In-line sensing of grape juice press fractioning with UV-Vis spectroscopy

Abstract

UV-Visible spectroscopy in conjunction with chemometrics, was successfully applied to objectively differentiate sparkling wine press juice fractions of Pinot noir. Two measurements methods were applied: reflectance using a fibre optic probe in-line and transmission using a benchtop spectrophotometer. Different wavelength ranges for UV-Visible spectroscopy were evaluated and their ability to measure total phenolic concentrations in press juice fractions was compared. The differentiation of free run, early and late press fractions shows promise as a tool for the rapid discrimination of fractions when grapes for sparkling wine are pressed. Calibrations for total phenolics were prepared from press fraction spectral data using partial least square regression (PLSR) with a large number of wavelengths (230-700 nm) and multiple linear regression (MLR) using a small number of key wavelengths. Calibration performance for both reflectance and transmission spectra was similar, but the best performing calibration used reflectance spectra. Reflectance spectroscopy can thus be used in-line to predict total phenolics in grape juice with an acceptable accuracy and to discriminate press fractions. Insights from this research will lead to the design and building of a fitting that can be attached to any press outlet, with the potential to automate press fractioning.