Due to their availability, easy accessibility and tunability, the protein based particles bare ample attention upon the manipulation of oil-water emulsification and droplet dispersion by forming O/W interfacial layer [1], [2]. Meanwhile, the effectiveness of these protein based macromolecular barrier is highly discussed regarding its close relationship with the bulk stability of O/W emulsions [3], [4], [5]. The property of adsorbed layer of proteins with tuned interfacial association and steadiness against applied extrinsic stresses is related originally to their structural flexibility [6], [7]. Experience has been accumulated upon the effects of both aqueous (protein pretreatment, pH, ionic strength) and oil conditions (polarity and viscosity) on the protein conformation and further its interfacial and emulsifying behaviors [8], [9]. These measures do play roles not only in the diffusion/orientation of protein hydrophobic moieties towards O/W interface, but their unfolding and lateral association to get well covered interface with viscoelasticity [1]. Some chemical modifications (glycosylation and succinylation), either with physical auxiliary (ultrasonication and high pressure) or not, are also accessible in transforming proteins into more active forms with enhanced flexibility at interface [5], [10], [11]. Phosphorylation, by selectively binding phosphate groups onto reactive side chains of proteins, is one representative chemical approach mostly used for food polymer modification [12], [13]. Especially, this specific binding mode renders the altered protein intrinsic structures by initially the introduced negatively charged groups (PO43-) [14], [15]. And the resultant emulsion stability is often the combined consequence of both steric and electrostatic stabilization, as can be found in systems of ovalbumin, myofibrillar protein, gelatin and peanut protein isolate [16], [17], [18], [19].

Apart from these well-studied protein forms, the application of phosphorylation for emulsifying fortification can also be seen in some by product proteins. Goose liver protein (GLP), extracted from the by product of its meat counterpart, is such an alternative with potentiated emulsifying activities [20]. Previous study upon GLP showed that the enlarged protein charging due to phosphorylation rendered promoted structural flexibility and surface hydrophobicity, and thus higher emulsifying indexes [21]. Meanwhile, the reduction of droplet approaching and aggregation resulted from increased interdroplet repulsive force was also observed [22]. It is basically established, from these limited studies, that the overall emulsifying elevation of GLP and interdroplet stability of emulsions could be reached by originally grafting more negatively charged groups onto the protein [20]. Despite the direct relation between these protein changes in bulk phase and improved emulsion stability, however, no research is implemented to explore concretely the O/W interfacial behaviors of GLP with different charging characteristics induced primarily by phosphorylation. And how the intradroplet polymeric association, interfacial film viscoelasticity and then the ultimate emulsion character were regulated by varying the GLP charging still remain to be known.

Sodium trimetaphosphate (STMP), sodium tripolyphosphate (STPP) and sodium pyrophosphate (TSPP) are three representative phosphates that commonly used for protein phosphorylation [23]. Particularly, differences including steric conformation, number of phosphate group and chain length among them would yield varied phosphorylation profiles and then extents of protein charging [14]. These characters make them suitable for the detailed exploration of phosphorylation on the interfacial behavior of proteins and then stabilizing of homogenized oil from the protein charging perspective. Nevertheless, there is no study focused on the phosphate type dependent phosphorylation upon the interfacial behavior of by product proteins like GLP. Therefore, the current study aims at 1) clarifying the role of phosphorylation on the interfacial behavior of GLP; 2) analyzing how phosphorylation induced charging impacts the formation and viscoelasticity of interfacial layer for GLP stabilized emulsions and 3) revealing the relationship between fortified functions (emulsifying and emulsion stabilizing) and tuned charging character of phosphorylated GLPs from an interfacial perspective. For these purposes, dynamic interfacial adsorptive and dilatational behaviors of GLP moieties were firstly explored. The ultrastructure of interfacial protein was observed to compare the intradroplet interaction and interface integrity of phosphorylated GLPs. Then, the capacity of phosphorylated GLPs to stabilize O/W emulsions was compared by monitoring emulsion droplet size, flocculation and micromorphology. Microrheology and multiple light scatter analysis were also introduced to show the emulsion stability from the interdroplet interaction perspective. It is demonstrated that phosphorylation renders both enhanced intradroplet association by favoring the interfacial adsorption of GLP and reduced interdroplet aggregation by restraining the droplet interaction electrostatically, therefore enhancing the emulsion stability.