Surface modification of zein colloidal particles with sodium caseinate to stabilize oil-in-water pickering emulsion

Zein colloidal nanoparticles can adsorb at the oil-water interface to form Pickering emulsion. However, zein Pickering emulsion is usually not stable due to the poor wettability of zein colloidal nanoparticles. The objective of this study was to modify the surface of zein nanoparticles using sodium caseinate (NaCas) and assess the properties of zein/NaCas nanocomplexes and the resultant oil-in-water Pickering emulsions. One percent (w/w) of zein/NaCas colloidal nanocomplexes were formed, with the zein:NaCas ratios (w/w) ranging from 10:1 to 10:4 at pH = 3 by an ultrasound treatment. The zeta-potential of the zein/NaCas nanocomplexes showed altered surface charges, indicating that NaCas adsorbed on the surface of the zein colloidal nanoparticles. Three-phase contact angle measurements suggested that the original zein colloidal nanoparticles were preferentially wetted in water. The incorporation of 0.1%-0.2% (w/w) NaCas significantly enhanced its wettability in the oil, and intermediate wettability was achieved at a zein:NaCas ratio of 10:3. Confocal laser scanning microscope (CLSM) images showed that the incorporation of NaCas improved the interfacial coverage of the Pickering emulsions. When the zein:NaCas ratio ranged from 10:1 to 10:3, the interface was composed of zein/NaCas nanocomplexes. At a zein:NaCas ratio of 10:4, NaCas can competitively adsorbed to the interface and formed a hybrid interfacial structure. The Zein/NaCas nanocomplexes stabilized the Pickering emulsions and exhibited greater centrifugal stability than plain zein emulsions at most pHs and ionic strengths. The underlying mechanisms of the improved emulsion stability are discussed in this paper. This study explored a novel approach to stabilizing Pickering emulsions via the surface modification method using a food-grade protein.