| In the processing of emulsified lipid food,the application of an oil-in-water(O/W)emulsion system could improve the oxidation stability of food.The effective encapsulation of oil by surfaceactive substances could improve the oxidation stability of the oil,effectively prevented lipid oxidation in food,and changed the sensory properties and nutritional value of products.Whey protein is a common natural emulsifier and has certain antioxidant activity.However,the inherent antioxidant activity of whey protein is very small,and the protein itself will be oxidized when it plays an antioxidant role,it will reduce some functional and nutritional characteristics of the protein.Recent studies had found that adding natural antioxidants into emulsion is an effective strategy to inhibit the oxidation reaction of lipid and protein.Polyphenol compounds are effective natural antioxidants,and polyphenols can covalently or noncovalent interactions with proteins,thus forming protein-polyphenol complexes,which can improve the emulsifying properties and antioxidant capacity of proteins.Therefore,the noncovalent and covalent complexes of Whey Protein Concentration(WPC)and Quercetin(Q)were prepared in this study,and the differences in the secondary structures and tertiary structures of the two complexes,as well as the solubility,foaming,emulsifying and antioxidant properties of the protein,were compared to explore the interaction principle between Whey Protein and Quercetin.On this basis,the complex oil-in-water emulsion was prepared,and the change of antioxidant capacity of the emulsion during storage was evaluated.This study showed that the noncovalent and covalent interaction between polyphenols and protein could effectively improve the properties of the protein,and polyphenols could also play a stable antioxidant activity in protein emulsion,which provided the scientific and theoretical basis for the progress in the field of functional food.(1)Preparation and basic properties of noncovalent and covalent complexes.Whey protein concentration and quercetin were used as raw materials to prepare noncovalent complexes at p H 7and covalent complexes at p H 9.The solubility,particle size,and zeta potential of the two complexes showed that with the addition of quercetin,the solubility of the noncovalent complexes decreased by 30.34%,the particle size decreased by 8.93%,and the zeta potential increased by 30.5%.The solubility and particle size of the covalent complexes increased by 78.03% and 18.04% respectively,and the zeta potential decreased by 21.5%.It could be seen that different interactions would have different effects on the basic properties of WPC.(2)Structural characterization of the complexes.Both covalent and noncovalent interactions between whey protein concentration and the binding of quercetin could significantly affect the secondary and tertiary structure of the protein.It was also found that the covalent interaction between protein and quercetin could significantly affect the structure and conformation of the protein.The polyphenol binding capacity and SH group content of the complexes were determined.It was found that the polyphenol binding capacity of the covalent complexes was 1.68 times that of the non-covalent complexes when the quercetin added was the largest(160 μmol/g protein),and the SH group content of the covalent and non-covalent complexes decreased by 87.68% and 85.90%,respectively.And it was found that the β-folded structure of the two compounds decreased significantly by fitting Fourier transform infrared spectrum and analyzing the content of the secondary structure(p>0.05),while the β-turn and random curl content increased significantly(p>0.05).The transformation from β-fold to β-corn and random curl made the rigid structure of protein changed into the flexible structure,and then affected the functional characteristics of the protein.The fluorescence spectrum results showed that the two interactions between quercetin and protein were static quenching,and the maximum fluorescence peak red shift was observed,which could be interpreted as the unfolding of WPC structure,thus exposing tryptophan residue to a more hydrophilic environment.(3)Functional characteristics of the complexes.After quercetin was combined,it affected the structure and conformation of whey protein concentrate and then affected the functional characteristics of the protein.Compared with unmodified whey protein concentrate,the emulsifying properties of the two compounds were better.When quercetin was added at 160 μmol/g protein,the emulsifying properties and emulsifying stability of the non-covalent compounds increased by 12.84%and 26.37%,respectively.Emulsifying property of the covalent complexes increased by 29.02%,while emulsifying stability decreased by 40.25%.Emulsifying property of the covalent complexes was better than that of the non-covalent complexes,but emulsifying stability was not as good as that of the non-covalent complexes.Besides,in terms of foaming performance,the results were similar to those of emulsifying property.Preliminary analysis showed that the difference in emulsifying stability and foam stability between the two compounds might be caused by the expansion of protein structure and the introduction of polyphenols,which changed the surface charge of the protein.The emulsification and foaming properties were improved because of the interaction between polyphenol and protein,which expanded the protein structure and exposed related groups to improve the interfacial activity of the complexes.Also,after polyphenol intervention,the antioxidant capacity of the compound was obviously improved(p<0.05).because the free hydroxyl group connected to the benzene ring could play an antioxidant role when quercetin was combined with WPC,which enabled protein-polyphenol compounds to participate in scavenging free radicals and stopping the chain reaction of free radicals,thereby enhancing the antioxidant activity of the compounds.(4)Characterization of whey protein concentrate-quercetin complexes stable emulsion and determination of antioxidant activity.Nano-emulsion was prepared by the high-pressure homogenization method,and the physical stability during storage was investigated by observing the delamination phenomenon and microscopic morphology of emulsion and testing the particle size of emulsion.The results showed that the non-covalent composite emulsion did not have oil-water phase separation after 21 days of storage,while a small degree of delamination was found in the covalent composite emulsion after 21 days storage.At the same time,it was observed by the microscope that droplet aggregation occurred in both emulsions,but there were still many small droplets.Besides,after 21 days,the particle size of noncovalent composite emulsion ranged from 561.51 nm to 612.32 nm,while that of covalent composite emulsion ranged from 684.56 nm to 791.50 nm,which indicated that all the emulsions in this study had excellent physical stability,but the physical stability of the noncovalent composite emulsion was obviously higher than that of covalent composite emulsion(p<0.05).On the antioxidant stability of the emulsion,the primary peroxide value and secondary peroxide value of noncovalent composite emulsion and covalent composite emulsion both increased with the extension of storage time and were smaller than those of whey protein concentrate emulsion in the control group.It was also found that the primary peroxide value and secondary peroxide value of covalent composite emulsion were significantly lower than those of noncovalent composite emulsion after 21 days(p<0.05).This showed that the two kinds of composite emulsion had a better antioxidant effect,and the antioxidant capacity of the covalent composite emulsion was stronger than that of non-covalent composite emulsion. |
PDF Full Text Request