Sonochemical effects on the structure and antioxidant activity of egg white protein-tea polyphenol conjugates

Epigallocatechin gallate modification of physicochemical, structural and functional properties of egg yolk granules

The aim of this study was to prepare protein-polyphenol covalent complexes by treating egg yolk granules (EYG) with alkali in the presence of epigallocatechin gallate (EGCG) and characterize the physicochemical, structural, and functional properties of these covalent complexes. Results revealed that the optimal covalent binding occurred when the concentration of EGCG reached 0.15% (w/w), resulting in a grafting rate of 1.51 ± 0.03%. As the amount of EGCG increased, corresponding increases were observed in the particle size and ζ-potential of the complexes, thereby enhancing their stability. Furthermore, our analysis using fluorescence spectroscopy, FTIR, SEM, and SDS-PAGE collectively demonstrated the formation of a covalent complex between EYG and EGCG. Notably, the covalent complexes exhibited improved antioxidant activity and emulsifying properties. Overall, this study establishes a theoretical framework for the future practical application of EYG, emphasizing the potential of EGCG to modify its structural and functional characteristics.

A chitosan/alginate coated nano-liposome to improve intestinal absorption of curcumin for oral administration

Attempts to improve low absorption and rapid metabolic conversion of curcumin were made by developing curcumin-loaded bilayer nanoliposomes coated with chitosan and alginate for intestinal-specific drug delivery. A curcumin-loaded nano-liposome was prepared with optimized formulations with phosphatidylcholine, curcumin, chitosan, and alginate. The particle size of the optimized formulation was approximately 400 nm, and the encapsulation efficiency was more than 99%. In the in vitro release study, curcumin release from the curcumin-loaded nanoliposome with double layers of chitosan/alginate (CNL-CH/AL) was suppressed in the simulated gastric fluid (SGF, pH 1.2) and enhanced in the simulated intestinal fluid (SIF, pH 6.8). In the in vivo pharmacokinetic study in rats, the CNL-CH/AL-treated group showed a prolonged absorption pattern of curcumin and the area under the plasma concentration-time curve from 0 to 24 h (AUC0-24) was improved 109-fold compared to the control group treated with a curcumin solution without a nanocarrier.

Food-packaging applications and mechanism of polysaccharides and polyphenols in multicomponent protein complex system: A review

With the increasingly mature research on protein-based multi-component systems at home and abroad, the current research on protein-based functional systems has also become a hot spot and focus in recent years. In the functional system, the types of functional factors and their interactions with other components are usually considered to be the subjective factors of the functional strength of the system. Because this process is accompanied by the transfer of protons and electrons in the system, it has antioxidant, antibacterial and anti-inflammatory properties. Polyphenols and polysaccharides have the advantages of wide source, excellent functionality and good compatibility with proteins, and have become excellent and representative functional factors. However, polyphenols and polysaccharides are usually accompanied by poor stability, poor solubility and low bioavailability when used as functional factors. Therefore, the effect of separate release and delivery will inevitably lead to non-significant or direct degradation. After forming a multi-component composite system with the protein, the functional factor will form a stable system driven by hydrogen bonds, hydrophobic forces and electrostatic forces between the functional factor and the protein. When used as a delivery system, it will protect the functional factor, and when released, through the specific recognition of the cell membrane receptor signal, the effect of fixed-point delivery is achieved. In addition, this multi-component composite system can also form a functional composite film by other means, which has a long-term significance for prolonging the shelf life of food and carrying out specific antibacterial.

Acidity-responsive polyphenol-coordinated nanovaccines for improving tumor immunotherapy via bidirectional reshaping of the immunosuppressive microenvironment and controllable release of antigens

The tumor immunosuppressive microenvironment (TIME) and uncontrollable release of antigens can lower the efficacy of nanovaccine-based immunotherapy (NBI). Therefore, it is necessary to develop a new strategy for TIME reshaping and controllable release of antigens to improve the NBI efficacy. Herein, an acidity-responsive Schiff base-conjugated polyphenol-coordinated nanovaccine was constructed for the first time to realize bidirectional TIME reshaping and controllable release of antigens for activating T cells. In particular, an acidity-responsive tannic acid-ovalbumin (TA-OVA) nanoconjugate was prepared via a Schiff base reaction. FeIII was coordinated with TA-OVA to produce a FeIII-TA-OVA nanosystem, and 1-methyltryptophan (1-MT) as an indoleamine 2,3-dioxygenase inhibitor was loaded to form a polyphenol-coordinated nanovaccine. The coordination between FeIII and TA could cause photothermal ablation of primary tumors, and the acidity-triggered Schiff base dissociation of TA-OVA could controllably release OVA to realize lysosome escape, initiating the body's immune response. More importantly, oxidative stress generated by a tumor-specific Fenton reaction of Fe ions could promote the polarization of tumor-associated macrophages from the M2 to M1 phenotype, resulting in the upregulation of cytotoxic T cells and helper T cells. Meanwhile, 1-MT could downregulate immunosuppressive regulatory T cells. Overall, such skillful combination of bidirectional TIME reshaping and controllable antigen release into one coordination nanosystem could effectively enhance the NBI efficacy of tumors.

Ultrasound-enhanced covalent reaction of gliadin: the inhibition of antigenicity and its potential mechanisms

BACKGROUND

Wheat proteins can be divided into water/salt-soluble protein (albumin/globulin) and water/salt-insoluble protein (gliadins and glutenins (Glu)) according to solubility. Gliadins (Glia) are one of the major allergens in wheat. The inhibition of Glia antigenicity by conventional processing techniques was not satisfactory.

RESULTS

In this study, free radical oxidation was used to induce covalent reactions. The effects of covalent reactions by high-intensity ultrasound (HIU) of different powers was compared. The enhancement of covalent grafting effectiveness between gliadin and (-)-epigallo-catechin 3-gallate (EGCG) was confirmed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, matrix-assisted laser desorption/ionization-time-of-flight-mass spectrometry and Folin-Ciocalteu tests. HIU caused protein deconvolution and disrupted the intrastrand disulfide bonds that maintain the tertiary structure, causing a shift in the side chain structure, as proved by Fourier, fluorescence and Raman spectroscopic analysis. Comparatively, the antigenic response of the conjugates formed in the sonication environment was significantly weaker, while these conjugates were more readily hydrolyzed and less antigenic during simulated gastrointestinal fluid digestion.

CONCLUSION

HIU-enhanced free radical oxidation caused further transformation of the spatial structure of Glia, which hid or destroyed the antigenic epitope, effectively inhibiting protein antigenicity. This study widened the application of polyphenol modification in the inhibition of wheat allergens. © 2024 Society of Chemical Industry.

