662 M. Dubernet
Characteristics of the Method
Intra-laboratory reproducibility: 0.4g/L
Inter-laboratory reproducibility: 0.6g/L12.3.4.6 Determination ofL-Lactic Acid in Wines and Musts
Principle
Enzymatic Method
L-Lactic acid, in the presence of NAD, is oxidised to pyruvate in a reaction catal-
ysedL-lactate deshydrogenase (L-LDH). The equilibrium of the reaction is forced
in the direction of the products by the elimination of pyruvate by reacting it with
L-glutamate, resulting in the formation ofL-alanine. This reaction is catalysed by
glutamate-pyruvate-transaminase (GPT):
L-lactate+NAD+−>pyruvate+NADH+H+
( in the presence ofL-lactate deshydrogenase)Pyruvate+L-glutamate−> L-alanine+α−cetoglutarate
( in the presence of GPT)The formation of NADH, measured by the increase in its absorbance at 340 nm,
is proportional to the quantity of lactate initially present.
Characteristics of the Method
Intra-laboratory reproducibility: 0.2g/L
Inter-laboratory reproducibility: 0.5g/L12.3.4.7 Determination of Total Phenolic Compounds in by the Folin-Ciocalteu
Index
Principle
Chemical Method
Total wine polyphenols are oxidised by the Folin-Ciocalteu reagent – composed of
a mixture of phosphotungstic and phosphomolybdic acids which are reduced by the
oxidation of the phenols, forming a mixture of blue oxides of tungsten and molyb-
denum. The blue coloration has an absorption maximum at approximately 750 nm,
the intensity of which is proportional to the level of phenolic compounds present
in the wine. The sequential analyser method is a direct automation of the manual
method and results are expressed as a unit-less index.