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History of Lactic Acid

      Lactic acid was discovered in 1780 by the experimental chemist Carl Wilhelm Scheele, who isolated “acid of milk” from sour whey [12, 13]. A further description of the history of lactic acid by Holten and Benninga shows that industrial production of lactic acid started in the United States in the 1880s [14, 15]. Avery patented and applied a process of fermentation of vegetable sugars [16]. The actual application was the use of a mixture of calcium lactate and lactic acid as baking powder. Unfortunately, this application was not a big success, but other applications in food and textile dyeing were developed.

      In 1950, the first commercial production of synthetic lactic acid started in Japan [15]. Lactonitrile was produced from acetaldehyde and hydrogen cyanide and hydrolyzed in the second stage to lactic acid. For some decades, synthetic lactic acid competed with lactic acid obtained by fermentation, but currently almost all lactic acid is produced by fermentation.

      1.2.2 Physical Properties of Lactic Acid

      1.2.3 Chemistry of Lactic Acid

Property Value Reference
CAS number General: 50‐21‐5
(S)‐Lactic acid: 79‐33‐4
(R)‐Lactic acid: 10326‐41‐7
Molecular weight (g/mol) 90.08
Formula C3H6O3
Melting point (°C) 18 (racemic) [17]
53 (chiral pure) [18]
Crystal structure (S)‐Lactic acid: orthorhombic, space group P212121 [19]
Solid density (g/mL) 1.33 (solid, 20°C) [20]
Solubility in water (wt%) 86 (20°C, monomeric (S)‐lactic acid) [20]
Heat of fusion (kJ/mol) (S)‐Lactic acid: 16.8 [21]
Boiling point (°C) 122 (at 14 mmHg) [22]
Liquid density (g/mL, 20°C) 1.224 (100% undercooled liquid) [22]
1.186 (80.8% solution in water) [23]
Viscosity (mPa s) 28.5 (85.3% solution in water, 25°C) [23]
pK a 3.86 [24]
Specific heat (J/(g K) at 25°C) Crystalline (S)‐lactic acid: 1.41 [25]
Liquid lactic acid: 2.34 [26]

      The condensation reactions are also the reason that it is quite difficult to obtain pure, solid, and enantiopure lactic acid. This can only be achieved by crystallization [27, 28]. The kinetics of the condensation reactions determine the stability of a solution of monomeric lactic acid and have a large influence on the stability of solid lactic acid.

      1.2.4 Production of Lactic Acid by Fermentation