Indirect Evaluation of Microbial Spoiling Activity in Grape Marcs by Near-Infrared Spectroscopy

Abstract

Ethanol, reducing sugars, and acetic acid content, related to yeast and bacteria spoiling activities, were chosen as indicators of technological suitability for grape marc devoted to the production of grappa. A fiber-optic probe was placed in contact with marc using visible near-infrared spectroscopy (NIRS) spectral range from 350 to 1800 nm. Analytical determination of sugar, ethanol, and acetic acid concentration followed standard laboratory methods and partial least squares regressions were performed using untreated and pretreated spectra data. Models were validated using full cross-validation, and their predictive ability was determined by coefficient of determination of cross-calibration and standard error of cross-validation. The best prediction models were developed using absorbance spectra for ethanol and for sugar content, and a medium predictive ability of NIRS was obtained for acetic acid. Results show that NIRS can be used for rapid quality assessment of grape marcs upon arrival at the distillery, in view of their potential technological transformation.