12 Automatic Analysers in Oenology 653
12.2.2.2 Determination ofL-Malic andL-Lactic Acids in Wines and Musts
According to Trossais and Asselin (1985), Battle et al. (1987a), Battle and Bouvier
(1986), and Curvelo-Garcia (1993).
Principle
The acids are oxidised, in the presence of nicotinamide-adenine-dinucleotide (NAD),
in a reaction catalysed by a specific enzyme:
Malate dehydrogenase (MDH) in the case of malic acid
Lactate dehydrogenase (LDH) in the case of malic acidThe equilibria of these reactions are forced in the direction of the products by the
addition of an alkaline hydrazine buffer and an excess of NAD.
MDH
L-malate+NAD+<==>oxaloacetate hydrazone+NADH+H+LDH
L-lactate+NAD+<==>pyruvate hydrazone+NADH+H+The quantity of NADH produced is proportional to the concentration of acid in
the sample, and its absorbance is measured at 340 nm.
Characteristics of the Method
Inter-laboratory reproducibility: 0.15g/L
12.2.2.3 Determination of Tartaric Acid in Wines and Musts
According to Battle et al. (1978b).
Principle
This involves the Rebelein method. A sample of dialysed wine is reacted with
metavanadate to produce a red colour at 37◦C in acid medium. The absorbance
is measured at 520 nm. High concentrations of malic acid and sugars can interfere
with the analysis.
Characteristics of the Method
Inter-laboratory reproducibility: 0.7g/L