Effect of ultrasonic frequency on the structural and functional properties of pea protein isolation

BACKGROUND

Pea protein, as a by-product of peas (Pisum sativum L.), is rich in a variety of essential amino acids that can meet the body's protein needs and is a valuable source of protein. Since the function of pea protein is closely related to its structure, pea protein has been subjected to different modifications in recent years to improve its application in food and to develop new products.

RESULTS

The effects of sonication frequency (primary and secondary time) on pea protein isolate's (PPI's) structural and functional properties were investigated. Sodium dodecyl sulfate polyacrylamide gel electrophoresis demonstrated that different sonication frequencies at the same power (600 W) treatment had no effect on PPI's molecular weight. Fourier-transform infrared spectroscopy revealed that treatment at different sonication frequencies caused secondary structural changes in PPI. The particle size distribution, foaming, stability, surface hydrophobicity, emulsification, and oxidation resistance of PPI were improved after primary and secondary sonication, but secondary sonication was not more effective than primary sonication for an extended period of time.

CONCLUSION

Overall, ultrasound is able to improve the structural and functional properties of pea proteins within a suitable range. It provides a theoretical basis for elucidating the modification of the structure and function of plant proteins by ultrasound and lays the foundation for the development of plant proteins in food applications as well as development. © 2023 Society of Chemical Industry.

The differential non-covalent binding of epicatechin and chlorogenic acid to ovotransferrin and the enhancing efficiency of immunomodulatory activity

Seeking safe and environmentally friendly natural immunomodulators is a pressing requirement of humanity. This study investigated the differential binding characteristics of two polar polyphenols (PP), namely epicatechin (EC) and chlorogenic acid (CA), to ovotransferrin (OVT), and explored the relationship between structural transformations and immunomodulatory activity of OVT-PP complexes. Results showed that CA exhibited a stronger affinity for OVT than EC, mainly driven by hydrogen bonds and van der Waals forces. Complexation-induced conformational variations in OVT, including static fluorescence quenching, increased microenvironment polarity surrounding tryptophan and tyrosine residues, and the transition from disordered α-helix to stable β-sheet. Furthermore, the structural conformation transformation of OVT-PP complexes facilitated the enhancement of immunomodulatory activity, with the OVT-CA (10:2) complex demonstrating the best immunomodulatory activity. Principal component analysis (PCA) and Pearson correlation analysis revealed the immunomodulatory activities of the OVT-PP complexes were influenced by surface hydrophobicity (negatively correlated), β-sheet percentage and polyphenol binding constants. It could be inferred that PP complexation increased the surface polarity of OVT, consequently enhancing its immunomodulatory activity by promoting cell membrane affinity and antigen recognition. This study provides valuable guidance for effectively utilizing polyphenol-protein complexes in enhancing immunomodulatory activity.

Covalent and non-covalent interaction of myofibrillar protein and cyanidin-3-O-glucoside: focus on structure, binding sites and in vitro digestion properties

BACKGROUND

The aim of this study was to investigate the effects of covalent and non-covalent interactions between myofibrillar protein (MP) and cyanidin-3-O-glucoside (C3G) on protein structure, binding sites, and digestion properties. Four methods of inducing covalent cross-linking were used in the preparation of MP-C3G conjugates, including tyrosinase-catalyzed oxidation, alkaline pH shift treatment, free radical grafting, and ultrasonic treatment. A comparison was made between MP-C3G conjugates and complexes, and the analysis included sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), C3G binding ratio, liquid chromatography-tandem mass spectrometry (LC-MS/MS), protein side-chain amino acids, circular dichroism spectroscopy, three-dimensional fluorescence, particle size, and in vitro simulated digestion.

RESULTS

Covalent bonding between C3G and amino acid side chains in MP was confirmed by LC-MS/MS. In covalent bonding, tryptophan residues, free amino groups and sulfhydryl groups were all implicated. Among the 22 peptides covalently modified by C3G, 30 modification sites were identified, located in lysine, histidine, tryptophan, arginine and cysteine. In vitro simulated digestion experiments showed that the addition of C3G significantly reduced the digestibility of MP, with the covalent conjugate showing lower digestibility than the non-covalent conjugate. Moreover, the digestibility of protein decreased more during intestinal digestion, possibly because covalent cross-linking of C3G and MP further inhibited trypsin targeting sites (lysine and arginine).

CONCLUSION

Covalent cross-linking of C3G with myofibrillar proteins significantly affected protein structure and reduced protein digestibility by occupying more trypsin binding sites. © 2023 Society of Chemical Industry.

Comparative study of dietary phenols with Tartary buckwheat protein (2S/13S): impact on structure, binding sites and functionality of protein

BACKGROUND

This research was to investigate the interaction mechanism between 2S albumin and 13S globulin (2S and 13S, the most important storage proteins in Tartary buckwheat seeds) and three phenols (rutin, quercetin and myricetin) regarding the structural and antioxidant properties of their complexes.

RESULTS

There are differences in the binding affinity of phenols for 2S and 13S. Rutin had a higher binding affinity for 2S, myricetin had a higher binding affinity for 13S, and 13S exhibited a higher affinity toward phenols than did 2S. Binding with phenols significantly changed the secondary and tertiary structures of 2S and 13S, decreased the surface hydrophobic value and enhanced the antioxidant capacity. Molecular docking and isothermal titration calorimetry showed that the binding processes were spontaneous and that there were hydrogen bonds, hydrophobic bonds and van der Waals force interactions between phenols and proteins.

CONCLUSION

These findings could provide meaningful guidance for the further application of buckwheat protein complex. © 2023 Society of Chemical Industry.

Construction of protein-, polysaccharide- and polyphenol-based conjugates as delivery systems

Natural polymers, such as polysaccharides and proteins, have been used to prepare several delivery systems owing to their abundance, bioactivity, and biodegradability. They are usually modified or combined with small molecules to form the delivery systems needed to meet different needs in food systems. This paper reviews the interactions of proteins, polysaccharides, and polyphenols in the bulk phase and discusses the design strategies, coupling techniques, and their applications as conjugates in emulsion delivery systems, including traditional, Pickering, multilayer, and high internal-phase emulsions. Furthermore, it explores the prospects of the application of conjugates in food preservation, food development, and nanocarrier development. Currently, there are seven methods for composite delivery systems including the Maillard reaction, carbodiimide cross-linking, alkali treatment, enzymatic cross-linking, free radical induction, genipin cross-linking, and Schiff base chemical cross-linking to prepare binary and ternary conjugates of proteins, polysaccharides, and polyphenols. To design an effective target complex and its delivery system, it is helpful to understand the physicochemical properties of these biomolecules and their interactions in the bulk phase. This review summarizes the knowledge on the interaction of biological complexes in the bulk phase, preparation methods, and the preparation of stable emulsion delivery system.

