Flow injection methods for determination of L-glutamate using glutamate decarboxylase and glutamate dehydrogenase reactors with spectrophotometric detection

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The use of glutamate decarboxylase and glutamate dehydrogenase reactors in flow injection (FI) systems for the determination of glutamate in food is described. In a gas diffusion FI system, glutamate was decarboxylated by glutamate decarboxylase immobilized on a carrier. The produced carbon dioxide was measured spectrophotometrically after gas diffusion using an acid–base indicator as acceptor. The linear range of calibration was from 2 to 20 mmol l–1 with r= 0.9978 and the RSD was 2.8%(n= 5) for 10 mmol l–1L-glutamate. Using a glutamate dehydrogenase reactor the NADH formed by the enzymic reaction was detected at 340 nm. For the normal FI approach the calibration graph was linear from 0.05–0.06 mmol l–1 with r= 0.9991 and the RSD was 2.4%(n= 5) for 0.3 mmol l–1L-glutamate. For the stop flow approach with a significant reduction in NAD+ consumption the linear range was from 0.2 to 1.0 mmol l–1L-glutamate with r= 0.9994. The RSD was 1.9%(n= 5) for 0.6 mmol l–1L-glutamate. A sample frequency of 30 h–1 using these three approaches was reached. Results from food samples using these three approaches were in good agreement with each other. Recoveries were 94.6–101.2%, 96.7–99.3% and 95.9–98.2% for L-glutamate decarboxylase, L-glutamate dehydrogenase with the normal FI approach and L-glutamate dehydrogenase with the stop flow approach, respectively. The assay results of one sample compared well with those given by the manufacturer.