Heat-induced cross-linking and degradation of wheat gluten, serum albumin, and mixtures thereof

Wheat gluten structure and (non-)covalent network formation during deep-fat frying

The aim of this work was to investigate wheat gluten protein network structure throughout the deep-frying process and evaluate its contribution to frying-induced micro- and macrostructure development. Gluten polymerization, gluten-water interactions, and molecular mobility were assessed as a function of the deep-frying time (0 - 180 s) for gluten-water model systems of differing hydration levels (40 - 60 % moisture content). Results showed that gluten protein extractability decreased considerably upon deep frying (5 s) mainly due to glutenin polymerization by disulfide covalent cross-linking. Stronger gliadin and glutenin protein-protein interactions were attributed to the formation of covalent linkages and evaporation of water interacting with protein chains. Longer deep-frying (> 60 s) resulted in progressively lower protein extractabilities, mainly due to the loss in gliadin protein extractability, which was associated with gliadin co-polymerization with glutenin by thiol-disulfide exchange reactions. The mobility of gluten polymers was substantially reduced during deep-frying (based on the lower T2 relaxation time of the proton fraction representing the non-exchanging protons of gluten) and gluten proteins gradually transitioned from the rubbery to the glassy state (based on the increased area of said protons). The sample volume during deep-frying was strongly correlated to the reduced protein extractability (r = -0.792, p < 0.001) and T2 relaxation time of non-exchanging protons of gluten proteins (r = -0.866, p < 0.001) thus demonstrating that the extent of gluten structural expansion as a result of deep-frying is dictated both by the polymerization of proteins and the reduction in their molecular mobility.

Effect of Microwave Treatment on Protease Activity, Dough Properties and Protein Quality in Sprouted Wheat

In this study, the effects of microwave treatment on protease activity, dough properties and protein quality in sprouted wheat were investigated. Microwave treatment led to a significant (p < 0.05) reduction in protease activity in sprouted wheat. Proteases with a pH optimum of 4.4 (cysteine proteinases) were more susceptible to microwave heating, which contributed mostly to protease inactivation. Significant improvements (p < 0.05) in the dough properties and gluten quality of sprouted wheat were observed, which are probably attributable to the synergistic effectiveness of protease inactivation and heat-induced gluten cross-linking. After microwave treatment, the decrease in the solubility and extractability of protein in sprouted wheat indicated protein polymerization, which was induced by intermolecular disulfide bond cross-linking. The changes in gliadin were less pronounced due to the relatively low temperature of the microwave treatment. The cross-linking in sprouted wheat that occurred after microwave treatment seemed to mainly involve glutenin, especially B/C low-molecular-weight glutenin subunits (B/C-LMW-GSs) in the range of 30-50 kD.

Biodegradable Fiber-Reinforced Gluten Biocomposites for Replacement of Fossil-Based Plastics

Biocomposites based on wheat gluten and reinforced with carbon fibers were produced in line with the strive to replace fossil-based plastics with microplastic-free alternatives with competing mechanical properties. The materials were first extruded/compounded and then successfully injection molded, making the setup adequate for the current industrial processing of composite plastics. Furthermore, the materials were manufactured at very low extrusion and injection temperatures (70 and 140 °C, respectively), saving energy compared to the compounding of commodity plastics. The sole addition of 10 vol % fibers increased yield strength and stiffness by a factor of 2-4 with good adhesion to the protein. The biocomposites were also shown to be biodegradable, lixiviating into innocuous molecules for nature, which is the next step in the development of sustainable bioplastics. The results show that an industrial protein coproduct reinforced with strong fibers can be processed using common plastic processing techniques. The enhanced mechanical performance of the reinforced protein-based matrix herein also contributes to research addressing the production of safe materials with properties matching those of traditional fossil-based plastics.

Characterizing lysinoalanine crosslinks in food systems: Discovery of a diagnostic ion in model peptides using MALDI mass spectrometry

Formation of lysinoalanine protein-protein crosslinks during food processing adversely impacts nutritional value. However, mapping lysinoalanine directly in food is challenging. We characterized the fragmentation pattern of lysinoalanine crosslinks in synthetic peptide models over a range of pH and time treatments using mass spectrometry. A putative diagnostic ion resulting from the cleavage of the α-carbon and β-carbon of lysinoalanine is identified in MALDI MS/MS spectra. This represents the first step in mapping lysinoalanine in real food samples with higher precision than currently identifiable through standard or customized software. We then determined a correlated trend in the reduction of disulfide bonds and formation of lysinoalanine with increasing pH and time. Mapping lysinoalanine formation is critical to enhance our understanding of molecular processes impacting the nutritional value of foods, including notably in the development of protein alternatives that use alkaline treatment to extract protein isolates.