Structure Characterization and Functional Properties of Flaxseed Protein-Chlorogenic Acid Complex

This study aimed to investigate the effects of the covalent binding of flaxseed protein (FP) and chlorogenic acid (CA) on the structure and functional properties of FP-CA complexes fabricated using the alkali method. The results suggested that the encapsulation efficiency of CA encapsulated by FP ranged from 66.11% to 72.20% and the loading capacity of CA increased with an increasing addition ratio of CA with a dose-dependent relationship, which increased from 2.34% to 10.19%. The particle size, turbidity, zeta potential and PDI of FP and the FP-CA complexes had no significant discrepancy. UV-Vis and fluorescence spectra showed the existence of the interaction between FP and CA. SEM images showed that the surface of the FP-0.35%CA complex had more wrinkles compared to FP. Differential scanning calorimetry analysis indicated the decomposition temperature of FP at 198 °C was higher than that (197 °C) of the FP-0.35%CA complex, implying that the stability of the FP-CA complexes was lower than FP. The functional properties suggested that the FP-CA complex with 1.40% CA had a higher water holding capacity (500.81%), lower oil holding capacity (273.495%) and lower surface hydrophobicity. Moreover, the FP-CA complexes had better antioxidant activities than that of FP. Therefore, this study provides more insights for the potential application of FP-CA covalent complexes in functional food processing.

Ultrasound-assisted preparation of protein-polyphenol conjugates and their structural and functional characteristics

Herein, ultrasound-assisted conventional covalent binding methods (alkali treatment, free radical mediation, and an enzymatic method) were used to prepare soybean protein isolate (SPI)-(-)-epigallocatechin gallate (EGCG) conjugates to investigate the enhancement effect of the ultrasound synergistic treatment. In addition, the influence of EGCG grafting on the structure and properties of SPI was evaluated via reactive group analysis, spectral analysis, surface hydrophobicity measurements, emulsification property assessment, and α-glucosidase inhibition analysis. The obtained results revealed that the enzymatic method produced the highest polyphenol grafting content among the conventional techniques. Meanwhile, ultrasound treatment increased the amount of grafted polyphenol species during the alkali treatment and free radical mediation procedure, decreased the grafting efficiency in the enzymatic method, and maximized the grafting efficiency during the alkali treatment. In addition, reactive group and spectral analyses demonstrated that EGCG formed C-N and C-S bonds with SPI and decreased the α-helix content in the protein structure, thereby increasing the molecular flexibility of SPI. It also produced hydrogen bonds and hydrophobic interactions, as demonstrated by the results of molecular docking. Furthermore, the EGCG grafting of SPI conducted under the ultrasound-assisted conditions endowed SPI with unique functional characteristics, including good emulsification and antioxidant properties and high α-glucosidase inhibitory activity, while the ultrasound-assisted alkali treatment resulted in the optimal functional properties. The results of this study provide new insights into the effective preparation of SPI-EGCG complexes with multiple functionalities, thereby expanding the scope of high-value SPI utilization.

Characterization of walnut protein isolate-polyphenol nanoconjugates for the developing a food-grade delivery system

This study investigated the effects of the interaction of walnut protein isolate (WPI) with epigallocatechin gallate (EGCG), chlorogenic acid (CLA), (+)-catechin (CA), and ellagic acid (EA) on the structural and functional properties of proteins. The results for polyphenol binding equivalents and content of free amino and sulfhydryl groups as well as those from sodium dodecyl sulfate‒polyacrylamide gel electrophoresis confirmed the covalent interaction between WPI and the polyphenols. The binding capacities of the WPI-polyphenol mixtures and conjugates were as follows: WPI-EGCG > WPI-CLA > WPI-CA > WPI-EA. Fourier transform infrared spectroscopy (FTIR) and fluorescence spectrum analysis identified changes in the protein structure. The conjugation process obviously increased the polyphenols' antioxidant properties and the surface hydrophobicity was substantially reduced. WPI-EGCG conjugates had the best functional properties, followed by WPI-CLA, WPI-CA, and WPI-EA. Lycopene (LYC) was loaded into nanocarriers by WPI-EGCG self-assembly. These results indicated that WPI-polyphenol conjugates can be utilized to develop food-grade delivery systems to protect chemically lipophilic bioactive compounds.

Graphical abstract

Supplementary Information

The online version contains supplementary material available at 10.1007/s13197-023-05768-2.

Biotransformation of white and black mustard grains through germination and enzymatic hydrolysis revealed important metabolites for antioxidant properties and cytotoxic activity against Caco-2 cells

Germination and enzymatic hydrolysis are biological processes with well-recognized positive effects on phenolic composition and antioxidant potential. This study aimed to apply those processes to white (Sinapsis alba) and black (Brassica nigra) mustard grains and to analyze the influences on the total phenolic content (TPC); phenolic and peptide profile determined by ultra-performance liquid chromatography-high resolution mass spectrometry (UPLC-HRMS); antioxidant potential (DPPH, ABTS, and FRAP assays); and cytotoxicity against Caco-2, a human colorectal adenocarcinoma cell line. Enzyme combinations for hydrolysis were different for each mustard grain, but for both species, enzymatic hydrolysis and germination showed a positive effect on antioxidant properties. From UPLC-HRMS analysis and molecular network studies, 14 peptides and 17 phenolic compounds were identified as metabolites released from mustard after processes application, which were strongly correlated with increased antioxidant activity. In addition, enzymatic hydrolysis applied in germinated mustard grains for both mustards increased the cytotoxic activity against Caco-2 human colorectal adenocarcinoma cell line.

Royal jelly mediates fibrotic signaling, collagen cross-linking and cell proliferation in cardiac fibroblasts

Royal jelly (RJ) is a multifunctional bee product with a unique composition and wide-ranging biological properties, including antioxidant, anti-inflammatory and antiproliferative activities. Still, little is known about the possible myocardial protective properties of RJ. Considering that sonication could enhance RJ bioactivity, this study aimed to assess the effects of non-sonicated (NS) and sonicated (S) RJ on fibrotic signaling, cell proliferation, and collagen production in cardiac fibroblasts. S-RJ was produced by ultrasonication at 20 kHz. Ventricular fibroblasts isolated from neonatal rats were cultured and treated with different concentrations of NS-RJ or S-RJ (0, 50, 100, 150, 200, and 250 µg/well). S-RJ significantly depressed the expression levels of transglutaminase 2 (TG2) mRNA across all the concentrations tested and was inversely associated with the expression of this profibrotic marker. S-RJ and NS-RJ displayed distinct dose-dependent effects on mRNA expression of several other profibrotic, proliferation, and apoptotic markers. Unlike NS-RJ, S-RJ elicited strong negative dose-dependent relationships with the expression of profibrotic markers (TG2, COL1A1, COL3A1, FN1, CTGF, MMP-2, α-SMA, TGF-β1, CX43, periostin), as well as proliferation (CCND1) and apoptotic (BAX, BAX/BCL-2) markers, indicating that RJ dose-response effects were significantly modified by sonification. NS-RJ and S-RJ increased the content of soluble collagen, while decreasing collagen cross-linking. Collectively, these findings show that S-RJ has a greater range of action than NS-RJ for downregulating the expression of biomarkers associated with cardiac fibrosis. Reduced biomarker expression and collagen cross-linkages upon cardiac fibroblast treatment with specific concentrations of S-RJ or NS-RJ suggests putative roles and mechanisms by which RJ may confer some protection against cardiac fibrosis.

