Evaluation of Grape Quality Parameters by a Simple Vis/Nir System
Visible/near-infrared (Vis/NIR) spectroscopy is a rapid and nondestructive technique requiring minimal sample processing before analysis, and coupled with chemometrics methods it appears to be one of the most convenient and straightforward analytical tools for studying fruit quality and ripeness. Chemometrics is applied to solve both descriptive and predictive problems in the chemical, pharmaceutical, and food sectors. With this aim, an optical, portable, experimental system (Vis/NIR spectrophotometer) for nondestructive and quick prediction of ripening parameters of fresh berries and homogenized samples of grapes in the wavelength range 450-980 nm was built and tested. A total of 156 grape samples, representing vintage years 2005 and 2006 and harvested in the Valtellina viticultural area of Italy, were evaluated by Vis/NIR spectroscopy for ripeness parameters (soluble solids content, titratable acidity, and pH value) and for phenol ripening parameters (anthocyanins and polyphenols content). Accurate and good calibrations to predict ripeness parameters were obtained for both the 2005 and 2006 vintage years. Calibrations for technological ripening and for anthocyanins had good correlation coefficients (r(CV)>0.90). These models were extensively validated using independent sample sets. Good statistical parameters were obtained for soluble solids content (r > 0.8, SEP < 1.24 degrees Brix) and for titratable acidity (r > 0.8, SEP < 2.00 g tartaric acid L-1), showing the validity of the Vis/NIR spectrometer. Similarly, anthocyanins could be predicted accurately compared with the reference determination. Finally, for qualitative analysis, spectral data on grapes were divided into two groups on the basis of grapes' soluble content and acidity in order to apply a classification analysis (PLS-DA). Good results were obtained with the Vis/NIR device, with 89% of samples correctly classified for soluble content and 83% of samples correctly classified for acidity. Results indicate that the Vis/NIR portable device could be an interesting and rapid tool for assessing grape ripeness directly in the field or upon receiving grapes in the wine industry.