Thermomechanical Stress Analysis of Hydrated Vital Gluten with Large Amplitude Oscillatory Shear Rheology

Vital gluten is increasingly researched as a non-food product for biodegradable materials. During processing, the protein network is confronted with increased thermal and mechanical stress, altering the network characteristics. With the prospect of using the protein for materials beyond food, it is important to understand the mechanical properties at various processing temperatures. To achieve this, the study investigates hydrated vital gluten under thermomechanical stress based on large amplitude oscillatory shear (LAOS) rheology. LAOS rheology was conducted at increasing shear strains (0.01-100%), various frequencies (5-20 rad/s) and temperatures of 25, 45, 55, 65, 70 and 85 °C. With elevating temperatures up to 55 °C, the linear viscoelastic moduli decrease, indicating material softening. Then, protein polymerization and the formation of new cross-links due to thermal denaturation cause more network connectivity, resulting in significantly higher elastic moduli. Beyond the linear viscoelastic regime, the strain-stiffening ratio rises disproportionately. This effect becomes even more evident at higher temperatures. Lacking a viscous contribution, the highly elastic but also stiff network shows less mechanical resilience. Additionally, at these elevated temperatures, structural changes during the protein's denaturation and network shrinkage due to water evaporation could be visualized with confocal laser scanning microscopy (CLSM).

Study on electrospinning of wheat gluten: A review

Electrospinning has attracted extensive attention among various nanofabrication technologies owing to its ability to produce nanofiber structures with unique properties, such as high specific surface area and porosity, as well as tunable fiber morphology and mechanical properties. The most representative spinning raw materials include natural polymers and synthetic polymers. Owing to the sustainable development strategies, more and more researchers focus on natural polymers. Among natural polymers, wheat gluten (WG) nanofibers have recently attracted much attention owing to its high specific surface area, superior biocompatibility, and unique viscoelasticity. This review summarizes the composition and characteristics of WG, the physical and chemical indicators of a WG electrospinning solution, the main influencing factors in the WG electrospinning process and a characterizations of WG nanofibers. Finally, the review also outlines the applications of WG nanofibers in drug release, biological scaffold, and active food packaging.

Impact of Different Frozen Dough Technology on the Quality and Gluten Structure of Steamed Buns

To advance the industrialization production of steamed buns, the current study explored the freeze-stability of unfermented, pre-fermented and par-steamed frozen dough. The results showed that the steamed bun made from unfermented dough with 2.0% yeast, the pre-fermented dough with a pre-fermented time of 30 min and the par-steamed dough with a pre-steamed time of 15 min showed the best sensory properties quality upon frozen storage. The gassing power of un- and pre-fermented dough gradually decreased, and dough with longer pre-fermented time exhibited more evident loss of gassing power. Freeze-induced depolymerization of gluten protein was the least distinct in the par-steamed dough, followed by the pre- and un-fermented dough, which was probably related to the superior freeze stability of glutenin-gliadin macro-crosslinks upon the pre-steaming stage. The surface hydrophobicity of gluten proteins of frozen dough decreased during the initial storage and was enhanced subsequently, which was related with the combined effects of the unfolding and synchronous aggregation induced by freezing and steaming, respectively. Moreover, the surface hydrophobicity of gluten in par-steamed frozen dough and steamed buns was more resistant to frozen storage, which was probably attributed to the established stable structure during the pre-steaming process.

