Simultaneous enzymatic determination of l(−) malic acid and l(+) lactic acid in wine by flow injection analysis

Abstract

A simultaneous determination of two organic acid species in wine by a flow injection analysis method using enzyme-immobilized open tubular reactors is described. The design of the optimal geometry of the FIA system is especially important to avoid the matrix fluorescence. Reproducible results were obtained in the determination of l(−) malic acid and l(+) lactic acid using l-MDH and l-LDH, respectively. The assay reactions utilize the fluorimetric determination of a common product, NADH. The sample was dialyzed on line and injected into the FIA-system. The system responds linearly (log-log calibration graphs) to the injected sample (80 μl) in concentrations ranging from 0·1 m m to 5 m m and 0·05 m m to 3 m m for malic and lactic acid, respectively. The maximum sample throughput is 15 h −1. The results obtained with both proposed and HPLC methods are in good agreement. The l-MDH and l-LDH reactors were stable for six months when used daily at optimal pH.