Malic acid (2-hydroxybutanedioic acid, C4H6O5) is a white, odorless, crystalline solid. In contrast to other fruit acids, it is very hygroscopic and has a tendency to lump. Malic acid is a dicarboxylic acid and has an asymmetric carbon and occurs as l(the natural)- and d-isomers. Malic acid is found in other fruits such as grapes, watermelons, cherries, and in vegetables such as carrots and broccoli. This acid is mainly used in food applications including candy and beverages. It gives a tart taste, lowers the pH, has antimicrobial effects, and confers special blending and flavor-fixing properties. There are also nonfood applications such as use for metal cleaning and finishing, textile finishing, electroless plating, pharmaceuticals, infusions, and paints.
Malic acid was first described by Sheele who, in 1785, isolated this acid from unripe apples. The name malic is from the Latin for apple, malum.
As early as 1928, Yuill reported that a small amount of malic acid was produced along with succinic acid and fumaric acid in the culture of Aspergillus flavus. Since then, countries around the world have successively carried out research on malic acid. Japan has never interrupted the research on the one-step conversion of starch directly fermented into L-malic acid since 1959, and achieved great results in the 1950s and 1960s.
In the early 1960s, countries such as England and America began to use chemical synthesis to produce DL-malic acid. In addition to the industrialized production of DL-malic acid by chemical synthesis, the industrialized production of L-malic acid has also been realized by immobilized enzyme and immobilized cell transformation methods. In 1974, Japan's Tanabe Pharmaceutical Company first used immobilized cell technology for continuous production, and it was successful. Since the 1980s, Chinese researchers have made great progress in the selection of enzyme-producing strains, immobilization materials and L-malic acid extraction methods.
The biological method to prepare D-malic acid has been developed in the past ten years, mainly including microbial decomposition method and enzymatic method. At present, Japan and the Netherlands have adopted the enzymatic conversion method to realize the industrial production of D-malic acid microbial conversion method. In addition, the coupled reaction and separation technology was also applied to the enzymatic synthesis of D-malic acid to achieve higher yields.