Determining citrate in fruit juices using a biosensor with citrate lyase and oxaloacetate decarboxylase in a flow injection analysis system

A biosensor system was constructed for the determination of citrate concentration in foods. The citrate biosensor system consisted of a sample injector, peristaltic pump, enzyme reactor, CO32-ion selective electrode (ISE), reference electrode, detector and recorder. Citrate lyase and oxaloacetate decarboxylase were immobilized for the enzyme reactor. The carbonate ions produced through the enzyme reactions of citrate were potentiometrically detected by an ISE. The optimum conditions for the biosensor system were investigated. A linear relationship between the potential difference and logarithmic citrate concentration was obtained in the range of 10−1–10−4 M. There was little interference effect observed regarding most sugars and organic acids on the citrate biosensor system. Citrate concentrations in fruit juices were determined by the biosensor and GC. There was no significant difference between the two analytical methods. Therefore, this citrate biosensor was considered to be a useful method for the determination of citrate concentration in fruit juices.