AMPEROMETRIC FLOW-INJECTION DETERMINATION OF CITRIC-ACID IN FOOD USING FREE CITRATE LYASE AND CO-IMMOBILIZED OXALACETATE DECARBOXYLASE AND PYRUVATE OXIDASE

A flow-injection analytical system for the determination of citrate in food has been developed. Citrate lyase, oxalacetate decarboxylase, an d pyruvate oxidase were used, and the latter two enzymes were co-immob ilized. Hydrogen peroxide, which was produced by three enzyme reaction s, was monitored amperometrically with a platinum electrode. The carri er solution used was 0.1 M phosphate buffer (pH 7.0) containing 10 mM MgCl2, 80 mu M thiamine pyrophosphate (TPP), and 10 mu M flavine adeni ne dinucleotide (FAD). The detection limit was 0.02 mM, and the range of determination was 0.1-1.0 mM. The relative standard deviation was 1 .23% for 10 successive determinations at the 0.5 mM level. The determi nation frequency was about 15 tests per hour. The present system was a pplied to the determination of citrate in several fruits. The results showed good agreement with those obtained using a conventional method (F-kit method).