Determination of citric acid and oxalacetic acid in foods by enzymic flow injection

Abstract

A reversed-flow injection method for the determination of citric acid in foods is proposed. Two enzymes, citrate lyase and malate dehydrogenase (MDH), are used, the latter being immobilized. This method, based on the decrease in the absorbance of reduced nicotinamide adenine dinucleotide, has a linear range between 1 and 20 mg dm^–3, with a relative standard deviation (RSD) of 2.2%, an average recovery of 100.9% and a sampling frequency of 20 h^–1. Additionally, a flow injection method for the determination of oxalacetic acid in fruit juices using immobilized MDH is also proposed. The features of the method are as follows: linear range of the calibration graph 1–40 mg dm^–3, RSD 1.4%, average recovery 96.8% and sampling frequency 40 h^–1.