Emulsifying Properties Of Myofibrillar Protein Of Pork

In a model system in which animal fat was replaced by plant oil,the emulsifying properties of myofibrillar protein?MP?of pork were studied.Electrical conductivity,representing emulsifying properties of MP,was measured when the ionic strength and pH of the medium were changed.In addition,the following measurements were performed to study the effect of muscle species on MP emulsifying properties,i.e.,protein-protein interaction,moisture mobility and protein structure,by SDS-PAGE,LF-NMR and Raman spectrometry.The results will not only guide the industry to optimize the technical conditions to process low-cholesterol and low-salt meat products,but also provide a theoretical basis to solve of quality defects.1.Change of electrical conductivity during MP emulsificationElectrical conductivity?EC?during the emulsification was tracked.The results showed that EC decreased as oil content increased and there were two breakpoints on EC curve.Before the first breakpoint,the O/W emulsion system was formed in which the oil droplets formed by emulsifier were surrounded by the interface membrane.After that,conductive path transferred from the continuous phase to interface membrane,and the W/O emulsion system was formed.At the second breakpoint,the formation of W/O system was completed and the conductive path was transferred from interface membrane to oil phase?continuous phase?.This was the real breakpoint for EC measurement,which was also the end point of emulsification.The ES of emulsion could be evaluated by the natural logarithmic function coefficient and stable time,and the latter was more accurate,especially in a system which had a constant initial conductivity.2.Effect of ionic strength on the emulsifying properties of MPPhysicochemical properties of MP under different ionic strength were measured,including solubility,surface tension,intermolecular force,rheological properties,water mobility,distribution characteristics and microstructure.The results showed that solubility increased but surface tension decreased with increasing ionic strength,and the solubility was positively correlated with EC?P<0.05?and to ES?P<0.01?,while surface tension was negatively correlated with EC and ES.The active sulfhydryl and hydrogen bond increased and surface hydrophobicity decreased.The active sulfhydryl and hydrogen bond were positively correlated with EC?P<0.01?,while the surface hydrophobicity was negatively correlated with EC?P<0.01?and ES?P<0.05?.Total sulfhydryl had no effect on emulsifying properties.The weight and fat content of emulsion layer increased as the concentration of NaCl increased,while the water content,protein content of emulsion layer decreased.Water and protein contents were negatively correlated with EC?P<0.01?and ES?P<0.05?,while the weight and fat content of emulsion layer were positively correlated with EC?P<0.01?.The NaCl concentration of 0.3-0.4 mol.L^-1 is a critical point for emulsifying properties.Dynamical rheology showed that MP and its emulsion had the greatest G' and G" at 0.6 mol/L NaCl.G' and G" did not change at 0.2 mol/L NaCl and the stable gel network structure cannot be formed,indicating that the ability to form emulsion gel became stronger with the increasing NaCl concentration.Emulsification increased gel-forming ability.LF-NMR showed that the T₂₁ and T₂₂ of MP emulsion and its gel decreased as NaCl concentration increased,and its peak area decreased as well.T23 was higher at 0.2 mol/L NaCl,indicating that the water mobility decreased and the immobilized water went out to be free moisture,resulting in a serious juice loss and the decreased WHC at low salt.The microstructure of MP-gel and its emulsion-gel confirmed the relationship between ionic strength and gel properties and WHC.Chemical constituents of emulsified layer can illustrate emulsifying properties accuratly.3.Effect of the pH on the emulsifying properties of myofibrillar proteinPhysicochemical properties of MP under different ionic strength were measured,including solubility,surface tension,intermolecular force,rheological properties,water mobility,distribution characteristics and microstructure of the mixture.The results showed that as pH increased,the solubility and the stability of hydrogen bond increased,but the surface tension,active sulfhydryl and the surface hydrophobicity decreased.The pH had no effect on free sulfhydryl.The solubility and hydrogen bond was positively related to EC?P<0.01?and ES?P<0.05?.Surface tension,active sulfhydryl and surface hydrophobicity was negatively correlated with EC and ES?P<0.05?.Total sulfhydryl had no effect on emulsifying