Colour effects of co-pigmentation of anthocyanins revisited—2.A colorimetric look at the solutions of cyanin co-pigmented byrutin using the CIELAB scale

Abstract

Spectral measurements (380–780 nm) were performed on 224 model solutions of cyanin co-pigmented by the flavonol rutin at pH between 2.5 and 5.5. The colour of the solutions was expressed as colorimetric co-ordinates in the CIELAB scale using the L * (lightness) C * (chroma) and h (hue angle) notation (for the D65/10° CIE Illuminant/Observer condition). Contrary to previous reports, all the most intensely co-pigmented solutions, although exhibiting the strongest bathochromic of their λ max, did not displayed the bluest hues; an inverse, i.e. yellowing, effect was observed in the co-pigmented solutions at the lowest cyanin concentrations. The hyperchromic effect of co-pigmentation always caused a decreasing lightness of the solutions coupled with either an increasing or a decreasing chroma for those at low or high pigment concentrations respectively. Based on the amplitude of total colour CIELAB differences ( ΔE*) at each co-pigment to pigment ratio, the most intense effects of co-pigmentation were observed at pH 3.5, except for the solutions at the highest pigment content (pH 4.5); the evolution of colour differences at increasing co-pigment to pigment ratios at pH between 2.5 and 5.5 are discussed. Colour differences calculations also showed that the maximum colour steps at successive increasing co-pigment to pigment ratios were observed at low values (0.5 or 1:1) in the most coloured solutions and at the highest values (up to 8–16:1) in the less coloured ones. In describing the colour effect of co-pigmentation on anthocyanins, the discrepancies found between the results of the present colorimetric approach and those previously reported based on spectral data at the λ max only are discussed. Only the first one—by considering the spectral variations over the complete range of visible wavelengths and using the psychometric correlates of the three attributes of colour specified in the CIELAB system—correctly describes the colour variations as these are perceived by the human eye.