Extraction Of Lysozyme From Salted Duck Egg White And Preparation And Application Of Edible Bacteriostatic Membrane

Salted duck eggs are one of the traditional Chinese egg products,which are popular in China and Southeast Asian countries.In food processing,salted duck egg yolk is often used as the filling for moon cakes,zongzi,pastries and other foods,while salted duck egg white is often abandoned by most food enterprises because of its high salt content.On the one hand,the high quality protein in the salted duck egg white is seriously wasted.On the other hand,the decomposition and corruption of the discarded salted duck egg white will cause serious pollution to the surrounding environment.In order to improve the utilization scope of salted duck egg high value,this study used salted duck egg white as raw material and innovatively adopted the method of isoelectric point precipitation,ultrafiltration and cation exchange to isolate and extract the enzyme（SDEWL）of salted duck egg white lysozyme.Its structure and enzymatic properties were preliminarily investigated.Using carboxymethyl chitosan（CMCS）as film forming substrate,a new edible antibacterial film of SDEWL-CMCS was prepared and applied in the preservation of nectarine.The main research conclusions are as follows:1.In this experiment,isoelectric point precipitation and primary ultrafiltration were used to remove other proteins in salted duck egg white,then secondary ultrafiltration was used for desalting and enrichment of salted duck egg lysozyme,and finally cation exchange method was used to separate and extract salted duck egg lysozyme.In this study,the effects of resin dosage,adsorption time,eluent concentration and elution time on the extraction of lysozyme from salted duck eggs by cation exchange method were investigated.The orthogonal response surface combination design with four factors and three levels was adopted to optimize the extraction process,and the optimal extraction conditions were obtained as follows:The resin dosage was 35.81 m L,the adsorption time was 6.32 h,the concentration of（NH₄）₂SO₄was 1.02 mol/L,the elution time was62.26 min.Finally,pure lysozyme of salted duck egg was obtained by electrophoresis.The relative molecular weight of the enzyme was about14k Da,the enzyme specific activity reached 18300 U/mg,and the yield was 0.36%.2.The enzymatic characteristics of lysozyme of salted duck egg showed that the optimum reaction temperature was 60℃,the optimum p H was 7,and the stability was good in the range of 30℃～60℃and p H4～7.Na⁺and Mg²⁺can activate lysozyme to some extent.Fe²⁺,Cu²⁺and Zn²⁺had strong inhibitory effects on lysozyme activity.Other metal ions(Mn²⁺,K⁺,Ca²⁺)had no significant effect on lysozyme activity.Different surfactants had certain inhibitory effects on lysozyme of salted duck eggs,and glycerol had the strongest inhibitory effect.The bacteriostatic effect of salted duck egg lysozyme on common pathogenic bacteria was tested by the bacteriostatic circle method.The results showed that the bacteriostatic effect of salted duck egg lysozyme on gram-negative bacteria was not obvious,but the bacteriostatic effect on Gram-positive bacteria was significant.3.On the basis of previous studies,a new type of edible antibacterial fresh-keeping film with high antibacterial activity and good packaging performance was prepared by using CMCS as film-forming substrate and adding SDEWL.The thickness,water solubility,moisture resistance,oil resistance,mechanical properties and antibacterial activity of SDEWL-CMCS composite films were measured to investigate the effect of the amount of SDEWL on SDEWL-CMCS composite films.By Fourier transform infrared spectroscopy,X-ray diffraction,scanning electron microscopy and ultraviolet absorption spectroscopy,the interaction between each component of SDEWL-CMCS composite film was analyzed,and the experiment of packaging and storage of small nectarine was carried out.The results showed that the introduction of SDEWL into CMCS significantly improved the properties of the composite film,and had better comprehensive packaging properties and outstanding antibacterial properties.Compared with pure CMCS films,the tensile strength of the composite films with SDEWL increased from37.39 MPa to 46.18 MPa,water solubility decreased from 31.66%to21.84%,and water vapor permeability decreased by 1.09×10^-10g·m^-1·s^-1·Pa^-1.Oil resistance coefficient decreased from 2.70%to 1.62%.Most importantly,the composite film added with SDEWL greatly improved the antibacterial performance of Escherichia coli and Staphylococcus aureus.In addition,we studied the use of SDEWL-CMCS composite film as active packaging to keep young nectarine fresh and achieved good results.Compared with the non-film preservation treatment of nectarine,the composite film preservation treatment with SDEWL added significantly increased the content of soluble solid and vitamin C in nectarine during storage.Reduced the weight loss rate of nectarine and improved the color change of nectarine skin.