Wine Chemistry and Biochemistry

12 Automatic Analysers in Oenology 661

HK

Glucose+AT P−→G6P+ADP

HK

Fructose+AT P−→F6P+ADP

Glucose-6-phosphate is oxidised to form gluconate-6-phosphate in the presence

of NAD in a reaction catalysed by glucose-6-phosphate deshydrogenase (G6PDH):

G6PDH

G6P+NAD−→gluconate-6-phosphate+NADH+H+

Fructose-6-phosphate is transformed into glucose-6-phosphate by phosphoglu-

cose isomerase (PGI):

PGI

F6P−→G6P

The increase in the absorbance at 340 nm is proportional to the quantity of

d-glucose and d-fructose.

Characteristics of the Method

Intra-laboratory reproducibility: 0.3g/L from 0 to 5g/L and 0.6g/L above 5g/L.

Inter-laboratory reproducibility: 0.35g/L from 0 to 2g/L

12.3.4.5 Determination ofL-Malic Acid in Wines and Musts

Principle

In the presence of NAD,L-malic acid is oxidised to oxaloacetate in a reaction catal-
ysed byL-malate deshydrogenase (L-MDH). The reaction equilibrium is forced in

the direction of the products by the elimination of oxaloacetate, via its reaction with

l-glutamate, resulting in the production ofL-aspartate. This reaction is catalysed by

glutamate-oxaloacetate-transaminase (GOT):

L-malate+NAD+−>oxaloacetate+NADH+H+

( in the presence ofL-malate deshydrogenase)

Oxaloacetate+L-glutamate−> L-aspartate+α−cetoglutarate

( in the presence of GOT)

The formation of NADH, measured by the increase in its absorbance at 340 nm,

is proportional to the quantity ofL-malate initially present.