Protein fractions were isolated from coconut: coconut skim milk protein isolate (CSPI) and coconut skim milk protein concentrate (CSPC). The ability of these proteins to form and stabilize oil-in-water emulsions was compared with that of whey protein isolate (WPI). The solubility of the proteins in CSPI, CSPC, and WPI was determined in aqueous solutions containing 0, 100, and 200 mM NaCl from pH 3 to 8. In the absence of salt, the minimum protein solubility occurred between pH 4 and 5 for CSPI and CSPC and around pH 5 for WPI. In the presence of salt (100 and 200 mM NaCl), all proteins had a higher solubility than in distilled water. Corn oil-in-water emulsions (10 wt %) with relatively small droplet diameters (d32 0.46, 1.0, and 0.5 μm for CSPI, CSPC, and WPI, respectively) could be produced using 0.2 wt % protein fraction. Emulsions were prepared with different pH values (3−8), salt concentrations (0−500 mM NaCl), and thermal treatments (30−90 °C for 30 min), and the mean particle diameter, particle size distribution, ζ-potential, and creaming stability were measured. Considerable droplet flocculation occurred in the emulsions near the isoelectric point of the proteins: CSPI, pH 4.0; CSPC, pH 4.5; WPI, pH 4.8. Emulsions with monomodal particle size distributions, small mean droplet diameters, and good creaming stability could be produced at pH 7 for CSPI and WPI, whereas CSPC produced bimodal distributions. The CSPI and WPI emulsions remained relatively stable to droplet aggregation and creaming at NaCl concentrations of ≤50 and ≤100 mM, respectively. In the absence salt, the CSPI and WPI emulsions were also stable to thermal treatments at ≤80 and ≤90 °C for 30 min, respectively. These results suggest that CSPI may be suitable for use as an emulsifier in the food industry.
- Oil-in-water emulsion; coconut protein; coconut skim milk protein; protein isolate; protein concentrate; ultrafiltration; isoelectric precipitation
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