On-line determination of lactic acid during kefir fermentation based on a fibre-optic lactic acid biosensor and flow-injection analysis

Abstract Lactic acid was monitored on-line for 13 h during a kefir fermentation by means of a fibre-optic lactic acid biosensor in combination with flow-injection analysis. The biosensor, which is based on an oxygen optrode with immobilized lactate oxidase (LOD), is described. The consumption of oxygen was determined via dynamic quenching of the fluorescence of an indicator by molecular oxygen. LOD was adsorbed on a sheet of carbon black and cross-linked with glutaraldehyde. Carbon black was used for optical isolation to protect the optrode from interference from ambient light and sample fluorescence. With the zone sampling technique the linear range (0.02–0.5 mM) for l -lactate was extended up to 60 mM. The maximum sample throughput is 20 h −1 . For five repeated measurements an r.s.d. of 3% at the 60 mM level was observed. It was possible to do continuous l -lactate analyses with this enzyme optrode for at least 2 days.

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