Prepn. of L-malic acid (I) from fumaric acid (II) using fumarate hydratase (A) is carried out in a reactor, filled with aq. soln., in which (A) is present in dissolved and stabilised form. The entire amount of (II) is present as an alkaline earth or heavy metal salt and the ratio solubility of this salt to that of the corresponding (I) salt formed is > the ratio of the concn. of these 2 salts at reaction equilibrium. A small amount of a freely soluble (II) salt is added to stabilise the reaction pH and when the reaction is complete, solids are removed and converted to (I).
