Robust l-malate bienzymatic biosensor to enable the on-site monitoring of malolactic fermentation of red wines

- Bienzymatic electrochemical biosensor for l-malate.
- Malate dehydrogenase and diaphorase enzymes entrapped into an electrosynthesized polypyrrole film.
- Optimized redox mediator also incorporated into the polymeric film.
- Long-term stability for over 37 days.
- Application to l-malic acid determination during the malolactic fermentation of red wines.

Monitoring the malolactic fermentation process is strictly required to guarantee the sensorial quality and freshness of red wines. This could be achieved by in-field and real-time continuous measurements of l-malate concentration in the fermentation tanks. The potential of a miniaturized amperometric bienzymatic biosensor as an analytical tool to be applied in such scenario is described in this paper. The biosensor comprises a thin-film gold electrode as transducer, malate dehydrogenase (MDH) and diaphorase (DP) enzymes together with nicotinamide adenine dinucleotide (NAD+) cofactor as the selective receptor and an adequate redox mediator to record the corresponding amperometric signal. Three different biosensor architectures are studied, whose main differences lie in the immobilization of the different chemical components onto the electrode surface. In all cases a fast-electrosynthethized polypyrrole (PPy) membrane is generated for this purpose. The experimental conditions are optimized and the best architecture shows a sensitivity of 1365 ± 110 mA M−1 cm−2 and a detection limit of 6.3 × 10−8 M in a concentration range of 1 × 10−7 M - 1 × 10−6 M. The biosensor presents an excellent working stability as it retains above 90% of its sensitivity after 37 days, thus enabling the monitoring of the malolactic fermentation of three red wines. The obtained results show excellent agreement with the standard colorimetric method.

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