Study On Stability And Loaded Lutein Of Tilapia Protein Isolate Emulsion

Tilapia protein isolate(TPI)is a high-quality animal protein with balanced amino acid ratio and easy digestibility.However,due to the limitation of low solubility in water,its emulsifying activity and emulsifying stability are poor,and the application of TPI as emulsifier is limited.The pH value is the most obvious external factor that affects the functional properties of protein.Heat treatment is an essential operation in food processing.In this study,O/W TPI emulsion was prepared by high pressure homogenization using TPI as emulsifier.The effects of pH value and heat treatment on the stability of the TPI emulsion were investigated.On this basis,the effects of pH value combined with heat treatment on the stability and in vitro digestion of the TPI emulsion loaded with lutein were investigated.And the purpose of this study is to provide the basis for the application of fish protein in emulsion and the development of new functional protein emulsion products.1.Studies showed that the pH value(3.0-10.0)had obvious effects on the particle size,zeta potential,centrifugal stability coefficient,creaming index and microstructure of TPI emulsion.In the experimental range,the emulsion droplets coalesced and flocculated in large quantities at pH 5.0 and 6.0(near the isoelectric point of TPI),and the stability of TPI emulsion was very poor.However,when pH values were 3.0 and 10.0,TPI has the best solubility,and the TPI emulsion had smallest particle size of emulsion and the greatest absolute value of zeta potential(P < 0.05).And the emulsions had no obvious stratification on the 28 th day of storage,and which indicated that the emulsion has good stability.Circular dichroism spectra analysis showed that the stability of the emulsion was positively correlated with the α-helix content of TPI(P < 0.01)and negatively correlated with β-folding content of TPI(P < 0.05).Preliminary analysis indicated that when the pH value was far from the isoelectric point of TPI,the increase of solubility and surface net charge of TPI leaded to the change of secondary structure of protein with the increase of interfacial protein flexibility and the improvement of emulsion stability.Therefore,the solubility and emulsification of TPI could be effectively adjusted by changing the pH value of the system.2.The effect of heat treatment(60,70,80,90 ℃,30 min)on the stability of TPI emulsion and its mechanism were further tested under the selected pH values(3.0,6.0,7.0,8.0,10.0).The results showed that the surface net charge and viscosity of the emulsion increased(P < 0.05)with the further unfolding of the molecular structure of interfacial protein during heat treatment,and the enhanced intermolecular interactions could effectively delay the droplet flocculation and coalescence and extent the storage period of emulsion.And the improvement effect was different under different pH conditions.Under the condition of extreme acid(pH 3.0)and alkali(pH 10.0),TPI emulsion was more resistant to high temperature,and the average particle size of emulsion was the smallest after heat treatment for 30 min at 90 ℃(P < 0.05).Under the condition of pH 6.0,heat treatment can lead to the increase of the particle size of the emulsion,but form a stable aggregate,the viscosity of the emulsion increases,and the stability of the emulsion was significantly improved.At pH 7.0,the TPI emulsion without heat treatment has poor stability,and after heated at 70 ℃ for 30 min,the emulsion had the best stability with small and evenly distributed droplets.Therefore,controlling the heat treatment conditions could effectively improved the stability of TPI emulsion.3.TPI emulsion loaded with lutein(200 μg/m L)was prepared by high pressure homogenization using TPI as emulsifier,and the effects of pH(3.0,7.0 and 10.0)and heat treatment(70 ℃,30 min)on the stability of the emulsion were investigated.The results showed that after heat treatment,the particle size of TPI emulsion loaded with lutein decreased(P < 0.05),the surface net charge increased(P < 0.05),and the emulsion droplets were more dispersed and evenly distributed.At pH 3.0,the protein layer of the emulsion interface was loose and uncompact,and the lutein degraded with obvious color change of emulsion,and the particle size of the bu emulsion increases due to the aggregation of oil droplets.However,at pH 7.0 and 10.0,no degradation of the lutein by heat treatment at 70 ℃,and there was no delamination on the 28 th day of storage at 4 ℃ and the emulsion was stable.Therefore,controlling the pH value of the system combined with heat treatment could effectively improve the stability of the lutein loaded TPI emulsion.4.Simulated gastrointestinal static digestion in vitro studies showed that at pH 7.0 and10.0,after heat treatment at 70 ℃ for 30 min,the flocculation degree and the size of flocculate TPI emulsion loaded with lutein were smaller in the stomach digestion.The smaller flocculation promoted the transport of oil droplets to the small intestine,the release rate of free fatty acids was accelerated,and the bioavailability of lutein was improved(P < 0.05).The release rate of free fatty acids was 94.22%,and the bioavailability of lutein was 35.69% at pH7.0.However,the acidic condition of pH 3.0 was not conducive to the encapsulation of lutein in emulsion,and lutein was easily degraded.The bioavailability of TPI emulsion loaded with lutein decreased to 20.52% after heat treatment at pH 3.0.Therefore,the moderate heat treatment can enhance the stability and bioavailability of lutein loaded TPI emulsion under neutral and alkaline conditions,which provides a basis for the transport of hydrophobic active components by TPI emulsion.