Mechanism Of Ions-Induced Egg White Protein Aggregation Behavior On The Gel Structure Formation And Stabilization

This research was funded by National Natural Science Foundation of China（no.32101993）.Egg white protein（EWP）is rich in a variety of essential amino acids,and has been widely used in food industry for its excellent nutrition and processing characteristics.In order to improve the quality of EWP in actual production,it is important to enhance the gelling and emulsifying properties of EWP by suitable additive such as salt ions.Nowadays,food industry in China put forward complex and variety requests on the gelling properties of EWP,however,the current technologies are hard to control the texture characteristics precisely.The gelation of protein could be defined as a process that the protein molecules form to continuously network structure through orderly aggregation.Hence,the investigate of ions-induced aggregates would be meaningful for the efficient utilization of salt in processing of production,which could contribute to the control of aggregation behavior,and the differential regulation of EWP products.This work established the aggregation kinetic model based on ovalbumin（OVA）,the main ingredient of EWP,and provided the analysis on the kinetic model parameters,the gelling properties and the driven force during the formation of gel.Meanwhile,considering that proteins existed in egg white as complexes instead of individual protein,we attempted to clarify the relationship between aggregation and gel behavior via ovalbumin-lysozyme（OVA-LYS）binary system.At last,we explored the effect of ions on co-aggregation of compound proteins,and selected Na Cl as freeze-thaw stabilizer to regulate the structure and digestive properties of emulsion-filled gel subjected to freezing and thawing.This work conduced to the quality improvement of emulsion-filled gel and the development of fat replacer.The main research results are as follows:（1）The structure and processing characteristic of ions-regulated gelation of EWP was evaluated via aggregation kinetics model,which was built by monitoring turbidity.Results showed that compared with Na Cl and KCl,the addition of Na₂SO₄ increased free sulfhydryl content,surface hydrophobicity and particle size of EWP significantly,while weakened the order of secondary structure.Hence,strengthened gel network structure was observed with higher porosity,which improved the texture profiles and rheological properties of EWP gels.Based on these phenomena above,the relationship between aggregation behavior and gelling properties with ions was further investigated by aggregation kinetics model and principal component analysis.Because of the enhancement of protein interactions,the aggregation growth rate with Na₂SO₄ was much faster than the samples with Na Cl,which reflected over-aggregation due to the accelerated nucleation process and resulted in firmed gel network structure.（2）The binary protein system on egg white ovalbumin-lysozyme was established to investigate the relationship between co-aggregation and co-gelation.We focused on the formation of OVA-LYS complex,the typical thermo-dynamically favored coacervation process,in terms of gelling properties,microstructure and thermodynamics.Benefited from synergistic effects during co-gelation,the thermally induced gels of OVA-LYS complex formed at extremely low protein concentration（18mg/m L）and showed higher storage modulus with increasing LYS concentration.Moreover,the rising particle size,reduced Zeta potential,unordered secondary structure and strengthened protein chain were observed with adding LYS.Remarkably,the divalent ions enhanced thermodynamic stability of OVA-LYS complex,although the growth of aggregates units were prevented by ions at room temperature.ITC and molecular docking analyses revealed the binding affinity stoichiometry and combination phase,which were closely related to the decrease of minimum energy resulted from the formation of hydrogen bond.（3）The freeze-thaw stability of emulsion-filled gel based on egg white protein was analyzed in terms of microstructure,digestive dynamics and processing properties in this work.The results showed that the initial spatial distribution of oil droplets and fine network of gel observed by confocal laser scanning microscopy（CLSM）were damaged during freeze-thaw cycles.These phenomena led to the rising of swelling ratio and the change of acid uptake process,which affected the p H value and pepsin activity inside gels.Hence,the digestion evaluated by the model of digestive kinetics was influenced significantly after freeze-thaw treatment,such as the slower release of amino acid.Meanwhile,the characteristic of emulsion and gel matrix was detected,illustrating the effect of oil content and ions on gel subjected to freezing and thawing.Furthermore,the lowering of relaxation time proved the improved stability caused by ions,which resulted from the enhanced limitation of network structure to fillings.