A novel biosensor for l-ascorbic acid determination in different beverages was elaborated. The ascorbate oxidase enzyme (AAO) from sp., EC 1.10.3.3, was immobilized on a screen-printed carbon electrode with poly(ethylene glycol) (400) diglycidyl ether (PEGDGE) as a crosslinking agent. The standards and samples were measured first with a blank electrode. An inert protein, bovine serum albumin (BSA), was immobilized on the surface of this electrode with PEGDGE. The BSA mass was equivalent to the mass of 10 U of AAO enzyme immobilized on the electrodes (0.021 mg). The linear measuring range for l-ascorbic acid was between 5 and 150 µmol/L. As l-ascorbic acid is a vital vitamin and a common antioxidant used in food industry, fruit juices and vitamin C effervescent tablets were examined. The results were compared to HPLC measurements.

A thermostable lactate oxidase (from Aerococcus viridans) based bioreactor was elaborated for l-lactate determination in dairy products. The enzyme was immobilised in a thin-layer enzyme cell connected into a flow injection system with an amperometric detector. The linear measuring range was 0.1-1 mmol/L l-lactate (R 2 = 0.99; linear regression equation: y = 81.2x + 3.3). The detection limit was 0.05 mmol/L. 40 Samples were analyzed per hour. Considering reproducibility: the signals measured were very stable for over 800 injections, after this period the activity of the cell decreased slowly. The method was used for the control of dairy products and for monitoring milk freshness.

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