Light and heat sensitivity of red cabbage extract in soft drink model systems

Anthocyanins of red cabbage (Brassica oleracea L.) display colour over a larger range of pH-values than the majority of anthocyanins from other natural sources, being pink at pH 3, violet at pH 5 and blue at pH 7. Using reversed phase high-performance liquid chromatography with diode array detection, cyanidin was found to be the only anthocyanidin of red cabbage extract and, tentatively, identified as cyanidin-3,5-diglucoside and cyanidin-3-sophoroside-5-glucoside, the latter found in various acylated forms. The apparent quantum yield for photobleaching has been determined for red cabbage extract at 25°C in air-saturated McIlvaine buffer, using monochromatic light at each of the irradiation wavelengths, 313, 366, and 436 nm, in continuous photolysis experiments, in order to provide an objective measure for the sensitivity of this food colorant to ultraviolet and visible light. The quantum yield was found to depend on both pH and irradiation wavelength, ranging from 0.2×10−4 mol einstein−1 for 436 nm light at pH 7.0 to 3.7×10−4 mol einstein−1 for 366 nm light at pH 5.0. The thermal stability at pH 3.0 in McIlvaine buffer of four different anthocyanin extracts was compared and, for the temperature range investigated (25–80°C) the degree of stability was red cabbage>blackcurrant>grape skin>elderberry. The thermal stability of the same anthocyanin extracts was also compared for a non-carbonated soft drink medium of pH 3.0 yielding the same order of stability but with rates of degradation approximately twice as high as in buffer, which may indicate a detrimental effect of sugar and ascorbic acid. Due to the high thermal stability of red cabbage extract in solution, photobleaching will be the primary destabilising factor for red cabbage anthocyanin-coloured products in display.