BACKGROUND: Lipase-catalyzed esterification of lactic acid with ethanol was investigated; however, some difficulties exist with such a system. Because of its high polarity, lactic acid is immiscible with non-polar organic solvents, which have generally been used for non-aqueous enzyme reactions. In addition, the strong acidity of lactic acid causes acid inactivation of enzymes and acid-catalyzed, non-enzymatic esterification. RESULT: In the present study, particular polar organic solvents, such as 1,4-dioxane, were found to suppress the enzyme inactivation and non-enzymatic esterification caused by the acidity. The magnitude of this effect varied with solvents and strongly correlated with the Kamlet-Taft parameter beta, which indicates the basicity of the solvents in non-aqueous systems. An immobilized lipase from Candida antarcitica was found to be the most active and stable enzyme for the reaction. Based on the findings, lipase-catalyzed esterification of lactic acid (1.0mol L-1) could be continued for up to 4 weeks without any loss of enzyme activity. CONCLUSION: In addition to miscibility with lactic acid, the effect of these polar solvents, which is to suppress the acidity of lactic acid and is presumably due to the basicity, appears to play a very important role in the efficient enzymatic reaction of lactic acid. (C) 2008 Society of Chemical Industry