Effects of lipid peroxide on the structure and gel properties of ovalbumin

Ultrasonic treatment treated sea bass myofibrillar proteins in low-salt solution: Emphasizing the changes on conformation structure, oxidation sites, and emulsifying properties

The physiochemical properties, structure characteristics, oxidation, and emulsifying properties of myofibrillar proteins (MPs) in low salt solution after treated by the ultrasound were investigated. The solubility, mean diameters, sulfhydryl content, and carbonyl contents of MPs after ultrasonic treatment increased, while the turbidity decreased. The surface hydrophobicity of MPs with 200 W-600 W treatment increased, but decreased at 800 W treatment. The circular dichroism analysis revealed that α-helix content increased, while β-sheet and random coil content decreased after ultrasonic treatment. Fluorescence spectroscopy indicated the fluorescence intensities of MPs were increased after ultrasonic treatment. SDS-PAGE results showed more protein polymers due to myosin heavy chain (MHC) aggregation via disulfide bonds. Based on LC-MS/MS result, the myosin heavy chain was susceptible to oxidation, with monooxidation being the main oxidative modification. Finally, the emulsions stabilized by ultrasonically treated MPs, especially those treated at 800 W, exhibited decreased particle size, improved uniformity, and enhanced stability.

Characterization and Mechanism of Gel Deterioration of Egg Yolk Powder during Storage

Egg yolk forms have several health and industrial applications, but their storage characteristics and gel mechanisms have not been thoroughly studied. In order to investigate the relationship between the changes in structure and properties of egg yolk gel and egg yolk powder during storage, in this paper, egg yolk powder was stored at 37 °C for 0, 1, 3, and 6 months in an accelerated storage experiment, and the influence of storage time on the gel properties of egg yolk powder was analyzed. The results showed that the contents of protein carbonylation and sulfhydryl in the yolk decreased gradually with the extension of storage time. Circular dichroism and fluorescence spectra showed that the ordered structure and structural stability of egg yolk proteins decreased gradually. Oxidation led to the formation of intermolecular crosslinking in the egg yolk proteins and oxidized aggregates, resulting in a decrease in surface hydrophobicity, which affected the gel properties of the egg yolk powder after rehydration, resulting in the phenomenon of lipid migration and gel degradation. The results provide a theoretical basis for improving egg yolk powder's overall quality and storage stability.

Effects of Packaging Materials on Structural and Simulated Digestive Characteristics of Walnut Protein during Accelerated Storage

Walnuts are rich in fat and proteins that become oxidized during the processing and storage conditions of their kernels. In this study, the effect of three packaging materials (e.g., polyethylene sealed packaging, polyamide/polyethylene vacuum packaging, and polyethylene terephthalate/aluminum foil/polyethylene vacuum packaging) were investigated on the oxidation, structural and digestive properties of walnut kernel proteins. Results showed that the amino acid content gradually decreased and carbonyl derivatives and dityrosine were formed during storage. The protein molecule structure became disordered as the α-helix decreased and the random coil increased. The endogenous fluorescence intensity decreased and the maximum fluorescence value was blue-shifted. After 15 days of storage, surface hydrophobicity decreased, while SDS-PAGE and HPLC indicated the formation of large protein aggregates, leading to a reduction in solubility. By simulating gastrointestinal digestion, we found that oxidation adversely affected the digestive properties of walnut protein isolate and protein digestibility was best for polyethylene terephthalate/aluminum foil/polyethylene vacuum packaging. The degree of protein oxidation in walnuts increased during storage, which showed that except for fat oxidation, the effect of protein oxidation on quality should be considered. The results of the study provided new ideas and methods for walnut quality control.

Protective Effects of Four Natural Antioxidants on Hydroxyl-Radical-Induced Lipid and Protein Oxidation in Yak Meat

The impacts of natural antioxidants, including ferulic acid, diallyl sulfide, α-tocopherol, and rutin, at a level of 0.2 g/kg on lipid and protein oxidation of minced yak meat in a hydroxyl-radical-generating system were investigated, and the effectiveness was compared with synthetic antioxidant 2,6-di-tert-butyl-4-methylphenol (BHT). The exposure of yak meat to oxidative stress from 12 h to 24 h elevated lipid and protein oxidation. Treatments with antioxidants resulted in significantly lower peroxides, conjugated dienes, and thiobarbituric acid-reactive substances, and were also effective in retarding the formation of carbonyl groups, reducing the loss of sulfhydryl groups and protecting α-helix contents, of which ferulic acid and rutin were the most effective. Myosin heavy chain underwent lower degradation in the samples treated with ferulic acid or rutin compared with the oxidized control and other antioxidant treatments, while that of the BHT treatment showed a similar intensity with oxidized control at 24 h of oxidation. The physical stability of myofibrillar proteins in samples with antioxidants from high to low was rutin, ferulic acid, α-tocopherol, and BHT~diallyl sulfide. These results indicate that rutin and ferulic acid may be promising antioxidants in inhibiting the oxidative reactions during the processing of yak meat.

