Lactic acid (2-hydroxy prop ionic acid) is the most widely occurring carboxylic acid in nature. The Swedish chemist Scheele first discovered it in 1780, but it was first produced commercially by Charles E. Avery at Littleton, Massachusetts, USA in 1881. Lactic acid can be manufactured by either a Chemical synthesis or a Carbohydrate fermentation. The commercial process for chemical synthesis is based on lactonitrile. Hydrogen cyanide is added to acetaldehyde in the presence of a base to produce lactonitrile. This reaction occurs in liquid phase at high atmospheric pressures. The crude lactonitrile is recovered and purified by distillation. It is then hydrolyzed to lactic acid, either by concentrated HCl or by H 2SO 4 to produce the corresponding ammonium salt and lactic acid. Lactic acid is then esterified with methanol to produce methyl lactate, which is removed and purified by distillation and hydrolyzed by water under acid catalyst to produce lactic acid and the methanol, which is recycled. Though chemical synthesis produces a racemic mixture, stereo specific acid can be made by carbohydrate fermentation depending on the strain being used. It can be described by a fermentation and neutralization of product lactic acid by calcium hydroxide. The broth containing calcium lactate is filtered to remove cells, carbon treated, evaporated and acidified with sulphuric acid to get lactic acid and calcium sulphate. The insoluble calcium sulphate is removed by filtration; lactic acid is further obtained by hydrolysis, esterification, distillation and hydrolysis.

Lactic acid is a three carbon organic acid whose one terminal carbon atom is part of an acid or carboxyl group; the other terminal carbon atom is part of a methyl or hydrocarbon group; and a central carbon atom having an alcohol carbon group. Lactic acid exists in two optically active isomeric forms L (+) and the D (-).Lactic acid is soluble in water and water miscible organic solvents but insoluble in other organic solvents. It exhibits low volatility. Lactic acid can be analyzed by NAD + linked lactate dehydrogenase enzyme assay. A colorimetric method has been described in many papers. Gas chromatography can be used after esterification of lactic acid but is unsatisfactory for quantitative estimation. Liquid chromatography and its various techniques can be used for quantitative analysis and separation of its optical isomers.

Lactic acid is used as acidulant/ flavouring/ pH buffering agent or inhibitor of bacterial spoilage in a wide variety of processed foods. In contrast to other food acids it has a mild acidic taste. It is non-volatile odourless and is classified as GRAS (generally regarded as safe) by FDA in the US. It is a very good preservative and pickling agent. Addition of lactic acid aqueous solution to the packaging of poultry and fish increases their shelf life (Anon, 1989). The esters of lactic acid are used as emulsifying agents in baking foods (stearoyl-2-lactylate, glyceryl lactostearate, glyceryl lactopalmitate). The manufacture of these emulsifiers requires heat stable lactic acid, hence only the synthetic or the heat stable fermentation grades can be used for this application. Technical grade lactic acid is used as an acidulant in vegetable and leather tanning industries. Various textile finishing operant and acid dying of food require low cost technical grade lactic acid to compete with cheaper inorganic acid. Lactic acid is being used in many small scale applications like pH adjustment hardening baths for cellophanes used in food packaging, terminating agent for phenol formaldehyde resins, alkyl resin modifier, solder flux, lithographic and textile printing developers, adhesive formulations, electroplating and electropolishing baths, detergent builders. Lactic acid has many pharmaceutical and cosmetic applications and formulations in topical ointments, lotions, anti acne solutions, humectants, parental solutions and dialysis applications, for anti carries agent. Calcium lactate can be used for calcium deficiency therapy and as anti caries agent. Its biodegradable polymer has medical applications as sutures, orthopaedic implants, controlled drug release etc. Polymers of lactic acids are biodegradable thermoplastics. These polymers are transparent and adjusting the composition, and the molecular weight can control their degradation. Their properties approach those of petroleum-derived plastics. Lactic acid esters like ethyl/butyl lactate can be used as green solvents. They are high boiling, non-toxic and degradable components. Poly L-lactic acid with low degree of polymerization can help in controlled release or degradable mulch films for large-scale agricultural applications.

Lactic acid bacteria are among the best-studied microorganisms. Important new developments have been made in the research of lactic acid bacteria in the areas of multidrug resistance, bacteriocins, quorum sensing, osmoregulation, autolysins and bacteriophages. Progress has also been made in the construction of food grade genetically modified Lactic acid bacteria. The desirable characteristics of industrial microorganisms are their ability to rapidly and completely ferment cheap raw materials, requiring minimal amount of nitrogenous substances, providing high yields of preferred stereo specific lactic acid under conditions of low pH and high temperature, production of low amounts of cell mass and negligible amounts of other byproducts. Sucrose (from syrups, juices and molasses), lactose (from whey), maltose (produced by specific enzymatic starch conversion processes), glucose (from starch conversion processes, mannitol etc have been commercially used. Molasses are cheap but give low yields of lactic acid and laborious purification procedures. Whey is also cheap and easily available but like molasses have expensive purification processes. These have stimulated the development of modern technologies like ultrafiltration and electrodialysis. Hydrolyzed potato starch, corn, straw, whey, cottonseed hulls, grapefruit, sulfite waste liquor etc. have also been investigated.

Lactic acid fermentation is known to be end product inhibited fermentation by an undissociated form of lactic acid. Several studies have been carried out to overcome this problem. It has found that using extractive lactic acid fermentation technique could give a lactic acid yield and productivity. Mainly the two reactor systems result in high yields and productivities of lactic acid: - continuous cells recycle fermentation process, and fed batch fermentation. A number of approached can be used for separation of lactate salt from fermented medium, which are extraction by solvents, ion-exchange separation, separation by adsorption, separation by vacuum distillation, and the membrane separation.

Lactic acid polymers consist of mainly lactyl units, of only one stereoisoform or combinations of D and L lactyl units in various ratios. A disadvantage of polycondensation is that a low molar mass polymer is obtained. There have been studies to obtain high molar mass polymer by manipulating the equilibrium between lactic acid, water and polylactic acid in an organic solvent or a multifunctional branching agent was used to give star-shaped polymers. In the presence of bifunctional agents (dipoles and diacids) they form telechelic polymers, which can be further linked to give high molar mass polymers using linking agents like diisocynate. The synthesis of polylactic acid through polycondensation of the lactic acid monomer gave weight average molecular weights lower than 1.6 x 10 4, whereas ring opening polymerisation of lactides gave average molecular weights ranging from 2 x 10 4 to 6.8 x 10 5.

Various efforts in manufacturing lactic acid and lactic acid based polymers have been made. Technological advancements in the major process components – fermentation, primary and secondary purification, polymerization, chemical conversion of lactic acid and its derivatives would enable low cost large volume and environment friendly production of lactic acid. Recent advancements in membrane based separation and purification would enable lactic acid production without producing salt or gypsum by products.

The L-lactic acid based polymers may produce polymer which is a linear homopolymer of the molecular size >70 kDa. The main application field of lactic acid polymer has been medical applications and a number of companies have made their efforts in manufacturing lactic acid based polymers and their products. These medical applications include its usage if carrying different properties in terms of tensile strength, viscosity, purity etc. L-lactic acid polymer exist in three different forms solids that can be used for filling the gaps in bones, solid with tensile strength to produce sutures (stitching material), and the glue form that is mainly applied in joining membranes or thin skins in humans. The proven degradability in biological systems, biocompatibility and the possibility of tailoring the properties to a wide range have made lactic acid derivatives well suited for a range of applications.

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