L-Malic acid is an intermediate of cell metabolism that is involved
in two respiratory metabolic cycles: the tricarboxylic acid cycle and the
glyoxylic acid cycle. It is also the predominant acid component in apple
and many other fruits. Having a greater acid taste and better taste retention
than citric acid, malic acid is commonly used as a food and beverage acidulent
(1), and is also used in pharmaceuticals, cosmetics, metal cleaning,
coatings, polymers, and resins (1). Extraction from apple juice (0.4-0.7%
L-malic acid) was the traditional method for L-malic acid preparation (2).
However, the relatively low concentrations of L-malic acid present in natural
sources makes its isolation expensive and impractical. Currently, malic
acid can be produced by chemical synthesis (D, L-racemate mixture) via hydration of maleic acid or fumaric acid at elevated temperature and pressure,
or by biosynthesis (L-isomer) from fumaric acid (1-3).