Production of 1-Malic Acid with Immobilized Thermophilic Bacterium, Thermus Rubens Nov. sp.

In order to establish an advantageous method for the production of L-malic acid, the conversion of fumaric acid to L-malic acid by thermostable fumarase was studied. A new thermophilic bacterium, Thermus rubens nov. sp., which has a high fumarase activity, was isolated from a hot spring in Japan. This bacterium could produce about twice the activity of fumarase in comparison with the mesophilic microorganisms, such as Brevibacterium ammoniagenes and Candida lipolytica. Past enzymatic production of L-malate from fumarate was done batchwise using Ca-fumarate as substrate (1); but the byproduct calcium sulfate was a disadvantage. Continuous production of L-malate with immobilized microorganism was developed (2, 3); with a maximum theoretical conversion of 81% (with Na-fumarate). The equilibilium constant (L-MA/FA) of L-malate and fumarate is 4.00 for Na, 4.75 for NH4, 7.00 for Mg, and 12.3 for Ca, all at 60°C and pH 7.0. When Mg- or Ca- fumarate is used, the conversion can be increased to 87.5% or 92.5%, respectively; but these salts cannot be used on a practical basis because the solubilities are very low at room temperature.