A Novel Thermal-driven Self-assembly Method to Prepare Albumin Nanoparticles: Formation Kinetics, Degradation Behavior and Formation Mechanism

Nanoparticles based on bovine serum albumin (BSA), which shares 76% homology with human serum albumin (HSA), have emerged as a promising candidate for the efficient delivery of anticancer drugs. Thermal-driven self-assembly is a novel organic solvent-free approach to produce albumin nanoparticles. In our previous study, some features of this nanoparticle such as drug loading efficiency, drug encapsulation efficiency and drug release kinetics have been evaluated. However, the formation mechanism that determines the above nanoparticle properties remains unclear. Here, we investigated the formation kinetics and mechanism using spectroscopic methods including fluorescence spectroscopy, circular dichroism (CD) and differential scanning calorimetry (DSC). We also applied chemical analysis methods that measured the content changes of albumin active groups and vanillin. To verify the covalent networks in the nanoparticles, trypsin and glutathione (GSH) were used separately to cleave the peptide bonds and disulfide bridges, and dynamic light scattering (DLS) and sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) were used to analyze the degraded samples. BSA nanoparticles started to form at 10 min and were completely formed at 120 min. With the digestion of trypsin, more than 50% of the nanoparticles were degraded within 60 min. CD spectra showed that α-helical structure of BSA decreased from 42.3% to 39.8% and 37.7% after heating for 10 and 60 min, respectively. In the DSC thermogram, the melting peak of BSA nanoparticles was 229.14℃, which is about 12℃ higher than the physical mixture of BSA and vanillin, indicating that chemical reactions occurred during the nanoparticle formation and formed a new more stable substance. Moreover, the results of active group assay, GSH degradation and SDS-PAGE experiments also proved that disulfide bonds and peptide bonds were formed between BSA molecules, whereas Schiff bases were formed between BSA and vanillin molecules. Formation kinetics and degradation behavior are important properties to characterize albumin nanoparticles and should be paid attention to. Not only that, this study also provides an effective way to study the formation mechanism of protein-based nanodrug delivery systems.

Cross-linking reactions in food proteins and proteomic approaches for their detection

Various protein cross-linking reactions leading to molecular polymerization and covalent aggregates have been described in processed foods. They are an undesired side effect of processes designed to reduce bacterial load, extend shelf life, and modify technological properties, as well as being an expected result of treatments designed to modify raw material texture and function. Although the formation of these products is known to affect the sensory and technological properties of foods, the corresponding cross-linking reactions and resulting protein polymers have not yet undergone detailed molecular characterization. This is essential for describing how their generation can be related to food processing conditions and quality parameters. Due to the complex structure of cross-linked species, bottom-up proteomic procedures developed to characterize various amino acid modifications associated with food processing conditions currently offer a limited molecular description of bridged peptide structures. Recent progress in cross-linking mass spectrometry for the topological characterization of protein complexes has facilitated the development of various proteomic methods and bioinformatic tools for unveiling bridged species, which can now also be used for the detailed molecular characterization of polymeric cross-linked products in processed foods. We here examine their benefits and limitations in terms of evaluating cross-linked food proteins and propose future scenarios for application in foodomics. They offer potential for understanding the protein cross-linking formation mechanisms in processed foods, and how the inherent beneficial properties of treated foodstuffs can be preserved or enhanced.

Thermally induced gluten modification observed with rheology and spectroscopies

The protein vital gluten is mainly used for food while interest for non-food applications, like biodegradable materials, increases. In general, the structure and functionality of proteins is highly dependent on thermal treatments during production or modification. This study presents conformational changes and corresponding rheological effects of vital wheat gluten depending on temperature. Dry samples analyzed by X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR) and thermalgravimetric analysis coupled with mass spectrometry (TGA-MS) show surface compositions and conformational changes from 25 to 250 °C. Above 170 °C, XPS reveals a decreased N content at the surface while FTIR band characteristics for β-sheets prove structural changes. At 250 °C, protein denaturation accompanied by a significant mass loss due to dehydration and decarbonylation reactions is observed. Oscillatory measurements of optimally hydrated vital gluten describing network properties of the material show two structural changes along a temperature ramp from 25 to 90 °C: at 56-64 °C, the temperature necessary to trigger structural changes increases with the ratio of gliadin to total protein mass, determined by reversed-phase high performance liquid chromatography (RP-HPLC). At a temperature of 79-81 °C, complete protein denaturation occurs. FTIR confirms the denaturation process by showing band shifts with both temperature steps.

