The Research Of Interaction Between Hen Egg Proteins And Soybean Protein Isolate And Its Gelation Properties

Hen egg is an excellent source of animal protein and it has great nutrition and good gelation properties.However,its application scope is relatively narrow,mostly be eaten before processing and used in the egg-based products such as egg pudding,dried egg and cake.It is seldom used in other protein-based systems,which limits the impact of egg’s nutritional value and gelation properties.Soybean protein isolate(SPI),a high-quality plant protein,has low price and there are plenty of its products on the market.But its methionine content is lower than the ratio recommended by the UN Food and Agriculture Organization(FAO)and World Health Organization(WHO),which brings down the amino acid score of the protein.Meanwhile,its gelation properties are insufficient to meet the needs of food production.Therefore,from the perspective of nutrition and gelation properties,the composite gel with high protein concentration was prepared by the hen egg and SPI in this paper,and the interaction between proteins was studied.It can make up for the disadvantages of SPI and provide some theoretical basis for the combination study of animal protein and plant protein.Meanwhile,by considering the influence of other components(like oil)of hen egg in the composite gel,it is closer to the actual situation of food production,which is more conducive to food production and development work to provide theoretical support.Firstly,the physicochemical properties of egg white(EW),egg yolk(EY)and the whole egg(TWE)with pH=7 were determined and compared.Meanwhile,gelation properties of their gels with different protein concentration(4%-11%)were also determined and compared.The purpose of these was to provide a better theoretical basis for the practical application of hen egg.The results showed that the trend of storage modulus(G’)of egg white and the whole egg was similar in the whole gelation process,while G’ of egg yolk in the insulation and cooling stages was much lower than that of egg white and the whole egg,because egg yolk had low protein solubility and high solid content.The texture results suggested that protein concentration has a great influence on the hardness and chewiness of the gel(Pearson correlation was 0.887 and 0.895,respectively,p<0.01),while the springiness and cohesiveness of the gels are mainly affected by the type of protein.When the protein concentration was 10%,the hardness of the whole egg gel was 20% and 53% higher than that of egg white gel and egg yolk gel;in terms of gel chewiness,the whole egg gel was 111%and 76% higher than that of egg white gel and egg yolk gel.Simultaneously,it maintained the high cohesiveness of egg yolk gel.Water-holding capacity increased as the protein concentration rose and it had positive correlation with the solid content of egg liquids(Pearson correlation was 0.877,p<0.01).The microscope pictures indicated directly the microstructure of egg gels.The details are as follows,egg white gel was rigid,granular and less uniform,while egg yolk gel presented a uniform,dense and smooth structure.The network structure of the whole egg gel is more like that of egg white gel,but the particles were smaller and the network was denser.Secondly,the interaction between the whole egg,egg yolk and SPI were chosen for study to illustrate the role of the whole egg and egg yolk in the complex process of gelation.The consequences of protein solubility illustrated that the protein solubility of the composite solution was highest when 25% the whole egg or egg yolk was added,increased by 20% or 9% respectively when compared with SPI.The low solubility and high solid content of egg yolk have significant effects on those of the composite solution.The amount increase of the whole egg,especially egg yolk,also resulted in the rise of the total sulfhydryl group content and the protein surface hydrophobicity of the composite solution.When the ratio of the whole egg/egg yolk was 75%,the protein surface hydrophobicity of the composite solution was the highest,which increased by132%/565% compared with SPI.The whole egg,compared with egg yolk,had greater enhancement effect on the G’ and hardness of composite gel,suggesting that egg white proteins were the main cause of increasing G’ and hardness of composite solution.Increasing the proportion of egg yolk had no significant effect on the springiness of composite gel,while more than 50% the whole egg would result in a significant decrease on the springiness of composite gel.The results about water state of gels declared that the addition of the egg,especially egg yolk,could improve the stability of the bound water.The interaction between proteins and the filling of non-proteins from the whole egg resulted in a compact linear network structure of TWE-SPI composite gel,while the excessive involvement of other components in the egg yolk contributed to the rough and sticky network structure of the EY-SPI composite gel.After selecting the best conditions for preheating and transglutaminase(TG)treatment,the individual and synergistic mechanisms of preheating and TG treatments on TWE-SPI composite gel were studied.The results suggested that the preheating treatment improved the protein solubility and protein surface hydrophobicity of SPI,and then caused the rearrangement of protein and other components,making the gel microstructure become flat and enhancing the G’ and hardness of the composite gel,but the later effect was small.Adding TG had no obvious effect on protein solubility of every solution and protein surface hydrophobicity of SPI,but it can cause 18%loss of the protein surface hydrophobicity of composite solution through reducing the protein surface hydrophobicity of egg proteins.It worked mainly by promoting crosslinking reaction between proteins,causing the gel network structure more apparent and significant increase in G’,the hardness of the composite gel.When preheating and TG were used at the same time,a synergistic effect between the two treatments could be seen.Compared with the control sample,the protein solubility of the composite solution rose about 27% and surface hydrophobicity of the composite solution only a 9% drop,meanwhile the crosslinking between proteins was good,thus leading to a compact network structure of composite gel and the obvious improvement of hardness.In addition to the springiness decrease of the composite gel caused by the addition of TG,the two treatments,either alone or in combination,had little effect on the springiness and cohesiveness of SPI gel,the whole egg gel and the composite gel.Although the addition of TG resulted in the decrease of water-holding capacity of SPI gel,the whole egg gel and the composite gel,all gels could maintain its original high water-holding capacity when preheating and TG were used together.Finally,the influence of oil adding amount(5%-20%)and oil type on the properties of the composite gel was studied based on the fact that the oil contained in the egg was embedded well.Then the composite gel was used to embed the oil-soluble active substance,β-carotene.It could not only improve the nutrition and gelling properties,but also increase the application scope of the system.Soybean oil,olive oil and menhaden oil with different ratio of saturated fatty acid,monounsaturated fatty acid and polyunsaturated fatty acids were selected for this research.The results proved that all the emulsions exhibited a shear-thinning behavior at the tested shear rate.In addition,the changes of emulsion viscosity were related to the viscosity of the selected oil itself.The higher the oil adding amount,the higher the viscosity of the emulsions,regardless of the types of oil added.The strain sweep and frequency sweep of emulsion gels with various oil adding amount and oil type were determined.It can be observed that the oil in the system belonged to the active filler and the G’ and G’’ of all emulsion gels increased along with the increase in oil adding amount.At the same time,there was an apparent increase of G’ between 10% and 15% oil content,suggesting that 10% oil content might be a turning point on emulsion gels’ property.The consequences of texture also indicated that the hardness and chewiness of the emulsion gels would be further increased,and the system still maintained good springiness and cohesiveness within the appropriate amount of oil addition.Water loss(WL)and oil loss(OL)were tested by extrusion method and the results showed that the gel system had good embedding effect on oil.Additionally,the increase of hardness and the decrease of WL were beneficial to the reduce of OL.The confocal laser images revealed that most of the oil was embedded by proteins and other components,and the oil adding amount and oil type had little effect on the microstructure of the gel system.The composite system of TWE-SPI was a good carrier and its maximum retention rate of β-carotene was more than 97%.