| This study was supported by the national key RESEARCH and development Program of the 13 th Five-Year Plan(2018YFD0400300).Egg white protein is widely used in food processing industry because of its balanced nutrition,high bioavailability,low price and abundant source.As a common processing method,heat treatment is widely used in food industry.Heat treatment can not only kill harmful bacteria in food,ensure human health,but also prolong the shelf life of food products,which is conducive to their transportation and storage.However,heating usually causes denaturation and aggregation of proteins,resulting in changes in their structural and functional properties.The purpose of this paper was to explore the effects of heat treatment on the structure,foaming and emulsifying properties of egg white protein under neutral conditions,and to explore its mechanism by means of circular dichroism,fluorescence,ultraviolet,dynamic light scattering,electrophoresis and turbidity.Meanwhile,egg white protein after heat treatment was used as carrier to load curcumin,and the changes of curcumin bioactivity during delivery and control were analyzed.In addition,the regulatory mechanism of polyphenols(caffeic acid and ferulic acid)on the structural and functional properties of heat-treated proteins was studied based on ovalbumin(54%),the highest content of egg white proteins.The results of this study can not only provide theoretical support for the change of structure and functional characteristics of egg white protein under different heat treatment conditions,but also have certain scientific research value and practical significance in improving the thermal processing level of egg products and the added value of products.The specific research contents and results are as follows:1.The effects of heat treatment on the structure and aggregation degree of egg X white protein were studied.Under neutral conditions,egg white protein was heated at55,75,95 and 120 ℃ for 15 min.The effects of different heating temperatures on the structure of egg white protein were analyzed by circular dichroism,fluorescence spectrum,SDS-PAGE and dynamic light scattering.The results showed that the surface hydrophobicity of egg white protein increased gradually when heated from room temperature to 75 ℃,but decreased significantly when heated above 75 ℃.This may be related to the aggregation behavior of egg white protein during heating(25-120 ℃),that is,egg white protein gradually forms large molecular weight aggregates with the increase of temperature from normal temperature to 75 ℃.When the heating temperature was higher than 75 ℃,the aggregates began to crack into small molecular weight oligomers with smaller particle size and more uniform distribution.2.The effect of heat treatment on the properties of egg white protein foam was studied.The results showed that the foaming ability of egg white protein decreased with the increase of temperature from room temperature to 75 ℃,while the foam stability increased gradually.At 75-120 ℃,the foaming ability of egg white protein increased gradually with the increase of temperature,while the foam stability began to decrease.In general,within the range of 25-120 ℃,the foam stability of egg white protein is the highest at 75 ℃,while the foaming ability is the strongest at 120 ℃.Combined with the analysis of egg white protein structure after heat treatment in the previous chapter,it can be inferred that the improvement of egg white protein foam properties(foamability and foam stability)by heat treatment is jointly regulated by the change of protein aggregation behavior and surface charge transfer.3.The effect of heat treatment on emulsifying properties of egg white protein was studied.The results showed that the egg white protein emulsion after heat treatment at 75 ℃ had the largest particle size,the strongest surface hydrophobicity and the worst emulsification.However,the emulsion formed by the egg white protein solution heated at 125 ℃ has smaller particle size and more uniform distribution,with the best emulsification.This may be because the egg white protein treated at 125 ℃ is mainly composed of a large number of oligomers with high hydrophobicity and electrostatic repulsion,which can rapidly diffuse to the oil-water interface to form a stable layer.However,the egg white treated at 75 ℃ is difficult to diffuse and stabilize at the oil-water interface due to its strong rigid structure and large aggregate composition,resulting in poor emulsification.The influence of oil ratio(θ = 0.1,03 and 0.5)on the emulsification of egg white protein at different heating temperatures was also analyzed.It was found that the increase of oil ratio was beneficial to the enhancement of emulsion stability.4.Based on the surface hydrophobicity of egg white protein increased significantly after heat treatment,a complex delivery system containing curcumin hydrophobic active substances was constructed on the basis of egg white protein after heat treatment,and egg white protein could be used as a new nanocomposite.The results showed that curcumin mainly bonded with the hydrophobic region of egg white protein through hydrophobic action to form a stable egg white protein-curcumin complex.Combining with egg white protein,the solubility of curcumin in aqueous solution was significantly increased by a maximum of 3800 times,and the stability and bioavailability of the complex were also significantly improved.Sds-page test of digestive fluid showed that curcumin could improve the digestibility of egg white protein in vitro.Finally,by analyzing the surface properties and structure of the complex after the interaction between egg white protein and curcumin,it was determined that the number of hydrophobic points was the most important factor affecting the interaction of the complex,and the egg white protein treated at 120 ℃had a more significant effect on the binding of curcumin.5.The effects of the combination of heated egg albumin with caffeic acid and ferulic acid on its structure and foam properties were studied under neutral conditions.The results showed that the effects of different temperatures on protein structure and surface activity of egg white were significantly different.The addition of polyphenols can improve the foaming ability and foam stability of egg albumin(unheated and heated),and the concentration of polyphenols has a great influence on the interface properties.Polyphenols can improve the foam properties of proteins by improving the solubility of proteins in aqueous solution and increasing the thickness of adsorbed film at gas-water interface by cross-linking proteins.In conclusion,the results of this study indicate that heat treatment can significantly affect the foamability and emulsification of egg white protein by changing the structure of egg white protein.Egg white protein aggregation was significant at 75 ℃.When heated at 120 ℃,the aggregate is decomposed into several oligomer particles with high hydrophobicity,high polymer flexibility and strong electrostatic repulsion.At the same time,it was found that heated egg white protein was an excellent nano-carrier,which could improve the stability,solubility and bioavailability of curcumin.Conversely,the addition of polyphenols can also improve the foaming ability and foaming ability of protein.This study provides important theoretical basis and reference significance for the development of new technological basis for commercial application of egg white protein. |
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