乳酸
Lactic acid is chiral, consisting of two enantiomers. One is known as -(+)-lactic acid or ("S")-lactic acid and the other, its mirror image, is -(−)-lactic acid or ("R")-lactic acid. A mixture of the two in equal amounts is called -lactic acid, or racemic lactic acid. Lactic acid is hygroscopic. -Lactic acid is miscible with water and with ethanol above its melting point, which is around 16, 17 or 18 °C. -Lactic acid and -lactic acid have a higher melting point. Lactic acid produced by fermentation of milk is often racemic, although certain species of bacteria produce solely ("R")-lactic acid. On the other hand, lactic acid produced by anaerobic respiration in animal muscles has the ("S") configuration and is sometimes called "sarcolactic" acid, from the Greek "sarx" for flesh.