Gum Arabic and Fe²⁺ Synergistically Improve the Heat and Acid Stability of Norbixin at pH 3.0–5.0

Thermal and acid stabilities of norbixin are challenges for its application as a food colorant. In this work, gum arabic and Fe²⁺ were studied for the possibility to improve the thermal and acid stabilities of norbixin. Norbixin was dissolved at 0.004% w/v in deionized water with and without 0.2% w/v gum arabic and/or 0.15 mM ferrous chloride, adjusted to pH 3.0–5.0, and heated at 90 or 126 °C for 30 min. Before heating, norbixin precipitated at pH 3.0–4.0, which was prevented by gum arabic. The thermal stability of norbixin was improved by the combination of gum arabic and Fe²⁺. Fluorescence analyses indicated the complex formation between norbixin and gum arabic with and without Fe²⁺. Particle size and atomic force microscopy results suggested Fe²⁺ and gum arabic synergistically prevented the aggregation of norbixin at acidic pH and during heating. It was hypothesized that the core of gum arabic–norbixin complexes was strengthened by Fe²⁺ to enable the synergy