Extrusion-Cooking Modifies Physicochemical and Nutrition-Related Properties of Wheat Bran

The potential of extrusion-cooking to change the physicochemical characteristics of wheat bran, increase its nutritional value and decrease its recalcitrance towards fermentation was investigated in this study. The conditions in a twin-screw extruder were varied by changing screw configuration, moisture content and barrel temperature. The former was not previously investigated in studies on bran extrusion. Extrusion-cooking resulted in an increased water-holding capacity and extract viscosity of bran, suggesting shear-induced structure degradation and structure loosening due to steam explosion at the extruder outlet. Modelling showed that the extent of these modifications mainly correlates with the amount of specific mechanical energy (SME) input, which increases with an increasing number of work sections in the screw configuration and a decreasing moisture content and barrel temperature. Extrusion led to solubilisation of arabinoxylan and ferulic acid. Moreover, it led to starch melting and phytate degradation. Upon fermentation of the most modified sample using a human faecal inoculum, small numeric pH decreases and short-chain fatty acid production increases were observed compared to the control bran, while protein fermentation was decreased. Overall, extrusion-cooking can improve the nutrition-related properties of wheat bran, making it an interesting technique for the modification of bran before further use or consumption as an extruded end product.

Wheat Gluten Amino Acid Analysis by High-Performance Anion-Exchange Chromatography with Integrated Pulsed Amperometric Detection

The present chapter describes an accurate and user-friendly method for determining amino acid composition of wheat gluten proteins and their gliadin and glutenin fractions. The method consists of hydrolysis of the peptide bonds in 6.0 M hydrochloric acid (HCl) solution at 110 °C for 24 h, followed by evaporation of the acid and separation of the free amino acids by high-performance anion-exchange chromatography with integrated pulsed amperometric detection (HPAEC-IPAD). In contrast to conventional methods, the analysis requires neither pre- or post-column derivatization nor a time-consuming oxidation or derivatization step prior to hydrolysis. Correction factors account for incomplete release of Val and Ile even after hydrolysis for 24 h and for losses of Ser during evaporation. Gradient conditions including an extra eluent allow multiple sequential sample analyses without risk of Glu accumulation on the anion-exchange column which otherwise would result from high Gln levels in gluten proteins.

Comparison of Gluten Extraction Protocols Assessed by LC-MS/MS Analysis

The efficiency of gluten extraction is of critical importance to the results derived from any analytical method for gluten detection and quantitation, whether it employs reagent-based technology (antibodies) or analytical instrumentation (mass spectrometry). If the target proteins are not efficiently extracted, the end result will be an under-estimation in the gluten content posing a health risk to people affected by conditions such as celiac disease (CD) and nonceliac gluten sensitivity (NCGS). Five different extraction protocols were investigated using LC-MRM-MS for their ability to efficiently and reproducibly extract gluten. The rapid and simple "IPA/DTT" protocol and related "two-step" protocol were enriched for gluten proteins, 55/86% (trypsin/chymotrypsin) and 41/68% of all protein identifications, respectively, with both methods showing high reproducibility (CV < 15%). When using multistep protocols, it was critical to examine all fractions, as coextraction of proteins occurred across fractions, with significant levels of proteins existing in unexpected fractions and not all proteins within a particular gluten class behaving the same.

Prediction of heat-induced polymerization of different globular food proteins in mixtures with wheat gluten

Egg, soy or whey protein co-exists with wheat gluten in different food products. Different protein types impact each other during heat treatment. A positive co-protein effect occurs when heat-induced polymerization of a mixture of proteins is more intense than that of the isolated proteins. The intrinsic protein characteristics of globular proteins which enhance polymerization in mixtures with gluten are unknown. In this report, a model was developed to predict potential co-protein effects in mixtures of gluten and globular proteins during heating at 100°C. A negative co-protein effect with addition of lysozyme, no co-protein effect with soy glycinin or egg yolk and positive co-protein effects with bovine serum albumin, (S-)ovalbumin, egg white, whole egg, defatted egg yolk, wheat albumins and wheat globulins were detected. The level of accessible free sulfhydryl groups and the surface hydrophobicity of unfolded globular proteins were the main characteristics in determining the co-protein effects in gluten mixtures.

