Malic acid, a kind of carboxylic diacid, was first isolated from
apple juice a number of years ago by Scheele. It has two stereoisomeric forms: D- and L- enantiomers. Natural malic
acid is the L- enantiomer. The synthetic malic acid is a racemic mixture
of the D- and L- enantiomers which is used as a food additive to prevent alteration and degradation by
microorganisms during its storage period. However, excessive concentrations
of malic acid may result in allergic contact dermatitis, convulsion and hives.