Light sensitivity of cochineal. Quantum yields for photodegradation of carminic acid and conjugate bases in aqueous solution

Abstract

The quantum yield for photobleaching of carminic acid, the major constituent of cochineal, has been determined at 25·0°C for air-saturated aqueous 1·0 m NaCl solution at different pH, using monochromatic light at each of the three wavelengths 254 nm, 366 nm, and 436 nm, respectively, in continuous photolysis experiments, in order to provide an objective measure of the sensitivity of this important food colourant to ultraviolet and visible light. For each wavelength of irradiation, the photobleaching increases with increasing pH, the effect being most significant for 254 nm irradiation. Reaction quantum yields for the fully protonated form of carminic acid and each of three deprotonated forms were calculated from the pH-dependence of the photobleaching in combination with potentiometrically determined acidity constants (LH 3: p K a,1 = 2·81 ± 0·09; LH − 2: p K 2 a,2 = 5·43 ± 0·04; LH 2−: p K a,3 = 8·10 ± 0·03, the constants being based on concentrations in 1·0 m NaCl), and have the value Φ LH 3 = 2 × 10 −5 mol einstein −1, Φ LH − 2 = 2 × 10 −5, Φ LH 2− = 6 × 10 −5, and Φ 3− = 4 × 10 −4 for 436 nm irradiation. For 254 nm irradiation, the values range from Φ LH 3 = 5 × 10 −5 mol einstein −1 to Φ L 3− = 0·01 mol einstein −1, with the values for 366 nm irradiation being intermediates. The increased photolability of the deprotonated forms and an apparent linear free energy relationship between lowest energy transition and photoreactivity resulting from 436 nm excitation are discussed in relation to the nature of the lowest electronically excited state.