Screen-printed enzyme electrodes for the detection of marker analytes during winemaking Prediction of phenolic compounds in red wine fermentations by visible and near infrared spectroscopy

Biosensors for malic acid and glucose have been developed, using screen-printed electrodes and two different classes of enzymes: NAD(P)+-dependent dehydrogenases and oxidases. The active surface of the electrodes was modified using Meldola Blue (malic acid) and Prussian Blue (glucose) and in this way sensitive, low cost and reliable NAD(P)H and H2O2 probes were obtained. Fixed potential amperometry was used for the detection of substrates in small volumes of sample (50 (mu)l). Immobilization of the enzymes in a polyethylenimine–glutaraldehyde cross-linking membrane allowed sensors to be obtained with sufficient operational stability. The detection limits were of 10^-5 M for malic acid and 10^-6 M for glucose. The sensors were applied in the analysis of different samples of wine. Near infrared (NIR) spectroscopy was used to simultaneously predict the concentrations of malvidin-3-glucoside (M3G), pigmented polymers (PP) and tannins (T) in red wine. A total of 495 samples from 32 commercial scale red wine fermentations over two vintages using two grape varieties (Cabernet Sauvignon and Shiraz), and also including as additional variables two types of fermenters, two different yeasts, and three fermentation temperatures were used. Samples were scanned in transmission mode (400–2500 nm) using a monochromator instrument (NIRSystems6500). Calibration equations were developed from high performance liquid chromatography (HPLC) and NIR data using partial least squares (PLS) regression with internal cross validation. Using PLS regression, very good calibration statistics (R^2cal>0.80) were obtained for the prediction of M3G, PP and T with standard deviation (S.D.)/standard error in cross validation (SECV) ratio (residual predictive deviation, RPD)) ranging from 1.8 to 5.8. It was concluded that near infrared spectroscopy could be used as rapid alternative method for the prediction of the concentration of phenolic compounds in red wine fermentations.