Background: Due to its excellent acidifier, antioxidant, and preservative properties, citric acid is added to or is a constituent in a broad range of foods and beverages. Its measurement should be performed to assure the food quality specifications are met.
Objective: To validate the performance of the Enzytec™ Liquid Citric acid test kit for the determination in food and beverages such as wines, juices, and tomato products.
Methods: The kit contains two ready-to-use components which makes handling very easy and suitable for automation. Citrate is cleaved into oxaloacetate and acetate by citrate lyase. Oxaloacetate reacts to L-malate by L-malate dehydrogenase and reduced nicotinamide adenine dinucleotide (NADH). Pyruvate, spontaneously formed from oxaloacetate, is converted by L-lactate dehydrogenase to L-lactate. The NADH consumed is equivalent to the amount of citric acid converted and is measured at a wavelength of 340 nm within 20 min.
Results: The test is specific to citric acid and shows no relevant interferences. Limit of detection and limit of quantification are 15 and 40 mg/L, respectively when using a test volume of 100 µL. The linear measurement range is from 40 to 1000 mg/L citric acid. Trueness was evaluated using materials from FAPAS, NIST, LGC, and two control wines from the German Wine Analysts. Matrix interference was evaluated by spiking tomato ketchup, tomato paste, orange juice and carbonated beverages and resulted in recoveries around 100%. Intermediate precision is between 6.2% and 8.5% for matrixes with extraction and below 4% for matrixes measured directly.For automation, three applications with different test volumes and different measurement ranges were validated. Linearity is given from 8 mg/L up to 5 g/L (depending on the test volume).
Conclusions: The method is robust and accurate for manual and automated applications. The method was approved as AOAC Official Method of Analysis℠.
Highlights: The ready-to-use components of the test kit have a shelf life of at least 24 months from date of manufacture.
© The Author(s) 2024. Published by Oxford University Press on behalf of AOAC INTERNATIONAL.