654 M. Dubernet
12.2.2.4 Determination of Citric Acid in Wines and Musts
According to Battle and Bouvier (1986).
Principle
Citric acid is transformed into oxaloacetate and acetate in a reaction catalysed by
citrate-lyase (CL).
CL
citrate←====>oxaloacetate+acetate (1)In the presence of malate deshydrogenase(MDH), oxaloacetate and pyruvate, its
decarboxylation derivative, are reduced toL-malate by reduced NADH. The equi-
librium of the reaction is forced in the direction of the products by the judicious
selection of operating conditions (buffer pH7.8 and an excess of NADH):
MDH
oxaloacetate+NADH+H+←===>L-malate+NAD (2)The quantity of NADH oxidised to NAD is proportional to the quantity of citrate
present and the oxidation of NADH is tracked by the reduction in its absorbance at
340 nm.
12.2.2.5 Determination of Reducing Sugars in Wines and Musts
According to Curvelo-Garcia (1993) and Curvelo-Garcia and Godinho (1988).
Principle
A copperIII -neocuproine chelate is reduced by reducing sugars in alkaline medium
at approximately 85◦C. A dialysis step eliminates macromolecules that can interfere
with the colour reaction, the absorbance of which is measured at 460 nm. Must
analysis requires a 40-fold dilution prior to analysis.
Characteristics of the Method
Inter-laboratory reproducibility: 20%
12.2.2.6 Determination of Glucose and Fructose in Wines and Musts
According to Battle and Bouvier (1986).