Investigation on complex coacervation between fish skin gelatin from cold-water fish and gum arabic: Phase behavior, thermodynamic, and structural properties

The study is aimed to investigate phase behavior, thermodynamic, and structural properties based on complex coacervation between fish skin gelatin (FSG) from cold-water fish and gum arabic (GA). Phase separation behavior between FSG and GA was investigated as a function of pH through varying mixing ratios from 4:1 to 1:4 at 25 °C and 1.0 wt% of total biopolymer concentration. The turbidity of FSG-GA mixture reached the maximum (1.743) at the 1:2 of mixing ratio and pHopt 3.5, and stabilized at zero. Then physicochemical properties of FSG-GA coacervates at pHopt 3.5 and FSG-GA mixtures at pH 6.0 (>pHc) were evaluated. Scanning electron microscope (SEM) and X-ray diffraction (XRD) showed that the interactions between FSG and GA occurred at pHopt 3.5 and were very weak at pH 6.0 (>pHc). The isothermal titration calorimetry (ITC) results including the negative Gibbs free energy change (ΔG = −18.71 ± 1.300 kJ/mol), binding enthalpy (ΔH = −41.81 ± 1.300 kJ/mol) and binding entropy (TΔS = −23.10 kJ/mol) indicated that the complexation between FSG and GA was spontaneous and driven by negative enthalpy owing to the electrostatic interaction and hydrogen bondings. The zeta potential (ZP) of FSG-GA coacervates at pHopt 3.5 was −9.00 ± 0.79 mV that was not close to electrically neutral, indicating other interactions besides electronic interaction. Hydrogen bondings in FSG-GA mixtures at pH 6.0 and 3.5 were found to be stronger than pure FSG at pH 6.0 and 3.5 owing to that the amide II peaks shifted to high wavenumbers. Electronic interaction was proven to exist in FSG-GA mixtures at pH 6.0 through the vanishment of asymmetric COO− stretching. However, the electronic interaction in FSG-GA coacervates at pHopt 3.5 was obviously stronger than FSG-GA mixtures at pH 6.0, resulting from the vanishment of asymmetric and symmetric COO− stretching vibration and the positively charged FSG and GA. The intrinsic fluorescence represented that the introduction of GA changed the microenvironment of tyrosine residues in FSG, which may be owing to the unfolding of the tertiary conformation. Moreover, the decrease of pH could promote the formation of random coils of FSG through circular dichroism (CD). Therefore the addition of GA into FSG and decrease of pH might enhance the conformation freedom of FSG, which would bring about favorable entropic effects and contribute to the complexation.