SENSING THE LACTIC ACID IN PROBIOTIC YOGURTS USING AN L-LACTATE BIOSENSOR COUPLED WITH A MICRODIALYSIS FIBER INSERTED IN A FLOW ANALYSIS SYSTEM

An amperometric biosensor for the determination of L-lactic acid in probiotic yogurts has been assembled using L-lactate dehydrogenase (EC 1.1.1.27, LDH) entrapped in 1% (v/v) neutralized Nafion (R) solution deposited on Variamine blue redox mediator modified screen-printed electrodes. The Variamine blue was previously covalently linked to oxidized single-walled carbon nanotubes and used for modifying screen-printed electrodes. The electrochemical cell, containing the L-lactate biosensor operating at an applied working potential of +200mV vs. AgAgCl, was coupled with a microdialysis fiber and connected with a flow system, thus obtaining a microdialysis based sampling experimental set-up. Various analytical parameters, such as the cofactor concentration (2mM, NAD(+)), the flow rate (10.5 mu L/min), the applied working potential (+200mV vs. Ag/AgCl), the working buffer (50 mM phosphate buffer +0.1M KCl), and pH (7.5), were optimized in batch amperometric experiments. The dynamic linear working range was comprised between 2 . 10(-4) and 1 . 10(-3) M. The proposed biosensor was challenged with real samples of yogurt, properly diluted in working buffer, and the performances of the L-lactate biosensor were compared with a commercially available kit for the determination of L-lactic acid in foodstuffs from R-Biopharm GmbH, Germany, showing a good agreement.