A colorimetric method was used to analyse the influence of procyanidin structure on colour changes of malvidin 3-O-glucoside (oenin) solution resulting from copigmentation. The study was performed in hydroalcoholic citrate/phosphate buffer solution (120 g L −1 ) at pH 3.6 and ionic strength 0.2 mol L −1 . Chromatic L * , a * and b * coordinates (CIELAB, D65/10 • illuminant/observer condition) obtained from spectral curves recorded between 360 and 830 nm allowed the calculation of lightness L * , chroma C * and hue angle h ab . In general, addition of copigment induced colour enhancement (loss of lightness and increased chroma). The prevailing parameters affecting colour changes were lightness and chroma for monomers and lightness and hue for procyanidins B5 and B8 (C4-C6 dimers). A small blueing effect was observed only for catechin monomer-copigmented solutions. For procyanidin copigments, as the structural complexity of the copigment increased, the hue angle moved to yellower values. The ester gallate of dimer B2 produced the strongest modification of colour attributes of oenin solution.
INTRODUCTIONChanges in anthocyanin and tannin composition are involved in the evolution of red wine colour. Purple tints of young red wines decline to a tilelike orange colour in mature wines. These colour changes result from the formation of more stable pigments during aging. Over the last few years, several groups of anthocyanin-derived pigments with different colours have been identified in red wines, namely vinyl-phenol derivatives of anthocyanins 1,2 and anthocyanin-pyruvic acid adducts, 3 -5 which have a brick-red colour, as well as compounds resulting from acetaldehyde-mediated condensation between anthocyanins and flavanols, e.g. ethyl bridge adducts with a purple colour, 6 -10 orange pyranoanthocyanin-flavanol pigments 11 and vinylpyranoanthocyanin-flavanols (portisins) with a blue colour. 12 -17 Colour expression of anthocyanin pigments also relies on copigmentation. Effectively non-covalent interactions of anthocyanins and wine constituents make an important contribution to red wine colour. 18 -21 Copigmentation of anthocyanins is responsible for variations in absorbance (hyperchromic shift) and increases the maximum absorption wavelength in the visible region (bathochromic shift). However, differences in the absorption spectrum are observed not only at λ max of the flavylium ion but also over the full range of visible wavelengths. The accurate description of colour should refer to lightness (L * ),