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

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 c olour of the solutions was expressed as colorimetric co-ordinates in the CI ELAB scale using the L* (lightness) C* (chroma) and h (hue angle) notation (for the D65/10 degrees CIE Illuminant/Observer condition). Contrary to pre vious reports, all the most intensely co-pigmented solutions, although exhi biting the strongest bathochromic of their lambda(max) did not displayed th e bluest hues; an inverse, i.e. yellowing, effect was observed in the co-pi gmented solutions at the lowest cyanin concentrations. The hyperchromic eff ect of co-pigmentation always caused a decreasing lightness of the solution s coupled with either an increasing or a decreasing chroma for those at low or high pigment concentrations respectively. Based on the amplitude of tot al colour CIELAB differences (Delta E*) at each co-pigment to pigment ratio , the most intense effects of co-pigmentation were observed at pH 3.5, exce pt 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 bet ween 2.5 and 5.5 are discussed. Colour differences calculations also showed that the maximum colour steps at successive increasing co-pigment to pigme nt ratios were observed at low values (0.5 or 1:1) in the most coloured sol utions 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 dis crepancies found between the results of the present colorimetric approach a nd those previously reported based on spectral data at the lambda(max) only are discussed. Only the first one-by considering the spectral variations o ver the complete range of visible wavelengths and using the psychometric co rrelates of the three attributes of colour specified in the CIELAB system-c orrectly describes the colour variations as these are perceived by the huma n eye. (C) 1999 Elsevier Science Ltd. All rights reserved.