Engineered Interface For Explaining Foaming Characteristics Of Egg White Protein Systems And Regulation Mechanisms

Egg white protein?EWP?is widely accepted as a good foaming agent in aerated food systems such as ice cream,mousse and bakery food based on its excellent foaming properties.During egg processing,various factors including egg freshness,pH values,foaming time,foaming methods and yolk residue could cause the differences of foaming properties of egg white protein.However,yolk contamination which was demonstrated to be the main problem in reducing foaming properties of egg white could be easily caused by a series of elements including the poor freshness of eggs,fragility of the eggshells or mistakes in manual operation.Fully separation of egg white and egg yolk could not be guaranteed by the existing separation technologies.Therefore,it would be a common problem for the reduction of foaming properties of egg white because of yolk contamination.Egg white protein would be used as the research object.Influences of kinds of physicochemical factors on the foaming properties and application in batter systems were studied.Then,egg white protein contaminated by a small amount of egg yolk was taken as the research model in order to simulate the actual egg white liquid system.Enzymatic modification was used in order to increase the foaming properties of egg white.Besides,the structure of components and the mechanism of improvement were analyzed in detail.At the same time,egg white protein microgel?EWPM?was successfully prepared to improve the foaming stability of egg white protein.The correlation between interfacial properties and foaming properties of egg white was clarified by analyzing the adsorption stability of EWP and EWPM at the air-water interface.Not only the theoretical support for the correlation among the component structure,interfacial properties and foaming properties was provided,but also the technical level of egg white liquid processing was improved in this research.They have important theoretical research meanings and practical application values.The detailed results were depicted as follows:1.Physicochemical factors have significant effects on the foaming properties of egg white protein.Therefore,the effects of egg freshness,pH values and yolk contamination on foaming properties of egg white protein at high protein concentration?12.5%?were investigated.It was determined that pH values and yolk contamination were the key factors affecting the foaming properties of egg white protein.After analyzing the microstructure,apparent viscosity and preliminary application in batter systems of egg white protein at different pH values,pH 7.0was found to be the best condition.The results of influences of yolk contamination on foaming properties of egg white protein showed that 0.5%yolk contamination would lead to a significant reduction in reducing the apparent viscosity and foaming properties of egg white protein.Under this condition,the weak interaction between egg white protein would be destroyed and the final product in batter system would be in poor quality.In order to clarify the deterioration mechanism of yolk contamination on foaming properties,plasma and granule were separated from egg yolk and studied.The results showed that yolk plasma was the main factor in reducing the foaming properties of egg white protein.2.Based on the results that egg yolk contamination could significantly decrease the foaming properties of egg white protein and yolk plasma was the main deterioration factor,egg white protein contaminated by 0.5%egg yolk was taken as the research model.Lipase and Phospholipase A₂ were used to improve its foaming properties.It was found that both lipase and phospholipase A₂ could improve the foaming ability of egg white under their optimal enzymatic hydrolysis conditions,while their foaming stabilities were not increased.In order to clarify the mechanism of enzymatic hydrolysis,effects of the enzymatic hydrolysis solutions of yolk components?yolk plasma and yolk granules?on foaming properties of egg white protein were compared.The structure of yolk components after hydrolysis was also characterized.Results showed that the structure of egg yolk could not destroyed significantly by lipase.Lipase could lead to the aggregation of yolk components to a certain extent and mainly focused on egg plasma.Phospholipase A₂ could destroy the structure of egg yolk significantly.After hydrolyzed by phospholipase A₂,mean size of yolk components decreased.Phospholipase A₂mainly acted on yolk granule.3.Due to the fact that enzymatic modification could not improve the foaming stability of egg white protein,physical modification was established and studied.Egg white protein microgel?EWPM?was designed using a top-down approach involving the formation of a thermally-crosslinked egg white protein microgel?90^oC/30 min,pH 7.0?.The hydrodynamic diameter of designed EWPM was 359 nm.It was demonstrated that 0.5%yolk contamination did not influence the foaming properties of EWPM.In order to further understand the mechanisms of foaming stability of EWP and EWPM,air-water interfacial adsorption stability of EWP and EWPM were investigated.Foamability was found higher in EWP-stabilized foams compared to EWPM-stabilized foams,irrespective of the protein concentrations tested.Microstructural evaluation at multiple length scales?confocal laser scanning microscopy and cryogenic scanning electron microscopy?indicated that the EWPM stabilized the aqueous foams via a Pickering-type mechanism.Based on this,foams stabilized by EWPM were confirmed to be more stable during disproportionation and coalescence.Changes of surface shear viscosities of EWP at the air-water interface indicated that EWP films were more brittle,exhibiting shear thinning during the measurements,whereas the viscosities of the EWPM films were independent of shear after 24 h of ageing.4.According to the advantages of the higher foamability of EWP and the higher foam stability of EWPM,complex systems combined with EWP and EWPM were established to compare their interfacial and foaming properties in order to provide a twofold benefit of high initial foam volume?dictated by rapid adsorption of EWP?and high foam stability?governed by Pickering stabilization by EWPM?.Confocal images revealed that EWPM were preferentially located at the air-water interface with the increasing ratio of EWPM-EWP,suggesting pure Pickering stabilization at 1.0 wt%EWPM.The EWP/EWPM systems generated similar initial foam volumes as compared to those prepared solely with EWP?p>0.05?,and foams generated with increasing ratios of EWPM/EWP showed higher stability to bubble shrinkage and coalescence at longer time scales?9 h?.Besides,combination of EWP and EWPM always showed better foam stability than EWP itself,irrespective of their processing conditions?freezing at-20?/30min,oven-heating at 80?/30 min or microwaving at 700 W/15 sec?.All the results suggested that the air-water interface would be dominated by EWP when protein ratio was more than 70%,whereas dominated by EWPM when protein ratio is less than 50%.