Ultrasound-assisted free radical modification on the structural and functional properties of ovalbumin-epigallocatechin gallate (EGCG) conjugates

The influence of ultrasound-assisted free radical modification on the structure and functional properties of ovalbumin-epigallocatechin gallate (OVA-EGCG) conjugates was investigated by experimental measurements and computer simulations. Compared with the traditional free radical condition, the ultrasonic-assisted processing significantly increased the conjugating efficiency of OVA and EGCG and shortened the conjugating from 24 h to 1 h without affecting the equivalent amount of EGCG conjugating. The sodium dodecyl sulfate-polyacrylamide gel electrophoresis and multi-spectroscopy analysis (Fourier transform infrared spectroscopy, intrinsic fluorescence spectroscopy, and UV spectroscopy) indicated that the covalent conjugates could be formed between OVA and EGCG. And modification in the conformation of OVA was induced by EGCG. Furthermore, molecular docking results demonstrated the possession of high-affinity EGCG binding location on OVA, supporting and clarifying the experimental results. In addition, the functional properties of OVA including emulsification (emulsifying activity and emulsion stability) and antioxidant properties (DPPH scavenging capacity and ABTS scavenging capacity) were significantly improved after conjugation with EGCG, especially in ultrasound-assisted conditions. Overall, OVA-EGCG conjugates produced by ultrasound-assisted free radical treatment could be applied as a potential emulsifier and antioxidant, thereby expanding the application of OVA as a dual-functional ingredient.

Ultrasound treatment enhanced the functional properties of phycocyanin with phlorotannin from Ascophyllum nodosum

Introduction

Phycocyanin offers advantageous biological effects, including immune-regulatory, anticancer, antioxidant, and anti-inflammation capabilities. While PC, as a natural pigment molecule, is different from synthetic pigment, it can be easily degradable under high temperature and light conditions.

Methods

In this work, the impact of ultrasound treatment on the complex of PC and phlorotannin structural and functional characteristics was carefully investigated. The interaction between PC and phlorotannin after ultrasound treatment was studied by UV–Vis, fluorescence spectroscopy, circular dichroism (CD) spectroscopy, fourier transform infrared (FTIR) spectroscopy. Additionally, the antioxidant potential and in vitro digestibility of the complexes were assessed.

Results

The result was manifested as the UV–Vis spectrum reduction effect, fluorescence quenching effect and weak conformational change of the CD spectrum of PC. PC was identified as amorphous based on the X-ray diffraction (XRD) data and that phlorotannin was embedded into the PC matrix. The differential scanning calorimetry (DSC) results showed that ultrasound treatment and the addition of phlorotannin could improve the denaturation peak temperatures (Td) of PC to 78.7°C. In vitro digestion and free radical scavenging experiments showed that appropriate ultrasound treatment and the addition of phlorotannin were more resistant to simulated gastrointestinal conditions and could improve DPPH and ABTS+ free radical scavenging performance.

Discussion

Ultrasound treatment and the addition of phlorotannin changed the structural and functional properties of PC. These results demonstrated the feasibility of ultrasound-assisted phlorotannin from A. nodosum in improving the functional properties of PC and provided a possibility for the application of PC-polyphenol complexes as functional food ingredients or as bioactive materials.

Identification of binding sites for Tartary buckwheat protein-phenols covalent complex and alterations in protein structure and antioxidant properties

To investigate the effects of structure, multiple binding sites and antioxidant property of Tartary buckwheat protein-phenols covalent complex, protein was combined with different concentrations of phenolic extract. Four kinds of phenols were identified by UPLC-Q/TOF-MS, which were rutin, quercetin, kaempferol and myricetin. UV-vis absorption spectroscopy and X-ray diffraction showed that the phenols can successfully bind to BPI. Fourier-transform infrared, circular dichroism and fluorescence emission spectroscopy showed that the binding of phenol can change the secondary/tertiary structure of protein. The particle distribution indicated that the binding of phenols could reduce the particle size (from 304.70 to 205.55 nm), but cross-linking occurred (435.35 nm) when the bound phenol content was too high. Proteomics showed that only rutin, quercetin and myricetin can covalently bind to BPI. Meanwhile, 4 peptides covalently bound to phenols were identified. The DPPH· scavenging capacity of complexes were from 8.38 to 33.76 %, and the ABTS·⁺ binding activity of complexes were from 19.35 to 63.99 %. The antioxidant activity of the complex was significantly higher than that of the pure protein. These results indicated that protein-phenol covalent complexes had great potential as functional components in the food field.

A review of the structure, function, and application of plant-based protein-phenolic conjugates and complexes

Interactions between plant-based proteins (PP) and phenolic compounds (PC) occur naturally in many food products. Recently, special attention has been paid to the fabrication of PP-PC conjugates or complexes in model systems with a focus on their effects on their structure, functionality, and health benefits. Conjugates are held together by covalent bonds, whereas complexes are held together by noncovalent ones. This review highlights the nature of protein-phenolic interactions involving PP. The interactions of these PC with the PP in model systems are discussed, as well as their impact on the structural, functional, and health-promoting properties of PP. The PP in conjugates and complexes tend to be more unfolded than in their native state, which often improves their functional attributes. PP-PC conjugates and complexes often exhibit improved in vitro digestibility, antioxidant activity, and potential allergy-reducing activities. Consequently, they may be used as antioxidant emulsifiers, edible film additives, nanoparticles, and hydrogels in the food industry. However, studies focusing on the application of PP-PC conjugates and complexes in real foods are still scarce. Further research is therefore required to determine the structure-function relationships of PP-PC conjugates and complexes that may influence their application as functional ingredients in the food industry.