Effects of Peroxyl Radicals on the Structural Characteristics and Fatty Acid Composition of High-Density Lipoprotein from Duck Egg Yolk

In this study, high-density lipoprotein (HDL) from duck egg yolk was subjected to oxidation with a system based on 2,2′-azobis (2-amidinopropane) dihydrochloride (AAPH)-derived peroxyl radicals. The effects of peroxyl radicals on the protein carbonyl, free sulfhydryl, secondary/tertiary structure, surface hydrophobicity, solubility, particle size distribution, zeta potential and fatty acid composition of HDL were investigated by using sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE), Fourier-transform infrared spectroscopy (FTIR), circular dichroism (CD), fluorescence spectroscopy, dynamic light scattering and ultra-high-performance liquid chromatography coupled to tandem mass spectrometry (UHPLC-MS/MS). The results indicated that the content of protein carbonyl was significantly increased, that of free sulfhydryl was obviously reduced, and the ordered secondary structure was also decreased with increasing AAPH concentration. In addition, the surface hydrophobicity and solubility of HDL showed apparent increases due to the exposure of hydrophobic groups and aggregation of protein caused by oxidation. The fatty acid composition of HDL exhibited pronounced changes due to the disrupted protein–lipid interaction and lipid oxidation by AAPH-derived peroxyl radicals. These results may help to elucidate the molecular mechanism for the effect of lipid oxidation products on the oxidation of duck yolk proteins.

Effect of high intensity ultrasound assisted glycosylation on the gel properties of ovalbumin: Texture, rheology, water state and microstructure

In this paper, the effects of ultrasonic assisted glycosylation on the gel properties of ovalbumin (OVA) were studied. The molecular characteristics of native ovalbumin, heated ovalbumin, traditional glycosylated ovalbumin, ultrasonic ovalbumin and ultrasonic assisted glycosylated ovalbumin were compared. The lowest free amino group content and the highest browning intensity indicated that ultrasonic can facilitate the Maillard reaction. The gel hardness of ultrasonic glycosylation and the traditional heating glycosylation groups individually increased to 653.2 and 526.9 g compared with the control (344.9 g). The transformation of protein structure was confirmed by FTIR and fluorescence spectrum, which prompted negatively charged groups to reach the protein surface and form more disulfide bond in sOVA-X gel. The interaction between the water and the protein was strengthened, thereby increasing the water holding capacity. These results supplied a theoretical basis for the application of ultrasonic to improve protein properties.

Lipid oxidation induced egg white protein foaming properties enhancement: The mechanism study revealed by high resolution mass spectrometry

Lipid oxidation often occurs during egg white protein (EWP) storage and processing periods. Here, 2,2'-azobis (2-amidinopropane) dihydrochloride (AAPH) was performed to simulate lipid oxidation to probe the oxidation effects on foaming and structural properties of EWP. Results indicated that EWP structure became unfolding and flexible after oxidation, resulting in more hydrophobic groups and negative charge exposed and soluble aggregates formed, which revealed by the results of DLS and AFM. Additionally, high resolution mass spectrometry results evidenced that ovotransferrin and lysozyme trended to be new oxidation targets with the AAPH concentration increasing, and the oxidation sites inside lysozyme proved that EWP unfolding and exposure of internal hydrophobic amino acids, which were related to the enhancement of EWP foaming properties. Overall, our study provided a further analysis of the lipid induced oxidation of EWP, which may contribute to provide a more accurate strategy for enhancing protein foaming properties in food industry.

Mechanisms of Change in Emulsifying Capacity Induced by Protein Denaturation and Aggregation in Quick-Frozen Pork Patties with Different Fat Levels and Freeze-Thaw Cycles

Herein, we discuss changes in the emulsifying properties of myofibrillar protein (MP) because of protein denaturation and aggregation from quick-frozen pork patties with multiple fat levels and freeze–thaw (F–T) cycles. Protein denaturation and aggregation were confirmed by the significantly increased surface hydrophobicity, turbidity, and particle size, as well as the significantly decreased solubility and absolute zeta potential, of MPs with increases in fat levels and F–T cycles (p < 0.05). After multiple F–T cycles, the emulsifying activity and emulsion stability indices of all samples were significantly reduced (p < 0.05). The emulsion droplets of MP increased in size, and their distributions were dense and irregular. The results demonstrated that protein denaturation and aggregation due to multiple F–T cycles and fat levels changed the distribution of surface chemical groups and particle sizes of protein, thus affecting the emulsifying properties.

