Lactic acid is chiral and has two optical isomers. One is known as L-(plus)-lactic acid or (S)-lactic acid and the other, its mirror image, is D-(-)-lactic acid or (R)-lactic acid. L-(plus)-Lactic acid is the biologically important isomer.Lactic acid (IUPAC systematic name: 2-hydroxypropanoic acid), also known as milk acid, is a chemical compound that plays a role in several biochemical processes.