Single and double Pickering emulsions stabilized by sodium caseinate: Effect of crosslinking density

Abstract

Pickering emulsions are attracting increasing attention with their superb stability and low toxicity. Over the years, the emulsifiers for Pickering emulsions have evolved from synthetic and inorganic particles to edible ones like protein, polysaccharides, fat crystals, etc. In this study, we utilized sodium caseinate (SC) as Pickering emulsifiers. Upon crosslinking with genipin which reacts with the primary amino groups of casein via the nucleophilic attack and nucleophilic substitution, the original flexible sub-micelle structure reassembled to form soft protein particles. Different amount of crosslinkers gave SC particles ranging from polymer-like to particle-like ones. It was found that crosslinking of SC undermined their ability to decrease the interfacial tension of the oil-water interface, indicating reduced interfacial activity. Also, SC particles with different crosslinking density could all stabilize high internal phase emulsions (HIPEs) with high storage stability, indicating little effect of crosslinking on the stability of oil-in-water (O/W) emulsions. When it comes to double emulsions, the effect of crosslinking density was revealed. A higher emulsion stability was achieved using SC with a higher crosslinking density. However, during simulated digestion, crosslinking of SC turned out to have an adverse effect on the emulsion stability. The low pH and the high salt concentration caused severe aggregation of highly crosslinked SC, undermining the stability of the formed double emulsions. On the other hand, loosely crosslinked SC with its preserved steric repulsion showed better protection for the double emulsions. Given the potential of Pickering emulsions in the field of foods and pharmaceuticals, this work emphasized that simply replacing conventional emulsifiers with Pickering emulsifiers would not eliminate the stability problem for practical applications. Further studies on the combination or conjugation of particle-like emulsifier with polymer-like ones might be necessary.