Determination of L-(-)-malic acid and L-(+)-lactic acid in wine by a flow injection-dialysis-enzymic derivatisation approach

A simultaneous flow injection method for the determination of malic and lac tic 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 featu res was previously carried out. After separation by dialysis, the malic and lactic acids are collected into a nicotinamide adenine dinucleotide (beta -NAD(+)) acceptor stream. The oxidation of the coenzyme is catalysed by mal ic and lactic dehydrogenases immobilised on controlled pore glass in two di fferent enzymatic reactors and the reduced form of the coenzyme (NADH) is m onitored either spectrophotometrically (340 nm) or fluorimetrically (lambda (ex) = 340nm, lambda (em) = 460 nm). Discrimination between the two analyt es is achieved by dividing the dialysed into two aliquots by trapping a por tion of the solution in the loop of an auxiliary injection valve and leadin g each aliquots to one or the bioreactors with the help of a selection valv e. 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 3 sigma limits of detection (LODs) of 0. 03 and 0.05 g l(-1), respectively, for spectrophotometric detection and 0.0 2-1.5 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 use d. When applied to different Spanish wines the results compared well with t hose of the official methods. (C) 2001 Elsevier Science B.V. All rights res erved.