Determination of L-malic acid by enzymatic means
This method is suitable for malt, wort, beer and beer-based beverages non-alcoholic beverages.
L-Malic acid (L-malate) is oxidized by nicotinamide adenine dinucleotide (NAD) in the presence of L-malate dehydrogenase (L-MDH) to oxaloacetic acid:
L-Malic acid + NAD+ \(^{\underrightarrow{L-MDH}}\) oxaloacetic acid + NADH + H+
The equilibrium of this reaction disproportionately favors malic acid. However, the oxaloacetic acid can be captured with the help of a downstream reaction involving the enzyme glutamate-oxaloacetate-transaminase (GOT) in the presence of L-glutamic acid, shifting the reaction in favor of oxaloacetic acid and NADH:
Oxaloacetic acid + L-glutamate \(^{\underleftrightarrow{GOT}}\)L-aspartate + 2-oxoglutarate
The amount of NADH produced during the reaction is equivalent to the quantity of L-malic acid and its absorption is determined photometrically at 334, 340 or 365 nm.
Determination of organic acids by means of reversed phase chromatography/ion chromatography
This method is suitable for wine, fruit juice and other non-alcoholic beverages.
The organic acids are separated using two combined columns, reversed-phase HPLC and an ion exchange column and are then determined using a UV detector.