Ultrasonication significantly enhances grafting efficiency of chitosan-ferulic acid conjugate and improves its film properties under Fenton system

Ultrasonication (US)-assisted Fenton-system (US-Fenton) with different US time was developed for synthesizing chitosan (CS)-ferulic acid (FA) conjugates. The optimal US-Fenton for a suitable time was selected for preparing a film with CS-FA conjugate and its structural, functional, rheological, and physical properties were also investigated. Compared with Fenton-system, US-Fenton enhanced the grafting ratio of the conjugates, which increased firstly and then decreased as US time. The conjugate obtained by US-Fenton for 1 min (FUS1) possessed the highest grafting ratio (121.28 mg FA/g) and its grafting time was also shortened from 12 h to 1 min contrasted with Fenton grafted method. Structural characterization results showed that FA was conjugated on CS via ester and amide bonds with decreased crystallinity. Scanning electron microscopy and molecular weight analysis indicated that the degradation degree of CS-FA conjugates increased with US time. The DPPH and ABTS radical-scavenging activities of FUS1 were the closest to ascorbic acid, and it also showed the best antibacterial effect among the test conjugates. Accordingly, FUS1 was selected to obtain the film for contrasting with CS film. FUS1 film solution exhibited a decreased viscosity. In comparison to CS film, UV transmittance of FUS1 film approached zero, and its moisture, oxygen, and carbon dioxide permeabilities significantly decreased (P < 0.05). Moreover, its water solubility and tensile strength increased by 58.09% and 25.72% than those of CS film, respectively. Therefore, US-Fenton for 1 min could be a promising method for efficiently preparing active food package materials and FUS1 film possessed broad application prospects.

Sonochemical effects on fabrication, characterization and antioxidant activities of β-lactoglobulin-chlorogenic acid conjugates

The β-lactoglobulin-chlorogenic acid (LG-CA) conjugate was explored to be formed through ultrasonication, redox-pair method and their combination, the ultrasonication used a probe ultrasonic machine with a 6 mm probe at 270 W, and the frequency was 20-25 kHz. The formation of the conjugate was confirmed by SDS-PAGE with a larger molecular weight. Besides, Fourier infrared spectroscopy (FTIR) and Circular dichroism (CD) indicated changes in the secondary structure of the LG-CA conjugate. The α-helix and β-sheet contents of LG decreased and the unordered content increased significantly after the formation of covalent complexes. In addition, both the ultrasonic treatment and its combination with redox-pair method could significantly improve the antioxidant properties of LG. The former increased to 23.16 μmol Trolox/g sample, the latter 82-106 μmol Trolox/g sample. Therefore, ultrasonication could be used both individually and in combination with the redox-pair method to produce LG-CA conjugates with stronger antioxidant activities.

Pasteurization induced protein interaction decreased the potential allergenicity of ovalbumin and ovomucoid in egg white

BACKGROUND

Approximately 9.9% of young children in China suffer from egg allergies. Ovalbumin (OVA) and ovomucoid (OVM) are both the main allergens with higher allergenicity in egg white. The previous studies mainly focused on the effects of pasteurization on the structure and allergenicity of the isolated protein itself. The effects of the interaction between OVA and OVM on their spatial structure and allergenicity under pasteurization are still unclear. Therefore, in this study, the spectroscopic, immunological, and cytological methods were used to investigate the effects on OVA and OVM by their interactions which were induced by the following pasteurization, heating for 10 min at 60, 65, and 70 °C, respectively.

RESULTS

Results indicated that OVA and OVM could form macromolecular aggregates by their interaction at 70 °C, and their solubility was decreased while turbidity was increased. The spatial structures of OVA and OVM were both changed by their interaction, when pasteurization temperature was at 70 °C the exposure of their hydrophobic groups and α-helix content were decreased while their β-sheet was increased. The potential allergenicity of OVA and OVM was also changed, which showed that the immunoglobulin G (IgG)-binding ability of OVA and OVM could be increased, and their IgE-binding ability was decreased a bit. The releases of interleukin 4 (IL-4), IL-6, β-HEX, histamine and tumor necrosis factor alpha (TNF-α) from OVA-OVM-induced KU812 cells were all decreased at 70 °C.

CONCLUSION

Therefore, according to the results, if the liquid egg products were pasteurized for 10 min, the temperature of 70 °C should be carefully considered. © 2022 Society of Chemical Industry.

Reducing the potential allergenicity of amandin through binding to (-)-epigallocatechin gallate

•

Amandin (AMP) binding to EGCG changed protein structure.

•

AMP bound to EGCG primarily through glutamate and cysteine residues.

•

Alkaline and free radical methods dented AMP allergenic, but the principles differed.

Potential allergenicity of amandin was reduced by binding amandin with (−)-epigallocatechin gallate (EGCG) via alkaline, free radical, ultrasound-assisted alkaline, and ultrasound-assisted free radical methods. These results of total phenol content, free sulfhydryl group, free amino group, surface hydrophobicity, and sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) indicated that amandin might be covalently bound to EGCG through reactive groups such as sulfhydryl and amino groups, or non-covalently through hydrophobic interactions. Fourier transformed infrared (FT-IR) spectroscopy and fluorescence spectroscopy revealed structural changes of amandin-EGCG conjugate, which also caused significant reduction in potential allergenicity of amandin. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) found that amandin bound to EGCG mainly through cysteine and glutamate residues, and linear epitope for amandin was reduced. This provided a new method and theoretical basis of hypoallergenic almond food.

Boosting the Bioaccessibility of Dietary Bioactives by Delivery as Protein-Polyphenol Aggregate Particles

Protein-polyphenol aggregate particles concurrently fortify a functional food product with healthy dietary proteins and concentrated polyphenols. However, what impact does ingestion of aggregate particles have on ultimate health relevance of either the polyphenolic molecules in the matrix or the protein molecules? Because human health benefits are contingent on bioavailability after ingestion, the fate of these molecules during transit in the gastrointestinal tract (GIT) will dictate their utility as functional food ingredients. This brief review explores diverse applications of protein-polyphenol particles in the food industry and the bioaccessibility of both bioactive polyphenolic compounds and edible proteins. Evidence to date suggests that complexation of phytoactive polyphenolics effectively enhances their health-relevant impacts, specifically because the phytoactives are protected in the protein matrix during transit in the GIT, allowing intact, non-degraded molecules to reach the colon for catabolism at the gut microbiome level, a prerequisite to realize the health benefits of these active compounds.

Covalent and Noncovalent Complexation of Phosvitin and Gallic Acid: Effects on Protein Functionality and In Vitro Digestion Properties

To investigate the effects of different binding modes on the structure, function, and digestive properties of the phosvitin (Pv) and gallic acid (GA) complex, Pv was covalently and noncovalently combined with different concentrations of GA (0.5, 1.5, and 2.5 mM). The structural characterization of the two Pv-GA complexes was performed by Fourier transform infrared, circular dichroism, and LC-MS/MS to investigate the covalent and noncovalent binding of Pv and GA. In addition, the microstructure of the two Pv-GA complexes was investigated by super-resolution microscopy and transmission electron microscopy. The particle size and zeta potential results showed that the addition of GA increased the particle size and the absolute potential of Pv. The determination of protein digestibility, polyphenol content, SH and S-S group levels, sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and antioxidant capacity of the digests indicated that noncovalent complexes had greater antioxidant and protective effects on polyphenols. Molecular docking revealed that GA was conjugated with Pv through hydrogen bond interactions.