Elucidating the Calcium-Binding Site, Absorption Activities, and Thermal Stability of Egg White Peptide-Calcium Chelate

With the current study, we aimed to determine the characteristics and calcium absorption capacity of egg white peptide-calcium complex (EWP-Ca) and determine the effect of sterilization on EWP-Ca to study the possibility of EWP-Ca as a new potential calcium supplement. The results of SEM and EDS showed a high calcium chelating ability between EWP and calcium, and the structure of EWP-Ca was clustered spherical particles due its combination with calcium. The FTIR and Raman spectrum results showed that EWP could chelate with calcium by carboxyl, phosphate, and amino groups, and peptide bonds may also participate in peptide-calcium binding. Moreover, the calcium absorption of EWP-Ca measured by the intestinal everted sac model in rats was 32.38 ± 6.83 μg/mL, significantly higher than the sample with CaCl2, and the mixture of EWP and Ca (p < 0.05) revealed appropriate calcium absorption capacity. The fluorescence spectra and CD spectra showed that sterilization caused a decrease in the content of α-helix and β-sheet and a significant increase in β-turn (p < 0.05). Sterilization changed the EWP-Ca structure and decreased its stability; the calcium-binding capacity of EWP-Ca after sterilization was decreased to 41.19% (p < 0.05). Overall, these findings showed that EWP could bind with calcium, form a peptide-calcium chelate, and serve as novel carriers for calcium supplements.

Effect of dry heating on egg white powder influencing water mobility and intermolecular interactions of its gels

BACKGROUND

Dry heat processing remains the most promising and simple approach for achieving better gelling properties of spray-dried egg white powder (EWP). Water mobility and intermolecular interactions in gels derived from EWP were investigated after subjecting EWP to various dry heating times (0-21 days).

RESULTS

The gel hardness and water-holding capacity significantly increased with an increase in dry heating time (P < 0.05), and both parameters were positively correlated with gel transparency. In contrast to the coarser structure of untreated EWP gel, the gel of EWP corresponding to 15 days of dry heating time had a fine-stranded and orderly network structure with smaller pores. An increase in the binding force between the gel and water was observed with an increase in dry heating time due to the formation of more 'protein-water' hydrogen bonds. Increasing the dry heating time resulted in an increase in the contribution of disulfide bonds, which in turn made a significant contribution to the rigidity of the EWP gels. By contrast, a decrease in the contribution of ionic bonds and hydrophobic interactions upon increasing the dry heating time promoted the formation of orderly networks.

CONCLUSIONS

Overall, gel corresponding to EWP dry heating for 15 days had better gel properties, the highest transparency and water-holding capacity, as well as a fine-stranded and orderly network structure. These results provide more information on improvement of the gel properties of EWP through dry heat treatment. © 2020 Society of Chemical Industry.

Changes in the thermal stability and structure of myofibrillar protein from quick-frozen pork patties with different fat addition under freeze-thaw cycles

Changes in thermal stability and structure of myofibrillar protein from pork patties with different fat addition (0%, 5%, 10%, 15% and 20%) under freeze-thaw (F-T) cycles were discussed. The results showed that the total sulfhydryl, reactive sulfhydryl, free amino group, α-helix and β-sheet contents, fluorescence intensity (FI), and protein thermal stability (T_max, ∆H_total) of samples with the same fat content were significantly decreased, while the β-turn and random-coil content and the maximum fluorescence emission wavelength (λ_max) were significantly increased with increasing F-T cycles (P < 0.05). These changes in samples with 20% fat at the 5th F-T cycle were obvious and were verified by the decreases in ∆H_total (26.1%), reactive sulfhydryl (16.1%), and FI (16.8%) compared with the patties without fat. Therefore, repeated F-T cycles could decline the thermal stability of protein, destroy the protein structure of patty, and the changes were positively correlated with fat content of patty.

Animal and Plant Protein Oxidation: Chemical and Functional Property Significance

Protein oxidation, a phenomenon that was not well recognized previously but now better understood, is a complex chemical process occurring ubiquitously in food systems and can be induced by processing treatments as well. While early research concentrated on muscle protein oxidation, later investigations included plant, milk, and egg proteins. The process of protein oxidation involves both radicals and nonradicals, and amino acid side chain groups are usually the site of initial oxidant attack which generates protein carbonyls, disulfide, dityrosine, and protein radicals. The ensuing alteration of protein conformational structures and formation of protein polymers and aggregates can result in significant changes in solubility and functionality, such as gelation, emulsification, foaming, and water-holding. Oxidant dose-dependent effects have been widely reported, i.e., mild-to-moderate oxidation may enhance the functionality while strong oxidation leads to insolubilization and functionality losses. Therefore, controlling the extent of protein oxidation in both animal and plant protein foods through oxidative and antioxidative strategies has been of wide interest in model system as well in in situ studies. This review presents a historical perspective of food protein oxidation research and provides an inclusive discussion of the impact of chemical and enzymatic oxidation on functional properties of meat, legume, cereal, dairy, and egg proteins based on the literature reports published in recent decades.

