This method is available in a kit only for the rAPID-T system. The range of the method is 0.05 to 3.0 g/L. Dilute samples above this range.
Wine samples are dispensed into reaction wells along with a glycylglycine buffer at pH 10.0. Enzymes in both the buffer and reagent react with NAD+ and L-malic acid to yield oxaloacetate and NADH. The measured absorbance of NADH at 340 nm is directly proportional to the L-malic acid concentration in the sample.
Blank readings in the method eliminate interference from colored wine samples. Polyvinylpyrollidone (PVP) has been incorporated into the assay format to prevent performance inhibition by polyphenolic compounds (tannins).