Study of grape-ripening process variability using mid infrared spectroscopy

Abstract

To obtain a quality wine, it is necessary to collect grapes in an optimal state of maturation, so the control of the ripening process is fundamental for the viticulturist. During this process, the grapes suffer different physiological and chemical changes that include berry softening, sugar accumulation and metabolism of different chemical compounds such as organic acids, polyphenols or aromatic compounds. As these changes occur within each berry, the same bunch may contain berries at different stages of maturity, making it difficult to determine a single optimal state. In addition, when the position of the bunch on the vine and the position of the vine within the vineyard are also considered, the difficulty to correctly determine the optimum ripening point becomes even greater. To solve this problem, a representative sampling of the vineyard is usually made and the average values of sugar contents, acidity (pH or titratable acidity) and phenolic compounds (mainly in red varieties) are determined towards the designation of harvest time.

The classical analytical methods used to determine these parameters are destructive, time consuming and cannot be applied on-site. Recent developments in equipment, such as infrared spectroscopy, hyperspectral imaging or specific sensors (i.e. DA-meter) allow obtaining real-time information about the maturity of the grapes. In this work, a strategy
consisting on coupling FTIR-ATR spectroscopy and chemometric tools is proposed for an effective ripening control, which implies knowing the real state of maturation of the berries and not a single average value. This information will make it possible to carry out the suitable viticultural practices to improve the quality of the grapes.

ANOVA-simultaneous component analysis (ASCA) was applied to factorize the ripening variability sources, such as the bunch-height in the plant or the grape-position in the bunch. The variability sources affecting the MIR spectra and the sugar content and pH were studied, showing an evolution over time and depending on the position of the berries. Moreover, prediction of sugar content and pH was achieved by measuring the grapes in the vineyard, showing the capability of the FTIR portable device to monitor the ripening process.