Double Synergistic Regulation Of Heavy Metal Compounds During Processing Of Preserved Eggs

Preserved eggs are traditional egg products in China.They are very popular among consumers because of their unique flavor,rich nutrition and therapeutic effects.However,the current research on the mechanism of heavy metal compounds in the processing of preserved eggs is relatively weak.Based on this,we first studied the changes in alkalinity,pH of preserved egg white（PEW）,black spots on the eggshell surface and heavy metal content in various parts of preserved eggs regulated by different heavy metal compounds during pickling,and further adopted environmental scanning electron microscopy（SEM）to directly observe the plugging effect on eggshell surface.In addition,we used the X-ray photoelectron spectroscopy（XPS）to clarify the main components of the black spots on the eggshell surface.In addition,the effects of different anionic copper salts on the physicochemical properties,protein microstructure,intermolecular interactions,protein secondary structure,and protein components of preserved eggs during pickling were studied.Finally,an extra-shell alkali-induced egg white protein gel model was established to investigate the effect of different anionic copper salts on the gelation characteristics,microstructure and protein conformation of alkali-induced egg white gel（EWG）.The main results are as follows:1.Clarified the role of different heavy metal compounds in dynamically regulating the alkali infiltration of preserved eggsAs the pickling time increased,the alkali concentration of the feed liquid decreased,and the decreasing rate gradually slowed down.The pH of PEW reached a maximum on the 7th day and then decreased.The black spots on the eggshell surface gradually increased on the 14th day.The metal ions corresponding to the added heavy metal compounds（CuSO₄,CuSO₄/ZnSO₄,and PbO）undergo a complex migration with the pickling time.The content of metal ions in the eggshell and egg yolk gradually increased,while the migration of metal ions in the protein was more complicated.The eggshell SEM results showed that the addition of different heavy metal compounds could produce a plugging effect.XPS results showed that the main components of the black spots on eggshells of copper pickling,copper-zinc pickling and lead pickling preserved eggs were Cu₂S,Cu₂S/ZnS and PbS,respectively.In short,different metal compounds were combined with H₂S generated by degradation within the egg to form insoluble compounds to block eggshell pores,mesh holes and corrosion pores,thereby dynamically regulating the infiltration of alkalinity and ensuring the quality of preserved eggs.2.Investigated the regulation effect of different copper salts on PEW during picklingDifferent copper salts（CuSO₄,CuCl₂,and Cu（CH₃COO）₂）have certain effects on the physico-chemical properties,microstructures and intermolecular interactions of preserved eggs.As the the pickling time increased,the alkalinity of the pickling liquids and the moisture of preserved egg white marinated by the three copper salts gradually decreased.The copper salt gradually produced a plugging effect on pores of eggshell land reacted with the egg white protein,causing the pH of egg white to increase first and then decrease,and fanally tended to be stable.The rheological results showed that the adding with the three copper salts caused the viscosity of preserved egg white to increase first,then decrease and then increase,the hardness and springiness tended to increase,and theα-helix structure transformed into a more stable β-structure.The main contributors to maintaining the formation of preserved egg white gels marinated with different copper salts were ionic bonds and disulfide bonds.The increase in disulfide bonds promoted the formation of gel strength and a regular and dense three-dimensional network structure.The protein components of the soluble part of preserved egg white protein were degraded during the pickling process,and polymer aggregates were formed by crosslinking.Preserved eggs pickled by CuCl₂ had a faster penetration rate of alkali in the early stage and obtained better gel strength and network structures,followed by CuSO₄ and then Cu（CH₃COO）₂,but the effects of the three copper salts were similar in the later stage of pickling.Due to the difference in solubility of the copper salts and in the binding site with proteins,different copper salts leaded to different rates of alkali penetration and binding degree with the proteins.3.Clarified the regulation effect of different copper salts on alkali-induced EWGWith concentration increased,the three copper salts（CuSO₄,CuCl₂ and Cu（CH₃COO）₂）all promoted the formation of alkali-induced egg white protein aggregates,but there was no significant difference in the average particle size and turbidity among the three types of EWGs.At low concentrations,the three copper salts all promoted the formation of three-dimensional gel networks,and improved the gel texture,water holding capacity and solubility.While at high concentrations,the gel deteriorated.In addition,the increase in ion concentration disrupted the hydrogen bonds that maintain the stability of the protein secondary structure,thereby reducing theβ-sheet content,but the protein component of the soluble part of EWGs was not affected by the ion concentration.The alkali-induced EWGs added with copper salts were mainly contributed by ionic bonds and disulfide bonds.The salt bridges mediated by Cu²⁺played an important role in the aggregation of EWGs.Due to the difference in charge density and anion sites of protein cross-linking,the three anions had different effects on the stability of proteins,and finally showed different gel characteristics（gel strength,water holding capacity,solubility）:CuSO₄>CuCl₂>Cu（CH₃COO）₂.Therefore,copper salts,especially CuSO₄,can be used to improve egg white protein aggregation.