic acid as a product of the intensely occurring glycolytic process in them.-d,l-lactic acid is found in normal human feces along with other lower organic acids. In fermentative dyspepsia, acute and chronic enterocolitis, the total amount of these organic acids sharply increases, d,l-lactic acid is formed during anaerobic respiration, during autolysis processes, during the action of alkalis on sugars. d,l-lactic acid is found in some plants and is contained in sauerkraut, pickles, kumis, kefir, yogurt, ayran, bread (poor white bread contains a significant amount of lactic acid), where it is formed as a result of lactic acid fermentation. The right and left lactic acids form prisms that deliquesce in air and melt at 25-26°; they have equal but opposite optical rotation [α]_D = +3.82° (for a 10% solution); upon prolonged heating to 130-150°, they racemize into d,l-lactic acid. Zinc salts crystallize with two molecules of water (inactive lactic acid crystallizes with three molecules of water). Both salts are more soluble than the corresponding salt of inactive lactic acid; aqueous solutions of l-lactic acid salts rotate to the right, aqueous solutions of d-lactic acid rotate to the left.-l-lactic acid is formed during the fermentation of alkaline solutions of cane sugar in the presence of Bac. acidi laevolactici, during the souring of milk under the influence of Bac. aerogenes, Bac. bulgaricus. Not found in the animal organism. It is formed in a sugar-peptone nutrient medium by cultures of typhoid bacilli and cholera vibrio, pneumococcus, Bact. coli communis type. It is formed from methylglyoxal, glyceraldehyde, and dihydroxyacetone under the action of macerated yeast juice and various cells of the animal organism.-d-lactic acid (para- or meat-lactic acid) was isolated by Liebig from meat extract; it is formed during the fermentation of cane, grape, milk sugar in the presence of the gastric mucosa of the pig and under the influence of various bacteria, including Bac. bulgaricus, Streptococcus lacticus, Micrococcus acidi paralactici, from d,l-lactate ammonium under the influence of Penicillium glaucum. It is normally contained in all organs, as well as in blood, urine, bile, pericardial fluid, in cancerous tumors. It is formed in the body during autolysis processes, during the glycolysis of carbohydrates by hydration of methylglyoxal under the action of ketoldehyde mutase, and conversely, lactic acid can be converted to glucose (Embden). (See Metabolism.) It is formed when blood is passed through a glycogen-rich surviving liver, as well as during the deamination of alanine. Technical application. Lactic acid and its salts are used in printing and textile production and in the leather industry. In the process of distillation and brewing, lactic acid is used to destroy bacteria. Concentrated aqueous solutions of sodium salt (perglycerin) and potassium salt (perkaglycerin) replace glycerin in hydraulic brakes of recoilless artillery, as well as in pharmaceutical and cosmetic production; they are suitable for preserving anatomical specimens with preservation of natural color and for microscopic practice. Lactic acid serves as a flavoring and preservative in the manufacture of lemonades, extracts, essences, etc.-Application in medicine. Free lactic acid, Acidum lacticum, an external caustic agent; used in 10-75% solution for lubricating mucous membranes in laryngeal tuberculosis, syphilis; in the form of inhalations in 5-6% solution in diphtheria, croup. In dermatomycosis, for removing pigment spots, warts, calluses (1 part lactic acid, 1 part salicylic acid, 8 parts collodion). Internal. Kestner and Schmidt recommend instead of hydrochloric acid in achylia Ac. lact. conc., 15 drops per 150 cm³ of water. In 1% solution-for washing the stomach in acute enteritis and enterocolitis. The therapeutic use of kefir and yogurt, lactobacillin and others in gastrointestinal diseases in adults and children is based on the desire to replace the putrefactive flora of the intestine with the flora of lactic acid fermentation. β-lactic acid, CH₂OHCH₂COOH, is obtained by the action of moist silver oxide on β-iodopropionic acid; syrup; easily loses water, giving acrylic acid.