(609a) Gum Arabic and Ferrous Ions Synergistically Improve the Heat and Acid Stability of Norbixin

Norbixin (annatto) is a commonly used yellow food colorant. However, norbixin precipitates at pH below about 4.0, and thermal processing as used in pasteurization and sterilization degrades norbixin, resulting in the loss of color. In the present study, gum arabic and ferrous ions were studied for the possibility to improve the thermal and acid stability of norbixin in aqueous dispersions. Norbixin was dissolved at 0.04% w/v in deionized water with and without 2% w/v gum arabic and/or 1.5 mM ferrous chloride, adjusted to pH 3.0-7.0, and heated at 90 or 126 °C for 30 min. Samples before and after heating were characterized for color, absorbance at 460 nm, particle size, fluorescence spectroscopy, zeta potential, and morphology. Before heating, norbixin precipitated at pH 3.0 and 4.0, which was prevented by gum arabic, while ferrous ions prevented norbixin precipitation at pH 4.0 only. Thermal stability of norbixin was improved at pH 3.0-6.0 by gum arabic, further improved by additional ferrous ions. Dynamic light scattering and atomic force microscopy results suggested that ferrous ions and gum arabic synergistically prevented the aggregation of norbixin at acidic pH and during thermal treatment. Fluorescence analyses indicated the complex formation between norbixin and gum arabic with and without ferrous ions. The addition of ferrous ions lowered the magnitude of (negative) zeta potential of gum arabic, which was opposite to the data after adding ferrous ions to the gum arabic-norbixin mixtures. It was hypothesized that the core of negatively charged gum arabic-norbixin complexes was strengthened due to ionic bridging by ferrous ions, which stabilized the complexes during thermal treatments and prevented aggregation by electrostatic and possibly steric repulsions. Findings from this study indicate that ferrous ions and gum arabic can be used together to improve the heat and acid stability of norbixin for use as a natural food colorant.