Indirect evaluation of microbial spoiling activity in grape marcs by near infrared spectroscopy (NIRS)

Ethanol, reducing sugars and acetic acid content, related to yeasts and bacteria spoiling activities, were chosen as indicators of technological suitability for marc devoted to the production of grappa. A fiber-optic probe was directly put in contact with marc using the visible near infrared (Vis-NIRS) spectral range from 350 to 1800 nm. Analytical determination of sugar, ethanol and acetic acid concentration was done following 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 while a medium predictive ability of NIRS was obtained for acetic acid. Results show that NIRS can be used for rapid quality assessment of marcs at their arrival at distillery, in view of their potential technological transformation.