Improved thermal stability of phycocyanin under acidic conditions by forming soluble complexes with polysaccharides

Abstract

Phycocyanin, a natural blue-colored protein–chromophore complex, forms insoluble aggregates under acidic conditions and loses blue hue during heating. Targeting these issues, phycocyanin was complexed with six polysaccharides (κ-/ι-/λ-carrageenans, xanthan gum, high-methoxyl pectin, and guar gum), respectively; the colloidal and color behaviors of the complexes were examined at pH 3.0 before and after heating (60 °C/80 °C, 30 min). A simple HCl titration strategy was developed to inhibit the associative phase separation commonly occurs at acidic conditions during complexation. Soluble complexes were achieved with ι-carrageenan, xanthan gum, and high-methoxyl pectin, demonstrating good colloidal stability without phase separation at pH 3.0 before and after heating. Fluorescence analyses suggest that the electrostatic complexation between phycocyanin and the mentioned three polysaccharides were reinforced by hydrophobic interactions. The ι-carrageenan–phycocyanin complex exhibits the best colloidal and color stabilities according to laser diffraction particle size analysis and colorimetry. Electrophoresis suggests that the highly negatively charged carrageenans, particularly ι-carrageenan, display high affinity and entrapment efficiency to phycocyanin. Circular dichroism further reveals the ability of carrageenans to stretch phycocyanin's tertiary structures while maintaining native-like secondary structures, which inhibits color changes in phycocyanin's blue hue resulted from the dense interchromophore couplings between the α-helical coiled-coils. Overall, this work dissects polysaccharide–phycocyanin complexation under common beverage processing conditions and provides fundamental insights into the conformational modifications on phycocyanin imparted by polysaccharide–phycocyanin interactions. The improved colloidal and color stabilities would facilitate future application of phycocyanin in foods/beverages as substitutes of artificial pigments.