Colour stability of anthocyanins in aqueous solutions at various pH values

This study focuses on the impact of anthocyanin structures such as 5-glucosidic substitution and aromatic acylation on anthocyanin, colour and stability at various pH values. Two concentrations (0.05 and 0.15 mM) of cyanidin 3-glucoside (1), cyanidin 3-(2″-glucosylglucoside)-5-glucoside (2), and cyanidin 3-(2″-(2^′′′-sinapoylglucosyl)-6″-sinapoylglucoside)-5-glucoside (3) in aqueous solutions were studied at fourteen pH levels between 1.1 and 10.5 during a 98 day period at 10 °C. The three pigments represent the structural variation of many anthocyanins isolated from fruits and vegetables. Colours were expressed by the CIELAB coordinates h_ab, (hue angle), C^* (chroma=saturation), and L^* (lightness), as well as by visible absorption maxima (λ_max) and molar absorption coefficients (ε). Limitations of using only spectral parameters, such as λ_max and ε, to express colour variations, were revealed. Pigment 2 was more unstable than 1 at most pH values, showing lower C^* and higher L^* values after storage for only hours in neutral and weakly acid solutions. Pigment 3 showed higher colour stability than the nonacylated forms at all pH values but pH 1.1, where all pigments retained their colours in the storage period, and in the most alkaline solutions, where all pigments experienced dramatic colour changes. Of potential importance for some food products, in solutions with pH 4.1 and 5.1, 3 maintained nearly the same h_ab, C^*, and L^* values during the whole measuring period (98 days), in contrast to pigments 1 and 2. The hue angle shift towards bluish tones in freshly made samples of anthocyanins with 5-glucosidic substitution, were amplified with aromatic acylation (3) throughout the entire pH range except pH 10.5. The variations in the results emphasizes the importance of structure for anthocyanin properties in fresh and processed fruits and vegetables.