Malolactic fermentation is carried out by lactic acid bacteria which convert L-malic acid, present in wine after the alcoholic fermentation, into L-lactic acid and CO2 in the presence of NAD and Mn++. Many researchers have tried to improve the control and yield of this conversion with different systems (i.e., selection of strains, stimulating bacteria, hyperproducing modified strains, bioreactors with free or gel-entrapped bacteria, bioreactors with immobilized enzyme). We propose in this paper a membrane reactor using free enzyme, extracted from lactic acid bacteria, and cofactor to carry out the malolactic fermentation. Experiments have been carried out, after the design of the reactor, to obtain a maximum yield as a function of the operating conditions. We can assert that a yield of 70% is possible in a period of about one week.