Ultrasound-assisted preparation of lactoferrin-EGCG conjugates and their application in forming and stabilizing algae oil emulsions

The aim of this study was to prepare lactoferrin-epigallocatechin-3-gallate (LF-EGCG) conjugates and to determine their ability to protect emulsified algal oil against aggregation and oxidation. LF-EGCG conjugates were formed using an ultrasound-assisted alkaline treatment. The ultrasonic treatment significantly improved the grafting efficiency of LF and EGCG and shortened the reaction time from 24 h to 40 min. Fourier transform infrared spectroscopy and circular dichroism spectroscopy analyses showed that the covalent/non-covalent complexes could be formed between LF and EGCG, with the CO and CN groups playing an important role. The formation of the conjugates reduced the α-helix content and increased the random coil content of the LF. Moreover, the antioxidant activity of LF was significantly enhanced after conjugation with EGCG. LF-EGCG conjugates as emulsifiers were better at inhibiting oil droplet aggregation and oxidation than LF alone. This study demonstrates that ultrasound-assisted formation of protein-polyphenol conjugates can enhance the functional properties of the proteins, thereby extending their application as functional ingredients in nutritionally fortified foods.

Evaluation of crossing-linking sites of egg white protein-polyphenol conjugates: Fabricated using a conventional and ultrasound-assisted free radical technique

There has been strong interest in developing effective strategies to inhibit lipid oxidation in emulsified food products such as ω-3 fatty acids, carotenoids, or carotenoids. Dual-functional protein emulsifiers with antioxidant and emulsifying properties are in the spotlight. Our aim was to investigate the influence of caffeic acid (CF), chlorogenic acid (CA) with a C3-C6 structure, epigallocatechin gallate (EGCG), catechin (CT), and quercetin (QE) with a C6-C3-C6 structure on the cross-linking sites and structure of egg white protein (EWP)-polyphenol conjugates fabricated by the free radical method under conventional water bath (WB) and ultrasound assisted (US) conditions. Results of structural analysis and liquid chromatography-tandem mass spectrometry indicated that the structure of EWP-polyphenol conjugates and the cross-linking sites of the two are influenced by the polyphenol structure and the free radical system. Our study provides important information about the mechanism of research into proteins and polyphenols using the free radical method.

Altering functional properties of rice protein hydrolysates by covalent conjugation with chlorogenic acid

•

Rice protein hydrolysate was covalently conjugated to chlorogenic acid by three methods.

•

Covalent conjugation resulted in an unfolded structure of rice protein hydrolysate.

•

LYS might be the binding sites for chlorogenic acid grafted on rice protein hydrolysate.

•

Conjugates formed by alkaline method exhibited highest functional property.

Proteins and phenolic compounds are common components in foods that readily interact with each other to yield complexes, leading to changes in the functional properties. In this study, we investigated the effect of covalent conjugation of rice protein hydrolysates (RPH) with chlorogenic acid (CA) on the structural and functional properties of RPH. Three RPH-CA conjugates were prepared by the alkaline, enzyme, and free radical methods, respectively. Covalent conjugation decreased the content of free amino, thiol, and tyrosine groups, and increased in the amount of CA bounds from 15.23 to 21.11 nmol/mg. Moreover, the circular dichroism analysis revealed that covalent conjugation resulted in an increase of random coils. The emulsifying activity and antioxidant capacity of RPH were also improved by the covalent conjugation with CA. This work provides a better understanding of the formation of hydrolysates-chlorogenic acid conjugates, contributing to improving the functional properties of foods.

Impact of covalent grafting of two flavonols (kaemperol and quercetin) to caseinate on in vitro digestibility and emulsifying properties of the caseinate-flavonol grafts

Covalent grafting of one of the two flavonols (kaemperol and quercetin) to caseinate was achieved by a reaction between the heat-oxidized flavonols and caseinate at flavonol-lysine molar ratios of 1:100 and 1:200. Grafted caseinate products (GCPs) showed - NH₂ content reduction and respective kaemperol and quercetin contents of 1.08-6.13 and 3.23-6.64 mmol/kg protein. Quercetin was more reactive than kaemperol under the same conditions, while long-time flavonol heat and higher flavonol-lysine molar ratio caused greater flavonol-grafting. GCPs subjected to 180-day storage had further flavonol-grafting, -NH₂ content decrease, and weak protein crosslinking. GCPs consistently had higher surface hydrophobicity but lower emulsification and digestibility than caseinate, while greater flavonol-grafting caused a remarkable value change. Meanwhile, the Kjeldahl method was more suitable than the UV-absorption method to evaluate protein digestibility, because the grafted flavonols in this case did not interfere with data results. Collectively, the covalent flavonol-grafting of proteins can impact the assayed protein functionalities.

Ultrasound-assisted assembly of β-lactoglobulin and chlorogenic acid for non covalent nanocomplex: fabrication, characterization and potential biological function

It is essential to understand the ultrasound-induced changes in assembly of proteins and polyphenols into non covalent nanocomplex. β-Lactoglobulin (LG) and chlorogenic acid (CA) with various biological activities can be combined to form food-grade nanocomplexes. This study systematically explored the role of high-intensity ultrasound pretreatment on the binding mechanisms of LG and CA, and the potential biological function for embedding curcumin (Cur). The scanning electron microscopy (SEM) revealed that ultrasound treatment could destroy the structure of LG, and the particle size of the protein was reduced to<50 nm. The change in secondary structure of the protein by ultrasound treatment could be revealed by the fourier transform infrared (FTIR) and fluorescence spectra. Besides, it was found that LG and CA were combined to form a complex under the hydrophobic interaction, and CA was bound in the internal cavity of LG with a relatively extended conformation. The result demonstrated that the ratio of Cur embedded in the ultrasonic sample could be effectively increased by 7% - 10%, the particle size in the emulsion was smaller, and the dispersion was more stable. This work contributes to the development of protein-polyphenol functional emulsion systems with the ability to deliver Cur.

