Preparation And Application Of High-emulsifying Liquid Egg Yolk

Egg yolk is a natural food emulsifier,which is widely used in the preparation of bakery,ice-cream and mayonnaise.However,the emulsifying properties of heat-sensitive egg yolk are markedly declined after heat treatment,resulting in a decrease in product quality.Over the past few decades,researchers have devoted themselves to improving the emulsifying properties of egg yolk powder,and have achieved some progress.Nonetheless,high-emulsifying liquid egg yolk product has not yet been developed so far.This paper had modified the emulsifying properties of liquid egg yolk using food additives,protease hydrolysis,and composite hydrolysis of phospholipase and protease,with an aim to develop a high-emulsifying liquid egg yolk.Changes in egg yolk protein structures and interface properties after enzymatic hydrolysis were also characterized through SDS-PAGE,CD,FTIR,surface hydrophobicity,and?-potential,so as to preliminarily elucidate the mechanism of action by which enzymatic hydrolysis improved the emulsifying properties of liquid egg yolk.The main research contents and results were as follows:?1?Indexes,such as emulsifying activity,emulsification capacity and creaming stability,were used to investigate the effects of XG solution,NaCl solution,or XG and NaCl compound solution on the emulsifying properties of liquid egg yolk.The results showed that all the 3 treatment methods could improve the emulsifying properties of liquid egg yolk to various degrees.The creaming stability of liquid egg yolk with XG addition was strengthened with the increase in XG concentration.The creaming stability had increased by 48.80%at the XG solution concentration of 1.5%.Meanwhile,the emulsification capacity of liquid egg yolk was enhanced with the increase in NaCl concentration,which had increased by 28.49%at the NaCl solution concentration of 5%.The addition of 0.9%XG solution and 5%NaCl solution into liquid egg yolk could improve the creaming stability and thermal stability of the liquid egg yolk.To be concrete,the optimal creaming stability and thermal stability of the liquid egg yolk could be achieved at both additive amounts of 5%?w/w?,which had increased by 59.72%and37.29%,respectively.?2?Firstly,the effects of concentrations of flavourzyme,papain,neutrase and trypsin on the emulsifying properties of liquid egg yolk were examined,with mean oil droplet diameter and flocculation degree in the O/W emulsion prepared with yolk before and after thermal treatment as the indexes.Our study found that all the 4 proteases could reduce the flocculation degree of emulsion after thermal treatment under different enzyme additive amounts,but there were differences in the effects on the emulsion droplet size.The mean oil droplet diameter of emulsion had increased when flavourzyme was 0.5%,indicating lowered emulsifying activity of liquid egg yolk.Papain hydrolysis would lead to aggravated oil droplet coalescence during thermal treatment and reduced thermostability of emulsion.Neutrase and trypsin could reduce the oil droplet diameter of emulsion and enhance the emulsifying activity of liquid egg yolk.Subsequently,the effects of dual enzymatic hydrolysis of neutrase and trypsin on the emulsifying properties of liquid egg yolk were studied through orthogonal experiments.Compared with the single protease hydrolyzed liquid egg yolk,emulsion prepared with liquid egg yolk in the orthogonal experiments exhibited lower flocculation degree.The optimal dual enzymatic hydrolysis conditions established by orthogonal experiments were that trypsin concentration was0.15%,neutrase concentration was 0.15%,hydrolysis time of 0.5 h and temperature at45°C.Compared with untreated liquid egg yolk,the emulsification capacity of yolk hydrolyzed under such conditions had increased by 11.2%,while the oil droplet diameters had reduced by 6%and 60%,respectively,before and after thermal treatment of emulsion,and the flocculation degrees had decreased by 97.8%and 32.7%,respectively.?3?To further understand the mechanism of dual enzymatic hydrolysis in improving the emulsifying properties of liquid egg yolk,changes in the emulsifying and structures properties of liquid egg yolk were studied under different hydrolysis time.The results suggested that,the emulsifying activity and emulsification capacity of liquid egg yolk and thermostability of yolk emulsion could be enhanced at hydrolysis time of 0.5-4.5h.However,the thermostability of yolk emulsion began to decrease at the hydrolysis time of4.5 h.The solubility of yolk protein was increased after hydrolysis,which had increased from 24.21%in untreated liquid egg yolk to 60%at the hydrolysis time of over 1.5 h.On the other hand,hydrolysis had no significant effect on the surface hydrophobicity of yolk,but it could increase the random coil content in yolk protein and residual protein concentration at the continuous phase of emulsion.These findings had indicated elevated protein flexibility after dual enzymatic hydrolysis and cover the same area of interface membrane required protein was reduced.At the same time,emulsion prepare with hydrolyzed liquid egg yolk had higher absolute value of?-potential,indicating enhanced electrostatic repulsion between the oil droplets.This contributed to reducing oil droplet coalescence during thermal treatment and improving the thermostability of emulsion.Overall,dual enzymatic hydrolysis could promote the emulsifying properties of liquid egg yolk,which were associated with the increase in protein solubility,molecular flexibility,and absolute value of?-potential of the emulsion.?4?The effects of composite hydrolysis of PLA₂ with proteases on the emulsifying properties of liquid yolk were studied.As the results showed,the droplet size of emulsion prepared with the composite enzymatic hydrolysis treated liquid egg yolk was comparable to that prepared with dual enzymatic hydrolysis treated liquid egg yolk,but the former had a larger emulsification capacity.The maximum emulsification capacity was found at30 min of composite enzymatic hydrolysis to be 161.71 g oil/g yolk,which had increased by 50.92%compared with that of the untreated yolk.SDS-PAGE analysis indicated that,the subunits of yolk protein after composite enzymatic hydrolysis was different from dual enzymatic hydrolysis,which could be attributed to the fact that PLA₂ destroyed the compact structure of yolk granules,so that some proteins that originally embedded within the internal of granules were exposed and hydrolyed.Second-derivative FTIR spectra revealed that the total contents of?-helix and random coil structure in the composite enzymatic hydrolyzed egg yolk had increased.Moreover,further analysis on the interface properties of oil droplets revealed that,emulsion prepared with the composite enzymatic hydrolysis treated liquid egg yolk had the largest absolute value of?-potential and residual protein concentration at the continuous phase.?5?The effects of composite enzymatic hydrolyzed liquid egg yolk on the quality of mayonnaise were studied.It was found that mayonnaise prepared with composite enzymatic hydrolyzed liquid egg yolk exhibited similar particle size as mayonnaise prepared with untreated liquid egg yolk,but the former had better thermostability,low-temperature stability,oxidative stability and hardness.When the additive amount of liquid egg yolk was 4%,the low-temperature stability and thermostability of the modified mayonnaise were 99.14%and 100%,respectively,while those of ordinary mayonnaise were 53.78%and 94.72%,respectively.At the additive amount of liquid egg yolk of 10%,the hardness of modified mayonnaise had increased by 146%compared with ordinary mayonnaise.The POV of modified and ordinary mayonnaise were 9.48mmol/L and 12.32 mmol/L,respectively,after storage at 37°C for 20 days.