: Becker, T.; Kittsteiner-Eberle, R.; Luck, T.; Schmidt, H.-L.

Abstract
A flow injection system for the determination of acetic acid in the 10-60 mM range was conceived and optimized on basis of the coupled reactions of acetate kinase/pyruvate kinase/lactate dehydrogenase. The enzymes were immobilized in series, reagent mixture containing PEP, ATP and NADH was pulsed into the analyte stream, and the decrease of the NADH absorption was determined. One injection cycle was performed within 90 s. Two identical parallel enzyme lines were installed, one of them being in use while the other was calibrated. Automated calibration, control of sample dilution, compensation for enzyme activity loss, the switching between the two enzyme lines and the control of all instruments were performed by an appropriate computer system. The analysis of vinegar samples resulted correct and reliable results. The semi-automated control of continuous vinegar production of 60 h yielded very satisfactory results in the range of 0.5-1.5 M after 40-fold dilution and was more reliable tha n the reference HPLC method. The extension of the analysis system to higher concentration ranges still demands an optimization of the dilution method. In total, the enzyme-based FIA for continuous acetic acid determination seems to be a very usefull and reliable method.