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Lactic acid (2‐hydroxypropanoic acid) is the simplest 2‐hydroxycarboxylic acid (or α‐hydroxy acid) with a chiral carbon atom and exists in two enantiomeric forms (Figure 1.1).

The chirality of lactic acid often results in confusion regarding nomenclature. A number of different names are used in the literature. This confusion is the result of mixing the molecular structure and a physical property (optical rotation). (S)‐Lactic acid (or L‐lactic acid) has a slightly positive‐specific optical rotation and is frequently named L‐(+)‐lactic acid [14]. However, a concentrated solution of (S)‐lactic acid at equilibrium contains lactic acid oligomers, which results in an overall negative optical rotation. Therefore, it is advised to use the structural R/S notation or the older notation of L and D and avoid the + and − of the optical rotation (Table 1.1).