Malic Acid: Preparation and Applications

Malic acid is a colorless, odorless, crystalline substance. It is highly soluble in water (100 g of water dissolves 126.3 g of malic acid at 20℃), methanol, ethanol, acetone, ether, and other polar solvents. Because it is a dibasic acid, it has two dissociation constants: at 20℃, KA1 =3.9×10^-4 and KA2 = 1.4×10^-5.

Applications

(R,S)-Malic acid closely resembles both citric and tartaric acids in its physical and chemical properties. However, the compound has a more neutral flavor, so it is often preferred when the flavor of citric acid is considered objectionable.

Malic acid is used primarily as an ingredient in hard candys and other sweets, jams, jellies, and various canned fruits and vegetables. Most countries authorize its use as an additive in foodstuffs.

Preparation

(R,S)-Malic acid is prepared commercially in the United States and Canada by hydration of maleic anhydride (→Maleic and Fumaric Acids). The sole manufacturer in the United States is Alberta Gas, with an annual capacity of ca. 5000 t. In this process maleic acid is heated at ca. 180℃ (under a pressure of ca. 1 MPa), malic acid is yielded as the main product. Byproducts are maleic and fumaric acids. The latter can be separated by filtration and returned to the process stream because of its low water solubility.

The filtrate is then concentrated; this causes separation of the malic acid, which is purified by multiple washings, evaporation, and recrystallization until the contents of fumaric and maleic acids are reduced to 7.5 and<500 ppm, respectively. Additional purification is required to prepare pharmacological-grade material.