Lactic acid (2-hydroxypropionic acid), CH3-CHOH COOH, is the most widely occurring hydroxycarboxylic acid, having a prime position due to its versatile applications in food, pharmaceutical, cosmetic, textile, leather, and chemical industries. Lactic acid was first found and described in sour milk by the Swedish chemist Karl Wilhelm Scheele in 1780. It was first isolated from sour milk by CW Scheele in 1780 and was first commercially produced in 1881 by CE Avery in Littleton, MA, USA. Pasteur, Lister, and Delbruck identified lactic acid as a microbial metabolite.
Lactic acid is chiral molecule and has two optical isomers. One is known as L-(+)-lactic acid and other is S-(-)-lactic acid. It is miscible with water or ethanol and is hygroscopic in nature. In animals, L-lactate is constantly produced from pyruvate by the enzyme lactate dehydrogenase (LDH) during normal metabolism and exercise. Its concentration does not increase until the rate of lactate production exceeds the rate of lactate removal. This equilibrium is governed by a number of factors, including monocarboxylate transporters, concentration along with isoform of LDH, and oxidative capacity of the tissues. The concentration of blood lactate is usually 1–2 mmol/L at rest, but can rise over 20 mmol/L during intense exertion.
The carbon source for microbial production of lactic acid can be either sugar in the pure form such as glucose, sucrose, lactose etc. or sugar-containing materials such as molasses, whey, sugarcane bagasse, cassava bagasse, starchy materials from potato, tapioca, wheat, barley, and carrot. The economics of production of lactic acid and its derivatives is dependent on many factors of which the cost of raw material is very significant.
Some agricultural by-products that are potential substrates for lactic acid production are cornstarch, cassava starch, lignocelluloses, hemicelluloses hydrolysates, cottonseed hulls, Jerusalem artichokes, corn cob, corn stalks, beet molasses, wheat bran, rye flour, sweet sorghum, sugarcane press mud, cassava, barley starch, cellulose, carrot processing waste, molasses spent wash, corn fiber Hydrolysates, and potato starch.
Many starch-degrading Lactobacillus spp., can be used for the one-step lactic acid production. The starch-degrading Lactobacillus spp. are L. amylophilus, L. amylovorus, amylolytic strains of L. plantarum, and L. amylolyticus. Semi-solid-state fermentation is adopted in the case of certain amylolytic bacteria because they prefer to grow at higher moisture level.
Several approaches have been considered in literature for lactic acid recovery, including solvent extraction and electrodialysis. However, comparatively high amounts of solvents are needed for extraction, and toxic effects are provoked by these solvents. In electrodialysis units, cells adhered to membranes during the process, leading to decreased efficiency in the overall recovery system. In this field, ion exchange separation provides an interesting alternative for lactic acid recovery from solutions having low concentration and complex composition, as in the case of fermentation media.
There are several issues that need to be addressed for the biotechnological production of lactic acid, such as the development of high-performance, lactic acid-producing microorganisms and the lowering of the costs of raw materials and fermentation processes.