Abstract: A flow injection analysis procedure was used for the determination of l-lactic acid during the production of mozzarella cheese. The apparatus consists of an electrochemical flow-through wall-jet cell assembled with a platinum sensor covered with the immobilized lactate oxidase enzyme and connected to an amperometer. This system was used to monitor the concentration of l-lactic acid produced by lactose fermentation catalyzed by selected starters during cheese manufacture. Lactate was detected in the range (5 x 10^-6)-(1 x 10^-4) mol/L with a detection limit of 2 x 10^-6 mol/L. l-Lactate has been measured in raw milk and during the manufacture of cow and water buffalo mozzarella cheese. The starter used was a commercially available strain of Streptococcus thermophilus. Real time analysis of lactate allowed a control of the curd-ripening evolution at different pasteurization temperatures of the milk. Values of lactic acid were compared with pH variation during the process. This method proved to be more sensitive than the pH measurement procedure for the control of the continuous production of lactic acid particularly near the "stretching point" when very slight pH variations were observed.

Abstract: Amperometric biosensors for the determination of Image-malic and Image-lactic acids were optimized and used to monitor micro-malolactic 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 Image-lactic acid, while Image-malic acid was detected via a new biosensor based on the malic enzyme immobilized in a reactor using phenazine methosulphate as mediator. After a preliminary optimization 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 Image-malic acid and Image-lactic acid, respectively. These optimized 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 Image-malic, Image-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 Image-malic and citric acids, and production of Image-lactic acid was found to be a function of the strains inoculated.