Portable NIR-AOTF spectroscopy combined with winery FTIR spectroscopy for an easy, rapid, in-field monitoring of Sangiovese grape quality

BACKGROUND A near infrared acousto-optically tunable filter (NIR-AOTF) spectrophotometer was tested for three seasons on four different vineyards with the aim of monitoring the ripening evolution of the Italian red wine grape variety Sangiovese. Predictive models for the estimation of several enological parameters were carried out applying the partial least squares chemometric approach. Reference analysis was conducted using Fourier transform infrared spectroscopy (FTIR). Spectral detections were obtained working on whole grape berries. A global set of 96 samples (n = 76 in 2009, and n = 20 in 2010) each one represented by 100 intact grape berries was tested. Finally, in 2011, an external validation on an independent data set of 25 samples (50 grape berries per set) was carried out. RESULTS Coupling the two spectroscopic applications, the following enological parameters were tested: degrees Brix, degrees Babo, total sugars (g L-1), glucose (g L-1), fructose (g L-1), density (g mL(-1)), titratable acidity (g L-1), tartaric acid (g L-1), pH, malic acid (g L-1), gluconic acid (g L-1), assumable nitrogen (mg L-1), anthocyanins (mg L-1), and total phenols (mg L-1). NIR-AOTF spectroscopy was able to predict with a high correlation versus the measured data: degrees Brix, degrees Babo, total sugars, glucose, fructose and density. The coefficient of determination (R-2) and the standard error in prediction were: 0.93 and 0.73 for degrees Brix; 0.93 and 0.62 for degrees Babo; 0.94 and 7.39 g L-1 for total sugars; 0.93 and 5.39 g L-1 for glucose; 0.92 and 5.07 g L-1 for fructose; and 0.91 and 0.004 g mL(-1) for density, respectively. Significant correlations were found in prediction for tartaric acid and pH value. Promising validation results were recorded for anthocyanins and total phenols, even though predictive models were affected by the method of sample preparation in compound extraction. CONCLUSIONS This study shows how NIR-AOTF spectroscopy can be used in viticulture to investigate, directly in-field, the most significant markers of ripening evolution and of Sangiovese grape quality definition. The use of the vinery FTIR spectroscopy provided the reduction of the time required by reference analysis, with the big advantage of obtaining many different parameters in just a few seconds and allowing easier calibration of the NIR spectrometer. (c) 2013 Society of Chemical Industry