Emulsifying Properties Of Hydrolyzed Egg Yolk And Stabilizing Effect Of Chitosan

Egg yolk is rich in nutrition,taken as a good source of high quality protein,lipid and various biological active substances.In addition,the emulsifying ability of egg yolk is excellent,determining its great potential to be used as nutrient and natural emulsifier in milk beverage system.However,in the food system with low viscosity the emulsification stability,limiting its application in food system.It is very important to explore a feasible method to improve the emulsifying stability of egg yolk and study on the characteristics of egg yolk emulsions to broaden the application in food industry.In this study,the effect of protease hydrolysis on the emulsification of egg yolk and the stabilizing effect of chitosan were studied.Firstly,the effects of neutral protease and alkaline protease(2,000 U/g protein)on the properties of egg yolk were studied.It was found that after enzymatic hydrolysis,the molecules of yolk protein became smaller,the solubility increased and the static charge increased.The decrease of molecular weight and the increase of static charge were beneficial for heat tolerance of egg yolk,which was manifested by the increase of gel point and the decrease of turbidity after high temperature treatment.It was found that the emulsion prepared by hydrolyzed egg yolk had smaller particle size,higher surface charge and better stability.There were some differences between the properties of egg yolk hydrolyzed by neutral protease and alkaline protease with the same enzyme activity,and the emulsification stability of alkaline protease treated egg yolk was better,which may be due to that the action sites of different proteases were various contributing the different effects on the properties of egg yolk.Then,the effect of alkaline protease adding amount(1,000-3,000 U/g protein)on the properties of egg yolk was studied.With the increase of alkaline protease amount,the molecular weight of egg yolk protein gradually became smaller,combined with the phenomenon that the solubility increased.In addition,the surface hydrophobicity of protein increased firstly and then decreased,and the surface charge showed obvious change when the amount of enzyme was 1,000 U/g.It was found that the emulsifying stability was better in the range of 1,800-2,200 U/g.The results showed that the increase of protein solubility and surface net charge after hydrolyzing was helpful to improve the emulsifying stability.However,excessive hydrolysis would induce the formation of small protein or polypeptide,which is negative for emulsifying properties of protein.In summary,the emulsions constructed at the optimum enzyme addition amount also had some stratification,indicating that protease enzyme hydrolysis could improve the emulsifying stability of egg yolk protein,but the effect was limited with slight creaming can be observed.This may be due to that these emulsions lacked sufficient electrostatic repulsion or space resistance between the droplets to maintain the stability.Next,aiming at the creaming problem of egg yolk stabilized emulsion,the effect of the multi-layer structure based on electrostatic interaction on the stability of the emulsion was further explored,and the yolk protein and chitosan with opposite charge at pH 6.0 were selected as the materials to construct the emulsion.The stable yolk-chitosan composite emulsion can be obtained by multi-layer structure.After adding chitosan,the emulsion potential changed from negative to positive,indicating that the yolk-chitosan structure was formed at the oil droplet interface.The decrease of net charge indicated that the electrostatic repulsion was not the main factor to stabilize the emulsion.The addition of chitosan may provide additional space resistance to stabilize the emulsion.The results of microstructure observation showed that there were droplet aggregates in the yolk emulsion and the aggregates were dispersed after the addition of chitosan.The rheological results showed that the static viscosity of the emulsion with better stability was lower,which may be related to the aggregation state and charge of the emulsion droplets.Finally,the effect of enzymatic hydrolysis of protease and chitosan on heat resistance and chemical stability of emulsion was investigated.The results showed that the addition of chitosan changed the droplet from aggregation state to uniform dispersion state,and the chance of droplet contact with oxygen-promoting ions increased,which led to accelerated oxidation of the emulsion.The thermal stability of the emulsion prepared by hydrolysed egg yolk was enhanced,possibly because the protein interaction became weaker after enzymatic hydrolysis.Hydrolysis also inhibited the intermolecular cross-linking of the emulsion interfacial yolk protein,which was not conducive to the formation of interfacial protein membrane,resulting in the reduced chemical stability of the emulsion.