Identification of barley-specific peptide markers that persist in processed foods and are capable of detecting barley contamination by LC-MS/MS

Consumers, especially those with allergies and/or intolerances, should have confidence in two critical areas of food safety: foods should be correctly labelled and free from contamination. To this end, global proteomic analysis employing LC-MS/MS of gluten-enriched extracts derived from 12 barley cultivars was undertaken, providing a foundation for the development of MS-based quantitative methodologies that would enable the detection of barley contamination in foods. Subsequently, a number of candidate barley-specific peptide markers were evaluated by multiple-reaction monitoring MS. From an initial panel of 26, 9 peptide markers were unique to barley, yet present in a wide range of barley varieties. The analytical method was then used to examine a range of breakfast cereals and was able to detect barley in a barley-based breakfast cereal and a muesli, but additionally allowed detection of contamination of cereals that were comprised of ancient grains and in commercially-sourced flours, including amaranth, chia, buckwheat, millet, rice, corn, oats, rye, spelt and green wheat (0.01-0.08%). LC-MS/MS provides an alternative to ELISA approaches to monitor food safety and the identification of robust and sensitive cereal-specific peptide markers is the first step toward the adoption of this technology.

SIGNIFICANCE

Coeliac disease is a serious health issue affecting up to 70million people globally for which there is no cure. The only treatment is a life-long gluten-free diet. Contamination of foods can occur at many stages of food production from farm to fork. As such, accurate quantification and identification of the source (i.e. cereal) and type (e.g. gluten) of contamination is critical to the health and well-being of a subset of the population, including those affected by coeliac disease and non-coeliac gluten sensitivity.

Impact of casein and egg white proteins on the structure of wheat gluten-based protein-rich food

BACKGROUND

There is a growing interest in texturally and nutritionally satisfying vegetable alternatives to meat. Wheat gluten proteins have unique functional properties but a poor nutritional value in comparison to animal proteins. This study investigated the potential of egg white and bovine milk casein with well-balanced amino acid composition to increase the quality of wheat gluten-based protein-rich foods.

RESULTS

Heating a wheat gluten (51.4 g)-water (100.0 mL) blend for 120 min at 100 °C increased its firmness less than heating a wheat gluten (33.0 g)-freeze-dried egg white (16.8 g)-water (100.0 mL) blend. In contrast, the addition of casein to the gluten-water blend negatively impacted firmness after heating. Firmness was correlated with loss of protein extractability in sodium dodecyl sulfate containing medium during heating, which was higher with egg white than with casein. Even more, heat-induced polymerization of the gluten-water blend with egg white but not with casein was greater than expected from the losses in extractability of gluten and egg white on their own.

CONCLUSION

Structure formation was favored by mixing gluten with egg white but not with casein. These observations were linked to the intrinsic polymerization behavior of egg white and casein, but also to their interaction with gluten. Thus not all nutritionally suitable proteins can be used for enrichment of gluten-based protein-rich foods.

Probing Conformational Change of Bovine Serum Albumin-Dextran Conjugates under Controlled Dry Heating

The time-dependent conformational change of bovine serum album (BSA) during Maillard reaction with dextran under controlled dry heating has been studied by small-angle X-ray scattering, fluorescence spectroscopy, dynamic light scattering, and circular dichroism analysis. Through the research on the radii of gyration (Rg), intrinsic fluorescence, and secondary structure, conjugates with dextran coating were found to inhibit BSA aggregation and preserve the secondary structure of native BSA against long-time heat treatment during Maillard reaction. The results suggested that the hydrophilic dextran was conjugated to the compact protein surface and enclosed it and more dextran chains were attached to BSA with the increase of the heating time. The study presented here will be beneficial to the understanding of the conformational evolution of BSA molecules during the dry-heating Maillard reaction and to the control of the protein-polysaccharide conjugate structure.

Accuracy of ELISA detection methods for gluten and reference materials: a realistic assessment

The determination of prolamins by ELISA and subsequent conversion of the resulting concentration to gluten content in food appears to be a comparatively simple and straightforward process with which many laboratories have years-long experience. At the end of the process, a value of gluten, expressed in mg/kg or ppm, is obtained. This value often is the basis for the decision if a product can be labeled gluten-free or not. On the basis of currently available scientific information, the accuracy of the obtained values with commonly used commercial ELISA kits has to be questioned. Although recently several multilaboratory studies have been conducted in an attempt to emphasize and ensure the accuracy of the results, data suggest that it was the precision of these assays, not the accuracy, that was confirmed because some of the underlying assumptions for calculating the gluten content lack scientific data support as well as appropriate reference materials for comparison. This paper discusses the issues of gluten determination and quantification with respect to antibody specificity, extraction procedures, reference materials, and their commutability.