| Pea protein has great nutritional value and functional characteristics,which has higher application value in the food industry.However,the low solubility of commercial pea protein limits its application in food products.In order to expand the application scope of commercial pea protein,its solubility could be improved by certain modification.This paper improved the solubility of commercial pea proteins by pH shift combined with ultrasonic.Then the modified pea protein was blended with whey protein to prepare mixed protein emulsions for delivering resveratrol.Finally,mixed protein emulsion gels were prepared to explore the application prospects of modified pea proteins in nutrient delivery.The main researches and results are as follows:(1)First,the influence of ultrasonic assisted pH shift on the functional properties,digestibility and protein structure of pH shift were investigated.The effect of ultrasonic frequency(mono-frequency: 20,28,40,60 k Hz;dual-frequency: 20/28,28/40,40/60,20/40,20/60 k Hz;triple-frequency: 20/28/40,20/40/60 k Hz),multi-freque nc y ultrasonic working mode(synchronous,sequential working mode)and ultrasonic involved period(ultrasound treatment during pH shift or after pH shift)on the solubilit y of pea protein was assessed.It was found that the ultrasonic treatment(28/40 k Hz,synchronous working mode,7.5min)during pH shift is the best modification to improve the solubility of pea protein.The solubility of pea protein under that processing conditions could increased by 79%(p <0.05)comparing to pH shift treated sample.The digestibility of pea protein modified by ultrasound assisted pH shift process was higher than the untreated samples,the final release of free amino(after digest for 4h)was increased by 46% comparing to pH shift treated sample.(2)The variation of pea protein structural characteristics was further analyzed by particle size,?-potential,hydrophobic,circular dichroism spectroscopy and fluorescence spectroscopy and SDS-PAGE.The results show that the particle size,hydrophobic and ?-helix of pea protein treated by ultrasound 7.5min during the pH shift were reduced,while the absolute value of ?-potential was increased.SDS-PAGE results showed that ultrasonic-assisted pH shift induce the increase of macromolec ules(> 150 k Da)and small subunits(35-25 k Da).These results suggested that pea protein solubility may be improved with increased surface static charges and reduced surface hydrophobic groups.The increase in modified pea protein digestibility may be related to its smal er particle size and partially unfolded structure,which were more conducive to the effect of digestive enzyme.(3)Secondly,the interfacial tension,interfacial protein content,composition of emulsifying properties and apparent viscosity index were studies to assess the influences of different modified pea protein and whey protein ratio on emulsif ying property.The results showed that improved the proportion of modified pea proteins would reduce the interface tension of the protein blends.And then improved the adsorption rate,the thickness of the o/w interface film,emulsification activity and apparent viscosity of the blended emulsion.The emulsion prepared by modified pea protein was 139% higher than the emulsion prepared by whey protein(p<0.05).Laser scanning confocal microscope(CLSM)demonstrated that the o/w interface film produced by modified protein was the thickest.The blended protein were more flexib le to quickly adsorb and deploy on interface therefore increase the emulsification activit y.(4)Next,the effects of modified pea protein and whey protein ratio on physical stability and oxidative stability of emulsions were investigated by particle diameter,?-potential,appearance and microscopic morphology and malondialde hyde content(TBARS).The results showed that the higher percentage of the pea protein,the larger the particle size of the emulsions,the smal er the absolute value of ?-potential.The high salt concentration and temperature could contribute to the larger size of oil droplet.The emulsification stability and emulsion appearance results showed no significant difference on different emulsions after heat or addition of salt.The oxidative stability of the blended protein emulsion was significantly improved.And when the ratio of pea protein and whey protein was 5:5(p:w=5:5),the TBARS of the emuls io n was reduced by 53% compared to the whey protein stabled emulsion(p<0.05).The physical stability and oxidative stability of different emulsions are related to the thickness of the interface protein film and the type of adsorption protein.(5)The release free amino group and free fatty acids,?-potential,particle size and morphology were investigated to analyze the influence of different protein ratio on the digestibility of emulsions.The results showed that the release of free amino group and free fatty acid of blended protein emulsions were higher than single protein emulsion.In the gastric digestive stage,the free amino release rate of the p:w=5:5emulsion was the fastest and its final release was 42% higher than the whey protein emulsion.The quickly digestion of surface protein lead to the significantly increase of particle size.In the intestinal digestion phase,free fatty acid final release of p:w=5:5was 48% higher than the pea protein,which may be related to the significant reduction in particle size during the intestinal digestion.(6)Before the preparation of the encapsulated emulsions,the interaction between resveratrol and different blended proteins was discussed.The results of fluoresce nce quenching mechanism showed that the quenching mechanism between resveratrol and p:w=5:5 was dynamic combined static quenching.The hydrogen bonding and van der waals force was the main force to form the complex.Fluorescence spectroscopy results showed that with increasing concentration of resveratrol,the fluorescent intensity of resveratrol-protein composites were induced and the maximum absorption wavelengt h occurred red shift.These indicated that the resveratrol could expose the hydrophobic group,which is more advantageous to the hydrophobic interaction between protein and resveratrol.(7)Then the physicochemical stability were studied to assess the influences of interaction between protein and resveratrol on emulsifying property.The interfac ia l tension results showed that the protein and resveratrol binding could enhance the reducing interfacial tension capability of protein.Compared with the emulsions without resveratrol,the interfacial protein content,the absolute value of the ?-potential and the particle size of the emulsion would increase with the addition of resveratrol,and the oxidative stability was improved.The TBARS of p:w=10:0 emulsion oxidation for 45 min was 12% lower than the emulsion without resveratrol.This may because resveratrol changed the interface composition of the composite emulsion.Then the interfacial film thickness and surface charge were changed,thereby forming a physical barrier for protecting the oil droplets.The encapsulation efficiency results showed that encapsulation efficiency of p:w=5:5 emulsion was the highest,and it was 41%(p<0.05)higher than p:w=10:0.There was no significant difference in bioaccessibility and stability between different encapsulated emulsions.The bioaccessibility was about 90%,however the stability was only 30-40%.(8)In order to improve the stability of resveratrol,the emulsion carried resveratrol was filled into whey protein gel.The results showed that the emulsion gels containing resveratrol was weak and its bound water content was low.CLSM observed that the emulsion particles formed large aggregations before gelation which was different with the control.It may be that the combination of resveratrol and proteins reduced the interaction between proteins,thereby weakens the gel structure.And the strong hydrophobic interaction resulted in a decrease of bound water.The final findings indicated that encapsulation efficiency and bioaccessibility of resvertrol gels were similar with emulsions,but its stability was significantly improved.There was no significant difference in the encapsulation efficiency,bioaccessibility and stability of different gels.In summary,the ultrasonic assisted pH shift changed the secondary and tertiary structures of pea proteins,improved the solubility,emulsification,foamability and digestibility of pea proteins.The blends of modified pea protein and whey protein could increase the physicochemical stability and the digestibility of whey protein emuls io n.The mixtures of p:w=5:5 could steadily bind with resveratrol.These composite could prepared emulsions and emulsion gels,which have better encapsulation efficiency and bioaccessibility of resveratrol.And the stability of resveratrol was further improved by emulsion gel.This may be related to the stable network structure of emulsion gel and the strong interaction between resveratrol and protein. |
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