Recent advances in food applications of phenolic-loaded micro/nanodelivery systems

The current relevance of a healthy diet in well-being has led to a surging interest in designing novel functional food products enriched by biologically active molecules. As nature-inspired bioactive components, several lines of research have revealed the capability of polyphenolic compounds (phenolics) in the medical intervention of different ailments, i.e., tumors, cardiovascular and inflammatory diseases. Phenolics typically possess antioxidant and antibacterial properties and, due to their unique molecular structure, can offer superior platforms for designing functional products. They can protect food ingredients from oxidation and promote the physicochemical attributes of proteins and carbohydrate-based materials. Even though these properties contribute to the inherent benefits of bioactive phenolics as important functional ingredients in the food industry, the in vitro/in vivo instability, poor solubility, and low bioavailability are the main factors restricting their food/pharma applicability. Recent advances in the encapsulation realm are now offering efficient platforms to overcome these limitations. The application of encapsulation field may offer protection and controlled delivery of phenolics in food formulations. Here, we review recent advances in micro/nanoencapsulation of phenolics and highlight efficient carriers from this decade, which have been utilized successfully in food applications. Although further development of phenolic-containing formulations promises to design novel functional food formulations, and revolutionize the food industry, most of the strategies found in the scientific literature are not commercially applicable. Moreover, in vivo experiments are extremely crucial to corroborate the efficiency of such products.

Effect of simultaneous treatment combining ultrasonication and rutin on gliadin in the formation of nanoparticles

Proteins, one of the vital nutritional compounds sensitive to the environment, can be modified by interaction with polyphenols. Ultrasonication has been applied for enhancing the functional properties of proteins. In this study, the interactions of gliadin (G) and rutin (R) in the absence and presence of ultrasonication (0, 150, 300, 450, and 600 W) for 20 min were investigated, with a focus on the properties of emulsions prepared by G-R complexes. Ultrasonication improved the interaction, which increased the content of β-type secondary structure. Ultrasonication at 450 W increased the particle size of the conjugates. For Pickering emulsions, treating the covering of R on G with ultrasonication improves the stability of the G-based emulsion significantly, owing to the strong films formed on the oil-water interfaces. The G-R complexes treated at 450 W ultrasonication formed emulsions that showed higher potential and storage modulus (G') and denser microstructures than those of the untreated emulsions. Nevertheless, ultrasound treatment at 600 W weakened the emulsion properties that were stabilized by the conjugates. Ultrasound combined R was shown to be a potential processing technology for changing the protein structure and producing stable emulsions. PRACTICAL APPLICATION: The interactions between proteins and polyphenols are able to preserve the stability of the functional compounds, allow targeted and controlled release, and improve the texture of these complexes employed in the food industry. Improvements in the functional characteristics of the protein-polyphenol complexes so that they possess high emulsifying stability during food processing is a crucial factor for employing them in the food industry. Therefore, the aim of this research is using a soluble complex of gliadin-rutin for the development of its functional characteristics.

Mechanism of ultrasound and tea polyphenol assisted ultrasound modification of egg white protein gel

In order to improve the characteristics of egg white protein gel and the stability of egg white protein gel products. In this work, the changes of the texture, physicochemical properties, secondary structure and microstructure of ultrasound modified egg white protein gel (UEP) and tea polyphenols (TP) assisted ultrasound modification of egg white protein gel (TUEP) with different ultrasonic power (0-360 W) were studied. With the increase of ultrasonic power, soluble protein, surface hydrophobicity and disulfide bonds of TUEP and UEP showed an increasing trend. In particular, with the increase of ultrasonic power, the content of intramolecular β-sheets and α-helices of TUEP showed an increasing trend, and significantly improved the relaxation time and microstructure, thus enhancing TUEP gel stability. In addition, the hardness and water holding capacity (WHC) of TUEP and UEP can be increased by ultrasonic treatment, and when the ultrasonic power reached 120 W, the hardness and WHC reached the maximum. The hardness and WHC of TUEP were significantly better than that of UEP. SDS-PAGE results showed that the peptide chain of protein without being broken under ultrasonic treatment. Ultrasonic treatment can improve the gel strength of egg white protein by promoting the cross-linking of proteins to form a dense gel structure, and egg white protein gel form more disulfide bonds and a more stable gel conformation under TP assisted ultrasound. In conclusion, ultrasound and TP assisted ultrasound modification of egg white protein gel is a reliable technique, which can improve the value of egg white protein in food processing.

Fabrication and characterization of quercetin loaded casein phosphopeptides-chitosan composite nanoparticles by ultrasound treatment: Factor optimization, formation mechanism, physicochemical stability and antioxidant activity

Ultrasound treatment was used to successfully prepare Quercetin (Qu)-loaded Casein phosphopeptides (CPP)/chitosan (CS) nanoparticles. Compared with the control, the above ternary nanoparticles with the smallest size (241.27 nm, decreased by 34.32%), improved encapsulation efficiency of Qu (78.55%, increased by 22.12%) when prepared under following conditions: ultrasonic frequency, 20/35/50 kHz; the power density, 80 W/L; the time, 20 min, and the intermittent ratio, 20 s/5s. Electrostatic interactions, hydrogen bonding, and hydrophobic interactions were the main driving forces for nanoparticles formulation, which were strengthened by ultrasound treatment. The compact, homogeneous and spherical composite nanoparticles obtained by sonication were clearly observed by scanning electron microscope and atomic force microscope. The environmental stability (NaCl, pH, exposure time, storage time, and simulated gastrointestinal digestion) and antioxidant activity of the ternary nanoparticles were remarkably enhanced after ultrasonic treatment. Furthermore, the ternary nanoparticles prepared by ultrasound exhibited excellent stability in simulated gastrointestinal digestion. The above results indicate that ultrasound not only increases the loading of the nanoparticles on bioactive substances but also improves the environmental stability and antioxidant activity of the formed nanoparticles. Ultrasound-assisted preparation of nanoparticles loaded with bioactive substances could be well used in the functional food and beverage industry.

The fabrication of novel zein and resveratrol covalent conjugates: Enhanced thermal stability, emulsifying and antioxidant properties

Novel zein and resveratrol conjugates were fabricated by alkaline and free radical grafting reactions. The grafting efficiency and total phenolic content of alkaline treated conjugates were slightly higher than those of free radical grafting. Compared to native and alkaline treated zein, the sulfhydryl contents of conjugates were obviously decreased, confirming that nucleophilic addition of resveratrol to sulfhydryl group of zein formed stable CS covalent bonds. The conformation changes of zein modified by resveratrol were revealed by fourier transform infrared spectroscopy and fluorescence spectroscopy. Moreover, covalent modification changed isoelectric point of zein from 6.5 to 5.4 (alkaline) or 5.6 (free radical grafting), and broadening the pH application range of zein. It was worth mentioning that the conjugates showed much higher thermal stability, antioxidant activity, and emulsify activity than those of native zein. This study provides an effective way for the design of novel delivery systems to encapsulate bioactive substances.

