Study On The Formation And Stabilization Mechanism Of Single Polysaccharide–stabilized High Internal Phase Emulsions

High internal phase emulsions(HIPEs)feature a high-volume fraction of the internal phase(Φ ≥ 0.74),exhibiting advantageous characteristics of both emulsions and gels,much effort is directed toward the development of new and efficient stabilizers for HIPEs and the exploration of their functions and potential applications.Considering that studies on the preparation of HIPEs using polysaccharides as sole interfacial active substances are still lacking,we herein examined the effects of common polysaccharides on HIPE performance and stability.The thus obtained HIPEs and the corresponding structure-activity relationships and stabilization mechanisms were similar,featuring the excellent interfacial film-forming properties.The mechanism of polysaccharide-induced stabilization significantly differed from those observed for traditional emulsifiers relying on amphiphilic emulsification and Pickering-type stabilizers.The main findings of this paper are as follows.(1)Cationic chitosan hydrochloride(CHC)was solely used as an efficient stabilizer of O/W HIPEs.At a CHC content of 0.08 wt%,one-step shear dispersion at pH 6.3 produced HIPEs with small droplets(～5 μm),high stability,strong gelation,and good plasticity.CHC generates three-dimensional(3D)network membrane framework structures to stabilize the oil droplets.When the content of CHC is 0.1 wt%,the network membrane structure between oil droplets is bridgable and flocculated to maintain the gel network structure of HIPEs.When the content of CHC is higher than 1.0 wt%,the CHC which is not closely bound to the interface further plays a stabilizing role in coordination with the interface membrane by forming the fiber branch chain structure,thus bridging the adjacent or distant oil droplets.The gel stability of HIPEs was further improved.(2)The regulation of pH on emulsification function of CHC was explored.CHC was used to stabilize variable-transparency HIPEs under different conditions.At pH 6.0–6.7produced milky white HIPEs with small droplets(～5 μm),high stability,strong gelation,and good plasticity,whereas highly transparent HIPEs with larger droplets(～100 μm)were formed at pH 6.8–12.7.The stabilization mechanism depended on pH,corresponding to the formation of(i)3D reticular membranes(pH 6.0–6.7)or(ii)fibrous Pickering particles(pH6.8–12.7).This dependence was ascribed to the effects of pH on CHC structure: CHC mainly existed as a nanofiber network with grids at pH 6.0–6.7,whereas fibrous aggregates were observed at higher pH.The aforementioned two kinds of HIPEs with different transparency featured high storage and thermal stability and good structural reversibility.(3)Anionic carboxymethyl chitosan(CMCS)was selected to explore whether charge difference will affect the emulsification function of polysaccharide.HIPEs with optimal properties(ultra-high stability,good gelation and structural reversibility,high plasticity and thermal stability)were obtained using single-step shear dispersion at a CMCS content of 0.3wt% and pH 6.8–7.2.CMCS functioned as a “structuring agent” to form three layers of jammed networks that immobilized the oil droplets enmeshed therein,through molecular rearrangement and characteristics such as flexible molecular entanglement chains and film-forming properties.Specifically,the hierarchical structures from the innermost layer(closest to the droplets)to the outermost layer were interfacial film,branching linear/membrane structures,and outer non-interfacial membranes,respectively.Interfacial membrane structures wrapped on the surface of the oil droplets;The branched structure of CMCS enabled bridging and,hence,the tight interconnection of oil droplets;The outer layers of reticular membranes(without adhering onto the droplets)improved the emulsion’s bulk viscosity and steric hindrance and further pushed the first layer closer to the dispersed droplets to create more compact adsorption,stabilize a large interfacial area,and improve the protective coating.Overall,the synergistic effects of various factors allowed CMCS to effectively stabilize HIPEs.(4)Both CHC and CMCS can stabilize HIPEs through 3D reticular membrane.In order to explore the universality of this stabilization mechanism to polysaccharide substances,we studied the emulsifying properties of polysaccharides or their derivatives commonly used in the food industry.Sodium carboxymethyl cellulose(Na-CMC),sodium alginate(SA),high-methoxyl pectin(HMP),and low-methoxyl pectin(LMP)are shown to effectively stabilize HIPEs at loadings as low as 0.1 wt% under acidic conditions,namely at pH 0.5–5.2(Na-CMC),0.5–4.0(SA),1.0–4.2(HMP),and 1.0–3.0(LMP).Despite their structural differences,the four abovementioned polysaccharides feature similar HIPEs stabilization mechanisms mainly involving the formation of 3D reticular interface membrane structures with fibrous branching chains.Within the pH range suitable for HIPEs formation,the polysaccharides mainly exist as nanofiber network structures with a grid width of 50–100nm in original aqueous environments but adopt flake-like and other structures at higher pH.Although the interfacial activity of polysaccharides is believed to be insufficient for their applications as individual emulsifying stabilizers,this study revealed the regularity that a single polysaccharide can stabilize HIPEs by 3D network membranes with fiber branches,and confirm the closely correlation between the polysaccharide fiber network in the aqueous phase and the reticular membrane structures at the interface.In addition,the inferred emulsification and stabilization mechanisms are shown to be significantly different from those observed for mainstream Pickering HIPE stabilizers.Thus,our work expands the understanding of the interfacial activity function of polysaccharides,provides conceptual thinking reference and design rules for the development of simple,efficient,safe,and cheap polysaccharide-based HIPEs,and supplements and furthers the theory of HIPE stabilization.