[Study on the optimal conditions in simultaneous reaction and separation for L-malic acid production]. 
Based on the principle of coupling reaction and separation process, free cells containing fumarase were used for producing L-malic acid.  The calcium fumarate was used as substrate to produce calcium malate directly.  This new method was more advantageous than the traditional immobilized cells conversion system in aspects such as simple equipment and operation, high conversion efficiency and the yield of product.  The results showed that at reaction temperature 40 degrees C, pH7.0-7.5, reaction time 20-28 h, the conversion efficiency was up to 99.9% and about 3.2 kg calcium fumarate was converted to calcium malate per liter enzyme suspension.  Also, L-malic acid produced in free fumarase system satisfied USP criterion, the residual fumaric acid was less than 0.1% and the cost was approximately to that of DL-malic acid produced by chemical synthesis.