Mechanisms of change in gel water-holding capacity of myofibrillar proteins affected by lipid oxidation: The role of protein unfolding and cross-linking

This work explored the effects of protein unfolding and cross-linking induced by lipid oxidation (linoleic acid, OLA) on the gel water-holding capacity (WHC) of beef myofibrillar proteins (MP). Medium concentration of OLA (≤6 mM) caused the increase of gel WHC from 55.2% to 65.1%, while relative high OLA concentration (>6 mM) decreased the gel WHC. When the OLA concentrations increased from 0 to 10 mM, the population of immobile water of gel decreased from 92.91% to 78.97%, whereas that of free water increased from 6.13% to 19.80%, suggesting that OLA treatment regardless concentration was harmful for gel WHC. However, medium OLA concentrations (≤6 mM) caused the shifting of α-helixes to β-sheets in MP gel, exerting positive effect on gel WHC. Protein unfolding and cross-linking jointly determined the increased gel WHC at moderate oxidative modification. Additionally, the protein aggregation at high OLA concentration resulted in decreased gel WHC.

Effects of oxidative modification on the functional, conformational and gelling properties of myofibrillar proteins from Culter alburnus

Protein oxidation is a critical process in the deterioration and spoilage of fish and related commodities during processing and storage. In this study, the hydroxyl radical generation system (HRGS) was used to simulate the effect of oxidation on the functional, conformational and gelling properties of topmouth culter (Culter alburnus) myofibrillar proteins (MP). Additionally, the effects of oxidation on the gel-forming abilities of MP were also systematically analyzed from the perspective of intermolecular interaction forces. Oxidation was shown to decrease the total sulfhydryl content, increase the surface hydrophobicity, and induce conformational changes in MP. Rheological analysis showed that oxidation reduced the gel strength. Water holding capacity (WHC) and low-field nuclear magnetic resonance (LF-NMR) analyses showed that low oxidation could enhance water binding of protein matrix, while high-degree oxidation could substantially reduce the gelling properties of MP. The selective solubility of MP gel proved that oxidation could reduce the content of ionic and hydrogen bonds and increase hydrophobic interactions. All the results indicate that oxidation could alter the intermolecular interactions between protein-protein and protein-water molecules, due to irregular unfolding and inhibition of the cross-linking of amino acid side chains, leading to reduction in the quality and function of fish and related products.

Variation in the structure and emulsification of egg yolk high-density lipoprotein by lipid peroxide

To further clarify the effect of the oxidation of egg yolk high-density lipoprotein (EYHDL) on the protein structure and emulsification, 2,2'-azobis (2-methylpropionamidine) dihydrochloride (AAPH) was selected as a representative lipid peroxidation-derived peroxyl radical. The results of Raman spectroscopy indicated that, with the increase in the concentration of AAPH, the EYHDL carbonyl content increased significantly and the free sulfhydryl content declined sharply. Circular dichroism spectroscopy and intrinsic fluorescence indicated that exposure of EYHDL to AAPH led to destruction of the orderly structure and reduction of the structural stability. The particle size distribution and zeta potential indicated that the peroxyl radical caused molecular aggregation. Moderate oxidizing conditions can enhance the emulsification of EYHDL, and high-intensity oxidation decreased emulsification. The research results indicated that EYHDL made a significant change in the oxidation system and led to a change in its structure and emulsification, providing a theoretical basis to clarify the EYHDL oxidation mechanism. PRACTICAL APPLICATIONS: Egg yolk powder is prone to emulsification degradation during storage. The emulsification of egg yolk powder is mainly derived from high-density lipoprotein in egg yolk. Moreover, egg yolk powder contains a large amount of lipids, and, during the processing and storage of egg yolk powder, many lipid peroxyl radicals are inevitably generated. Therefore, it is desired to combine the lipid peroxyl radicals generated during the storage of egg yolk powder with the decrease in emulsifiability. In this paper, we first investigated the effects of peroxyl radicals on the structure and emulsifying properties of high-density lipoproteins and provided a theoretical basis to solve the problem that the emulsifiability of egg yolk powder is significantly reduced during storage.