Recent Progress on Protein-Polyphenol Complexes: Effect on Stability and Nutrients Delivery of Oil-in-Water Emulsion System

Oil-in-water emulsions are widely encountered in the food and health product industries. However, the unsaturated fatty acids in emulsions are easily affected by light, oxygen, and heat, which leads to oxidation, bringing forward difficulties in controlling emulsion quality during transportation, storage, and retail. Proteins are commonly used as emulsifiers that can enhance the shelf, thermal and oxidation stability of emulsions. Polyphenols are commonly found in plants and members of the family have been reported to possess antioxidant, anticancer, and antimicrobial activities. Numerous studies have shown that binding of polyphenols to proteins can change the structure and function of the latter. In this paper, the formation of protein-polyphenol complexes (PPCs) is reviewed in relation to the latters' use as emulsifiers, using the (covalent or non-covalent) interactions between the two as a starting point. In addition, the effects polyphenol binding on the structure and function of proteins are discussed. The effects of proteins from different sources interacting with polyphenols on the emulsification, antioxidation, nutrient delivery and digestibility of oil-in-water emulsion are also summarized. In conclusion, the interaction between proteins and polyphenols in emulsions is complicated and still understudied, thereby requiring further investigation. The present review results in a critical appraisal of the relevant state-of-the-art with a focus on complexes' application potential in the food industry, including digestion and bioavailability studies.

Conjoint application of ultrasonication and redox pair mediated free radical method enhances the functional and bioactive properties of camel whey-quercetin conjugates

Ultrasonication, redox-pair generated free radical method and their combination (Ultrasonication/redox-pair method) was used for production of camel whey-quercetin conjugates. FTIR and SDS-PAGE confirmed successful production of whey-quercetin conjugates using ultrasonication and ultrasonication/redox-pair method. FTIR suggested existence of covalent (appearance of new peak at 3399 cm-1) and non-covalent linkages (shifting of peak at 3271 cm-1, 1655 cm-1 (amide I), 1534 cm-1 and 1422 cm-1 (Amide II)) in the whey-quercetin conjugates. Moreover, SDS-PAGE of conjugates produced by ultrasonication as well redox-pair method indicated shifting of protein bands slightly towards high molecular weight due to increase in the mass of proteins due to the binding of polyphenols. All conjugates showed improved techno-functional and bioactive properties in comparison to whey proteins. Conjugates produced through ultrasonication showed smaller particle size, improved solubility, emulsifying and foaming properties while conjugates produced through ultrasonication/redox-pair method depicted superior antioxidant properties in comparison to whey. Furthermore, conjugated samples showed higher inhibition of enzymatic markers involved in diabetes and obesity with highest potential recorded in conjugates produced using ultrasonication. Therefore, ultrasonication can be successfully used individually as well as in combination with redox-pair for production of whey-quercetin conjugates with enhanced bioactive and techno-functional properties.

Protein-polyphenol conjugates: Preparation, functional properties, bioactivities and applications in foods and nutraceuticals

Protein is a crucial nutritional ingredient in the daily human diet. Polyphenols (PPNs) are the abundant phytochemicals in plants, which are associated with health promotion as well as affect functionality in food systems. Both ingredients possess different types of functionalities (crosslinking, gelling, emulsifying, film-forming, etc.) and bioactivities (antioxidant, antimicrobial, anti-inflammatory, etc.). In the past decade, various methods have been implemented to enhance the functionalities and bioactivities of foods. Conjugation or grafting methods has been introduced widely. Conjugations of PPNs with proteins through various methods have been performed for the synthesis of the protein-polyphenol conjugate. Those potential grafting methods are alkaline associated, free-radical mediated, enzyme catalyzed, and chemical coupling methods. Several factors such as reaction conditions, type of proteins, and PPNs also influenced the conjugation efficiency. Various technologies, e.g., mass spectroscopy, fluorescence spectroscopy, UV spectroscopy, Fourier transform infrared spectroscopy, circular dichroism, and sodium dodecyl sulfate polyacrylamide gel electrophoresis have been used to elucidate conjugation and structural alternation of proteins and some properties of resulting conjugates. The prepared protein-PPN conjugates have been documented to enhance the bioactivities and functional properties of an initial protein. Moreover, conjugates have been employed in emulsions or as nanoparticles for nutraceutical delivery. Edible-films for food packaging and hydrogels for controlled drug release have been developed using protein-PPN conjugates. This chapter focuses on the methodologies and characteristics of protein-PPN conjugates and their applications in various food systems and nutraceutical field.

Effect of ultrasound on protein functionality

The review focus on the effect of ultrasound on protein functionality. The presence of transient ultrasonic mechanical waves induce various sonochemical and sonomechanical effects on a protein. Sonochemical effects include the breakage of chains and/or the modification of side groups of aminoacids. Sonomechanical modifications by enhanced molecular agitation, might lead to the transient or permanent modification of the 3D structure of the folded protein. Since the biological function of proteins depends on the maintenance of its 3D folded structure, both sonochemical and sonomechanical effects might affect its properties. A protein might maintain its 3D structure and functionality after minor sonochemical effects, however, the enhanced mass transfer by sonomechanical effects might expose internal hydrophobic residues of the protein, making protein unfolding to an irreversible denatured state. Ultrasound enhanced mass transport effects are unique pathways to change the 3D folded structure of proteins which lead to a new functionality of proteins as support shield materials during the formation microspheres. Enzymes are proteins and their reactions should be conducted in a reactor set-up where enzymes are protected from sonic waves to maximize their catalytic efficiency. In this review, focused examples on protein dispersions/emulsions and enzyme catalysis are given.

Effects of antioxidants, proteins, and their combination on emulsion oxidation

Lipid oxidation largely determines the quality of emulsion systems as well as their final products. Therefore, an increasing number of studies have focused on the control of lipid oxidation, particularly on its mechanism. In this review, we discuss the factors affecting the efficiency of antioxidants in emulsion systems, such as the free radical scavenging ability, specifically emphasizing on the interfacial behavior and the influence of surfactants on the interfacial distribution of antioxidants. To enhance the antioxidant efficiency of antioxidants in emulsion systems, we discussed whether the combination of antioxidants and proteins can improve antioxidant effects. The types, mixing applications, structures, interface behaviors, effects of surfactants on interfacial proteins, and the location of proteins are associated with the antioxidant effects of proteins in emulsion systems. Antioxidants and proteins can be combined in both covalent and non-covalent ways. The fabrication conditions, conjugation methods, interface behaviors, and characterization methods of these two combinations are also discussed. Our review provides useful information to guide better strategies for providing stability and controlling lipid oxidation in emulsions. The main challenges and future trends in controlling lipid oxidation in complex emulsion systems are also discussed.