Heat and light stability of three natural blue colorants for use in confectionery and beverages

Abstract

The stabilities of three natural blue colorants—gardenia blue, phycocyanin and indigo—toward heat and light were studied in simple solutions of various pH. Gardenia blue was found to be stable at temperatures up to 80 °C in aqueous solution at pH 3, 5 and 7. Exposure to light of 3×10⁵ lux for 24 hours resulted in approximately 50% degradation of gardenia blue in aqueous solution. Phycocyanin was found to be unstable to heat and light in aqueous solution. Phycocyanin is insoluble in acidic solution (pH 3) and denatures at temperatures above 45 °C at pH 5 and 7, leading to a color change. Exposure to light of 3×10⁵ lux for 24 hours in aqueous solution at pH 5 and 7 caused ~80% degradation. Indigo was found to be stable in medium-chain-triglyceride oil for temperatures up to 90 °C, but to photodegrade (by approximately 70% after five hours of exposure to 3×10⁵ lux), corresponding to a quantum yield of 1.8×10⁻⁴ mol Einstein⁻¹ for visible light and 1.4×10⁻² mol Einstein⁻¹ for UV, as determined for monochromatic light of 600 and 313 nm, respectively. Exploratory studies have been carried out with the three blue colorants in application media such as soft drink, jelly gum, hard candy and sugar coating for soft candy. Despite its lower stability towards heat and light, phycocyanin was concluded to be the more versatile blue food colorant among the three studied, showing a bright blue color in jelly gum and coated soft candy.