Determination of l-(−)-malic acid and l-(+)-lactic acid in wine by a flow injection-dialysis-enzymic derivatisation approach Original Research Article

A simultaneous flow injection method for the determination of malic and lactic acids in wines based on the coupling of dialysis-enzymic derivatisation with either photometric or fluorimetric detection is proposed. A study of the dialysis selectivity and efficiency as a function of the membrane features was previously carried out. After separation by dialysis, the malic and lactic acids are collected into a nicotinamide adenine dinucleotide (β-NAD+) acceptor stream. The oxidation of the coenzyme is catalysed by malic and lactic dehydrogenases immobilised on controlled pore glass in two different enzymatic reactors and the reduced form of the coenzyme (NADH) is monitored either spectrophotometrically (340 nm) or fluorimetrically (λex=340 nm, λem=460 nm). Discrimination between the two analytes is achieved by dividing the dialysed into two aliquots by trapping a portion of the solution in the loop of an auxiliary injection valve and leading each aliquots to one or the bioreactors with the help of a selection valve. The linear determination range is 0.05–1.0 g l−1 for malic acid and 0.1–1.0 g l−1 for lactic acid with 3σ limits of detection (LODs) of 0.03 and 0.05 g l−1, respectively, for spectrophotometric detection and 0.02–1.5 g l−1 for malic acid and 0.05–1.5 g l−1 for lactic acid and LODs of 0.015 and 0.01 g l−1, respectively, when fluorimetric detection is used. When applied to different Spanish wines the results compared well with those of the official methods.