Determination of L-malic acid by enzymatic means.
This method is suitable for malt, wort, beer, malt-based drinks, nutritional beer, beer-based mixed drinks, non-alcoholic soft drinks, NAB, fruit juice, soft drinks, juices, drinks
L-Malic acid (L-malate) is oxidized by nicotinamide adenine dinucleotide (NAD) in the presence of L-malate dehydrogenase (L-MDH) to oxaloacetic acid:
\(\text{L-Malic acid + NAD}^+ \space ^{\underrightarrow{\text{L-MDH}}} \space \text{Oxaloacetic acid}\text{ + NADH + H}^+\)
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 + H+:
\(\text{Oxaloacetic acid + L-Glutamate} \space ^{\underrightarrow{\text{GOT}}} \space \text{L-Aspartate + 2-Oxoglutarate}\)
The amount of NADH+H+ produced during the reaction is equivalent to the quantity of L-malic acid and its absorption is determined photometrically at 340 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.