properties?P>0.05?.Dynamic rheology showed that pH could change the rheological characteristics of MP and its emulsion.If pH was far from the isoelectric point,both MP and its emulsion had a good gel-forming ability.MP and its emulsion had the greatest G' and G" at pH6.5,but G'and G" changed little and the stable gel network structure was not formed at pH4.5.LF-NMR showed that the T₂₁ and T₂₂ and its peak area of MP emulsion and its gel at pH4.5 decreased as compared to the mixture at pH5.5 and pH6.5,but T₂₃ increased at pH4.5,indicating that the water mobility decreased and the immobilized water went out to be free moisture and this resulted in a serious juice loss and the decrease of WHC near to isoelectric point.The microstructure of MP-gel and its emulsion-gel confirmed the relationship between pH and gel properties and WHC.Chemical constituents of emulsified layer could be a good measurement of emulsifying properties under the different pH.4.Effect of muscle type on the emulsifying properties of myofibrillar proteinExtraction rate,solubility,surface tension,intermolecular force and protein components of MPs of three muscle types?Longissimus dorsi,LD;Psoas major,PM;M.supraspinatus,SS from pig?were measured.Rheological properties,water mobility,distribution characteristics,microstructure of MPs-gel and their emulsion-gel of three muscle types the mixture were analyzed at the same pH?pH 6.5?and ionic strength?0.6M?,EC and ES of LD?white muscle?were higher than the SS?red muscle??P<0.05?;the EC of PM?intermediate muscle?was similar to SS?P>0.05?,but the ES was higher than SS?P<0.05?.There were significant difference among viscosity,G' and G" and water holding capacity of MPs of three type muscles,and the order of them from high to low is LD>PM>SS.Extraction rate and solubility of MP of LD were higher than PM and SS.SDS-PAGE showed that myosin heavy chain?MHC?;actin myosin,light chain?MLC ??and MLC ?were of the prime importance.Protein components of SS and LD had significant difference,while the protein components of PM were similar to both LD and SS;Differences in extraction rate,solubility and amount and components of MP were the main reasons for the difference in emulsifying properties for different muscle types.Hydrogen bonds and hydrophobic force were the main intermolecular force,and hydrogen bond was another one of main reasons for emulsifying properties difference under the different muscle types.The water distribution characteristics of MP of three muscle types were different.The immobilized water went out to be free moisture,resulting in a serious juice loss and the decrease in WHC in MP-gel and its emulsion-gel of SS.Emulsification improved WHC of MPs.The result was consistent with that of effect of muscle type on WHC.The MPs and its emulsion gel of LD had orderly,uniform and compact three-dimensional network,and the interface of protein membrane was smooth and complete,which were consistent with that of effect of muscle type on rheological properties and WHC.Muscle types and emulsification did not change MP secondary structure that was not the cause to the difference in emulsifying properties among three muscle types.Heating caused the decrease in ?-helical content and the increase in ?-sheet content of MPs in order of LD,PM and SS.Heating did not change secondary structure of protein in MP emulsion.Muscle type had a significant effect on the Trp microenvironment,but not on Tyr.Emulsification and heating altered microenvironment of Trp and Tyr of LD and PM,but not for SS.Muscle type and emulsification changed the amount and conformation of-S-S-and sulfhydryl.Oil affected-S-S-,sulfhydryl and protein conformation.In summary,there was a corresponding relationship between the emulsification processing and electrical conductivity of emulsion system.There were two breakpoints,respectively corresponding to the formation of the O/W and the W/O emulsion system,the second breakpoint could be used for the judgment of emulsifying end-point.The stability of emulsion could be evaluated according to the change of the emulsion conductivity under certain conditions via the methods of coefficient and the stable time.The protein solubility,surface tension and protein molecular interactions changed along with the change of medium ionic strength and pH.These changes significantly affected EC,ES,rheological properties and the water distribution of MP.The difference in extraction rate,solubility,surface tension,intermolecular hydrogen bond force,protein components,the water distribution and microenvironment of MPs were the main causes to the difference emulsifying properties among different muscle types.