The objective of the QUALI-JUICE Collective Research Project financed by the European Union under the 6^th Framework Programme for Research and Technological Development (FP6) (COLL-CT-2005- 012461) is to develop an early warning system alerting to microbiological contamination for use in the European fruit juice industry. For the purpose of this Project, four commercially available biosensors were selected, i.e. Senzytec 1 (Tectronic srl., Italy), Biosen_C Line sport (EKF-Diagnostic GmbH, Germany), LactatProfi 3000 (ABT GmbH, Germany), and Olga (Sensolytics GmbH, Germany). The objective of the research conducted by the authors of this paper was to determine the concentration rate of L – lactates in the commercial apple juices and in concentrates using two biosensors: Biosen_C Line sport (EKF, Germany) and LactatProfi 3000 (ABT, Germany). Twelve commercial apple juices and twelve concentrations supplied by the VINKON company (Konin, Poland) were analysed. The concentration rates of L-lactates were measured using both the biosensors and Megazyme enzyme kits (Ireland). It was found that the biosensors tested could be applied to measure the concentration rates of L-lactates in apple juices and concentrates provided that specific corrections be incorporated into the instruction manuals attached by their manufacturers. The Biosen_C Line sport biosensor is characterized by a high reproducibility and repeatability of measurements taken; however, it is a typical laboratory device. With the LactatProfi 3000 biosensor applied, the accuracy of measurements taken is lower, but this instrument is very practical and can be used at any stage of the fruit juice production.