COLOUR EVALUATION ON RED WINEGRAPES BY CIELAB PARAMETERS

Abstract

Grape must and wine colour is traditionally assessed by several indexes calculated after measuring absorbance at 420, 520 and 620 nm. In the last years tristimulus colorimetry (mainly CIELab colour space) is increasingly used in research and wineries, since it seems to be a more accurate method to evaluate wine colour, especially at hue junctions when visual colour is quite subjective. The aim of our work is to compare CIELab parameters (L*, c*, h) to classical colour indexes in grape must. Total (TAnt) and extractable (EAnt) anthocyanin content was measured in more than 1600 samples of 4 cultivars ('Cabernet Sauvignon' - CS, 'Grenache' - GA, 'Graciano' - GR, and 'Tempranillo' - TE) at 3 stages of ripening by Glories method. In the same macerated extracts, tristimulus colour measurements were made with a Minolta reflectance spectrophotometer, using 2 and 10 mm path length quartz glass cells. Luminosity has been the parameter that has allowed the best estimation of TAnt and EAnt. However, the correlation coefficient found for EAnt was relatively low (R2=0.75) due to differences between cultivars; GA and GR showing lower luminosity for a given EAnt value than CS and TE. These differences are probably a result of differences in the anthocyanic profile of these cultivars, which may affect directly (differences in absorbance pattern) or indirectly (co-pigmentation and self association process) the colour when the whole spectrum is taken into account. Colorimetic measurements have been shown to allow estimating the classic colour parameters (provided varietal differences are considered) and to detect more subtle differences between samples.