Enzymatic Determination of D-Malic Acid in Selected Foods: AOAC Official Method 2026.02 First Action

: D-malic acid is one of the two stereoisomers that make up racemic (DL-) malic acid, which is commonly produced through chemical synthesis. In contrast, naturally occurring malic acid found in fruits, fruit juices, and wines consists almost exclusively of the L-isomer, with little or no D-malic acid being present. Therefore, the presence of D-malic acid in a natural product such as wine or fruit juice serves as an indicator that racemic DL-malic acid has been artificially added.

To validate the performance of the D-Malic acid test kit (Enzytec™ Liquid D-Malic acid) for the determination of D-malic acid in food such as soft drinks, fruit juices, tomato juice, and wine.

D-malic acid is oxidized by NAD+ in the presence of D-malate dehydrogenase to pyruvate and carbon dioxide. The NADH produced is equivalent to D-malic acid converted and is measured at 340 nm.

α-Ketoglutaric acid, sulfite, and meso-tartaric acid do not interfere at or below 0.5 g/L, 0.5 g/L and 0.2 g/L, respectively. L-tartaric acid reacts and leads to a creep reaction which can be eliminated by precipitation or extrapolation. LOD is at 4 mg/L for 100 µL. The linear measurement range is 14 mg/L to 500 mg/L for a test volume of 100 µL. Recovery was checked in grape juice, tomato juice, lemonade, pineapple juice, currant juice, bitter lemon and apple juice and resulted in mean recoveries between 90 and 103%. Trueness was also evaluated using reference wines and a fruit juice CRM. For automation, two applications with different test volumes were validated. Linearity is given from 5 up to 2500 mg/L.

The method is robust and accurate for manual and automated applications. The method was approved as AOAC Official Method of Analysis℠.

The kit contains two ready-to-use components which make handling easy and suitable for automation.