Effect of pH and NaCl on the Emulsifying Properties of a Napin Protein Isolate

The physicochemical and emulsifying properties of a napin protein isolate (NPI) were examined as a function of pH (3.0, 5.0 and 7.0) and NaCl content (0, 50 and 100 mM). Specifically, surface charge and hydrophobicity, interfacial tension (IT), solubility, and the emulsifying activity (EAI) and stability (ESI) indices were studied. Surface charge in the absence of NaCl ranged between +10 mV to -5 mV depending on the pH, becoming electrically neutral at pH 6.6. Overall, surface hydrophobicity decreased as the pH increased, whereas it increased as NaCl levels were raised. Solubility was high (93-100 %) regardless of the conditions. NPI's ability to reduce IT was enhanced at higher pHs, however the effect of NaCl was pH dependent with the addition of NaCl enhancing and decreasing NPI's ability to reduce IT at pH 3.0 and 7.0, respectively. Overall, EAI values were similar in magnitude at pH 3.0 and 5.0, and lower at pH 7.0. The effect of NaCl on EAI was similar at pH 3.0 and 7.0, where EAI at the 0 mM and 100 mM NaCl level were similar in magnitude, but increased significantly at the addition of 50 mM NaCl. However, the EAI values at pH 5.0 decreased as the level of NaCl increased. Overall, the stability of NPI-stabilized emulsions degraded rapidly and the addition of salt induced faster emulsion instability.