Influence Of Particle Size Of Emulsions And Salt Ions On The Freeze-thaw Stability Of Emulsions Stabilized By Whey Protein Isolate

Emulsion system is ubiquitous in foods.Some emulsion-based foods will undergo the freezing in the process of production and storage,while undesired sensory quality such as creaming or phase separation may occur after thawing.Therefore,finding ways to improve the freeze-thaw（FT）stability of emulsions and study its stability mechanism are of great value to the development and application of emulsion foods.In this paper,firstly,the effect of the particle size of emulsions coupled with ionic strength on the FT stability of emulsions stabilized by whey protein isolate（WPI）was investigated.Secondly,the effect of Hofmeister series anions with different ionic strengths on the FT stability of emulsions was studied.Finally,the effect of Hofmeister series anions on the interfacial rheological properties of emulsions,and the changes of the content of protein fractions in the emulsion interface before and after FT treatment was studied.The information can provide reference and data support for the study of the FT stability of protein-stabilized emulsions.The main results are as follows:1.Under the condition of protein concentration c=4%（w/v）and oil phase volumeφ=0.4,the emulsions with different particle size（8.92μm,1.28μm,0.83μm and 0.65μm）were prepared by different homogenization process,and the emulsion properties before and after FT treatment were characterized.The results showed that the FT stability of emulsions increased with the decrease in particle size of initial emulsions.When the particle size of initial emulsion was 0.65μm,the emulsion had the largest flocculation index（FI）,the smallest coalescence index（CD）and the largest adsorbed protein（AP）.After three cycles of FT treatment,the FI increased significantly（p<0.05）,the decrease of AP was the smallest,and the FT stability of emulsions was the best.Reducing the particle size of initial emulsions can improve the FT stability of emulsions by inhibiting the coalescence between emulsions during the FT process.Different ionic strengths（0-70 m M）Na Cl was added to WPI and the emulsions were prepared before emulsification.With the increase of ionic strength,the particle size of emulsions increased,zeta potential decreased,FI increased,CD decreased,apparent viscosity and viscoelastic modulus increased,and interfacial pressure（π）increased.The CD of emulsions with 50 m M was the smallest and fewer aggregates occured in the micrographs.After three cycles of FT treatment,the increase in the particle size of emulsions was the smallest,and the FT stability was the best.The salt addition changed the WPI properties,the particle size of WPI decreased,the zeta potential decreased,the surface hydrophilicity increased,and the protein solubility increased with30-50 m M,which was beneficial to promote the compact accumulation and arrangement of protein molecules at the interface,increase the adsorption concentration of interfacial proteins,and improve the interfacial stability.Simultaneously,the salt addition promoted the flocculation between emulsions and effectively inhibited the movement of emulsions during the FT process.Notably,when the particle size of initial emulsions was reduced,the30-50 m M Na Cl can maintain the good FT stability of emulsions.2.The emulsions were prepared by the same homogeneous conditions and the effect of Hofmeister series anions(Kosmotrope of SO₄^2-,Cl^-and Br^-,Chaotrope of Cl O₄^-and SCN^-)with different ionic strengths（0-200 m M）on the FT stability of emulsions stabilized by WPI were investigated.The results showed that the emulsions added Kosmotrope（Cl^-and Br^-）remained stable while all other emulsions demulsified after three cycles of FT treatments.With the salt addition,the particle size of WPI decreased,the zeta potential decreased,the surface hydrophobicity and proteins solubility increased.After the emulsions formed,the particle size and FI of emulsions increased,the zeta potential decreased,AP and protein capability（τ）increased,and the apparent viscosity and viscoelastic modulus increased.Compared with Chaotrope,the particle size,FI and apparent viscosity of initial emulsions added Kosmotrope were relatively larger,while the zeta potential and viscoelastic modulus were relatively smaller.The AP andτof the emulsions added Kosmotrope（Cl^-and Br^-）were relatively larger.With the increase of ionic strength,the particle size,FI,AP andτof initial emulsions increased firstly and then decreased,the zeta potential decreased gradually,while the apparent viscosity and viscoelastic modulus changed differently.The salt addition increased the flocculation degree of emulsions,and the increase in the apparent viscosity of emulsions was beneficial to promote the formation of gel network structure in the emulsion system.The increase of the surface hydrophobicity and protein solubility was beneficial to the rapid adsorption of protein molecules to the oil-water interface,which was conducive to improving the adsorption concentration of interfacial proteins.The decrease of particle size and zeta potential of WPI was conducive to the compact accumulation and arrangement of protein molecules at the interface,thus improving the interfacial stability.3.The interfacial rheological properties of emulsions added Hofmeister series anions and the content of interfacial protein fractions before and after FT treatment were characterized.Compared with Chaotrope,the interfacial pressure（π）of emulsions added Kosmotrope and the diffusion rate(K_diff)were relatively larger,while the rearrangement rate（K_R）was relatively smaller,but both were higher than the control.For the sample added Na Cl,π,K_diff and K_R increased with the increase of ionic strength,while the sample added Na SCN was on the contrary.The elastic component（E_d）of emulsions added Kosmotrope was less than Chaotrope,indicated that the interfacial strength of initial emulsions was lower than other emulsions.The changes in the content of interfacial protein fractions were studied by quantitative proteomics technique.The results showed that salt addition made the transferrin（TRF）and immunoglobulin（Ig）,which have large molecular weight and poor emulsification ability,performed as the interfacial dominant proteins,and the interfacial content of TRF and Ig on the emulsion added Na Cl was higher than other emulsions.The loose interfacial arrangement and accumulation of proteins will lead to form the larger steric hindrance at the interface,and the thickness of the interfacial layer was larger while the strength was weaker.After three cycles of FT treatment,the proteins with low molecular weight and high emulsification ability,includingβ-lactoglobulin（β-Lg）,α-lactalbumin（α-La）,bovine serum albumin（BSA）and casein（CN）,on the emulsion added Na Cl replaced other proteins as dominant proteins for the extrusion of ice crystals,while the TRF and Ig on the emulsion added Na SCN were still interfacial dominant proteins.There were obvious differences in the content of interfacial protein fractions for the emulsions added Na Cl and Na SCN before and after three cycles of FT treatment,which may directly affect the FT stability of emulsions.