Electrochemical biosensors for monitoring malolactic fermentation in red wine using two strains of Oenococcus oeni

Amperometric biosensors for the determination of L-malic and L-lactic acids were optimised and used to monitor micromalolactic fermentations (micro-MLFs) in red wine. Platinum-based probes, coupled with appropriate enzymes, were assembled in electrochemical flow-injection analysis systems. A classical lactate oxidase based sensor was used for L-lactic acid, while L-malic acid was detected via a new biosensor based on the malic enzyme immobilised in a reactor using phenazine methosulphate as mediator. After a preliminary optimisation phase, a recovery study to evaluate the effect of the matrix (red wine) on biosensor performance was carried out by the addition of different standard solutions of the two analytes to the samples. Recoveries from 93 to 100% and from 94 to 102% were observed for L-malic acid and L-lactic acid, respectively. These optimised biosensors were finally employed to monitor micro-MLFs induced by inoculation of two different strains of Oenococcus oeni into red wine. During the micro-MLFs, samples of wine were collected and assayed for L-malic, L-lactic, and citric acids by use of both biosensors and spectrophotometric techniques. In parallel the viable bacterial cell count was also evaluated. The kinetics of bacterial growth, degradation of L-malic and citric acids, and production of L-lactic acid was found to be a function of the strains inoculated.