Effect Of Yolk Lipids On Yolk Coagulability Under Extreme Processing Conditions

Salted and preserved eggs are traditional egg products in China.In the process of pickling,high salt or strong alkali could destroy lipoproteins in the yolk,release free lipids and make the yolk form gel without heating.This paper took salted and preserved eggs as the research object,through studying the changes of yolk lipid characteristics and the migration of lipid in yolk during the extreme processing such as high salt,high salt-heat synergism,strong alkali,to explore the effect on the gelation of yolk of salted and preserved eggs.1.The changes of yolk lipid properties of duck eggs were studied under the conditions of high salt,high salt-heat and strong alkali processing.The results showed that with the prolongation of picking time,the water content of egg yolk decreased and the oil exudation increased(P<0.05)after high salt treatment and high salt-heat treatment.The iodine value of egg yolk lipid decreased(P<0.05).However,the water content of egg yolk was increased by strong alkali treatment(P<0.05),and the oil exudation was firstly increased and then decreased,the iodine value of egg yolk lipid was increased at first and then decreased,but the saponification value of egg yolk was lower than that of egg yolk treated with high salt.According to the indexes of lipid oxidation(Conjugated Diene Acid and Thiobarbituric Acid value),egg yolk lipid was oxidized to different degrees under the three extreme processing conditions.The Conjugated Diene Acid value(primary oxidation degree)of egg yolk lipid reached the maximum value on the 35th,7th and 21th day,respectively.While the TBA value(secondary oxidation degree)all reached the highest value on the 7th day under the three different treatments.At the end of pickling,the fatty acids in the yolk were elevated.It can be concluded that different extreme processing conditions had certain effects on the unsaturated degree,hydrolysis degree and oxidation degree of lipid in duck egg yolk.2.The migration of lipid and the changes of gelatin structure in yolk during salting were studied by using texture spectrometry(TPA),nuclear magnetic resonance imaging(MRI),low-field nuclear magnetic resonance imaging(LF-MNR),laser scanning confocal microscopy(CLSM),X-ray diffraction(XRD)and infrared spectroscopy(FTIR).As the pickling time contuined,the hardness of yolks of raw salted egg increased(P<0.05),the hardness of yolks of cooked salted egg decreased first and then increased,the elasticity and cohesion of both of them decreased(P<0.05).During pickling,the high density hydrogen protons in the raw salted egg yolks decreased gradually,but did not change much in cooked salted egg yolks but higher than that in raw salted egg yolk.There were 3 and 4 different fluidity peaks in the raw and cooked salted egg yolks by T₂ relaxation time,respectively.T₂₁ in the raw salted egg yolks increased overall(P<0.05),T₂₂ decreased 3 weeks after pickling with salt(P<0.05),and T₂₃ decreased(P<0.05).T₂₁ in the cooked salted egg yolks disappeared after two weeks of pickling,T₂₂ and T₂₃ decreased(P<0.05),but T₂₄ had no significant change rule.CLSM observed that the structure of lipid and protein in the raw salted egg yolks became denser,and the diameter decreased gradually.The lipids and proteins in the cooked salted egg yolks changed from uniformly distributed continuous phases to dispersed granular structures of varying sizes.The crystallinity in the raw salted egg yolks firstly decreased and then increased,and decreased in the cooked salted egg yolks.Compared with the yolks of the fresh egg,the?-sheets and?-turns of the raw salted egg yolks were increased,and the?-helices and random curls showed an opposite trend.The changes of protein structure in cooked salted egg yolks compared with fresh egg yolks were opposite to that in raw salted egg yolk.In general,both salt and salt-heat synergism promoted the release of free lipids in egg yolks,but heat treatment could release more lipids.Moreover,the hardness of egg yolk gel under salt-heat synergism was lower than that under salt,indicating that the production of lipids might reduce the gel strength of egg yolk.3.The migration of lipid and the changes of gelatin structure in yolk during salting were studied by using texture spectrometry(TPA),nuclear magnetic resonance imaging(MRI),low-field nuclear magnetic resonance imaging(LF-MNR),laser scanning confocal microscopy(CLSM),X-ray diffraction(XRD)and infrared spectroscopy(FTIR).The hardness,elasticity and cohesion of preserved egg yolks increased at first and then decreased during pickling.MRI showed that the yolk of fresh egg yolks had a large amount of high density protons with low mobility,but the high density protons in the inner layer of yolk gradually decreased during the pickling process.Three peaks with different fluidity were observed in the yolk by T₂relaxation time.T₂₁ showed no significant change(P>0.05),T₂₂ showed an upward trend(P<0.05),and T₂₃ showed an upward trend(P<0.05),followed by a downward trend(P<0.05)and then a stabilization phenomenon.With CLSM,it was observed that the amount of free lipids released from egg yolks decreased after 28 days of pickling,possibly due to the saponification of lipids and alkali.During pickling,the crystallinity of protein decreased.Orderly secondary structures(?-turns and?-sheets)in egg yolks increased and then decreased,while?-helices and random curls generally show an opposite trend.Therefore,strong alkali treatment could change the migration of water and lipid in egg yolk and the gel structure of egg yolk,the increase of total lipid content might reduce the gel strength of egg yolk to a certain extent.