Optimization of L‐malic acid production by Aspergillus flavus in a stirred fermentor

Abstract Effects of various nutritional and environmental factors on the accumulation of organic acids (mainly L ‐malic acid) by the filamentous fungus Aspergillus flavus were studied in a 16‐L stirred fermentor. Improvement of the molar yield (moles acid produced per moles glucose consumed) of L ‐malic acid was obtained mainly by increasing the agitation rate (to 350 rpm) and the Fe z+ ion concentration (to 12 mg/L) and by lowering the nitrogen (to 271 mg/L) and phosphate concentrations (to 1.5 m M ) in the medium. These changes resulted in molar yields for L ‐malic acid and total C 4 acids ( L ‐malic, succinic, and fumaric acids) of 128 and 155%, respectively. The high molar yields obtained (above 100%) are additional evidence for the operation of part of the reductive branch of the tricarboxylic acid cycle in L ‐malic acid accumulation by A. flavus . The fermentation conditions developed using the above mentioned factors and 9% CaCO 3 in the medium resulted in a high concentration (113 g/L L ‐malic acid from 120 g/L glucose utilized) and a high overall productivity (0.59 g/L h) of L ‐malic acid. These changes in acid accumulation coincide with increases in the activities of NAD + ‐malate dehydrogenase, fumarase, and citrate synthase.