MICROSTRUCTURE OF EGG YOLK

Salt induced slowdown of kinetics and dynamics during thermal gelation of egg-yolk

We investigated the effect of the NaCl concentration (0.3-2M) on the structure and dynamics of hen egg yolk at room temperature and during thermal gelation at temperatures in the range of 66-90 °C utilizing low-dose x-ray photon correlation spectroscopy in ultra-small angle x-ray scattering geometry. With an increase in the salt concentration, we observe progressive structural and dynamic changes at room temperature, indicating the disruption of yolk components such as yolk-granules and yolk-plasma proteins. Temperature- and salt-dependent structural and dynamic investigations suggest a delay in the gel formation and aggregation of yolk low-density lipoproteins with increasing ionic strength. However, the time-temperature superposition relationship observed in all samples suggests an identical mechanism underlying protein aggregation-gelation with a temperature-dependent reaction rate. The sol-gel transition time extracted from kinetic and dynamic information follows Arrhenius's behavior, and the activation energy (460 kJ/mol) is found to be independent of the salt concentration.

New insight into yolk sphere microgel structure impacted by lipid and protein distribution changing under heating processing

The gel structure of boiled, shelled egg yolk is formed by the accumulation of yolk spheres, which are rich in lipids and proteins, and investigating the properties of the lipid-protein complex gel structure of the yolk sphere under heating is important. In this study, we used SEM and CLSM to confirm lipid migration and protein aggregation. We observed that during the heating process, the thermal stability decreased, and there was an increase in the content of β-turns and the degrees of freedom of water and lipids. G' increased during the frequency sweep but decreased after heating for 120 min. The various yolk gel structures exhibited varying degrees of resistance to compression from external forces. Prolonged heating resulted in the presence of gaps and increased surface roughness of the spheres. In conclusion, heating induced lipid migration and protein aggregation in the sphere microgels, thereby altering the structural properties of the gels.

Exploring non-equilibrium processes and spatio-temporal scaling laws in heated egg yolk using coherent X-rays

The soft-grainy microstructure of cooked egg yolk is the result of a series of out-of-equilibrium processes of its protein-lipid contents; however, it is unclear how egg yolk constituents contribute to these processes to create the desired microstructure. By employing X-ray photon correlation spectroscopy, we investigate the functional contribution of egg yolk constituents: proteins, low-density lipoproteins (LDLs), and yolk-granules to the development of grainy-gel microstructure and microscopic dynamics during cooking. We find that the viscosity of the heated egg yolk is solely determined by the degree of protein gelation, whereas the grainy-gel microstructure is controlled by the extent of LDL aggregation. Overall, protein denaturation-aggregation-gelation and LDL-aggregation follows Arrhenius-type time-temperature superposition (TTS), indicating an identical mechanism with a temperature-dependent reaction rate. However, above 75 °C TTS breaks down and temperature-independent gelation dynamics is observed, demonstrating that the temperature can no longer accelerate certain non-equilibrium processes above a threshold value.

A comparative study of unpasteurized and pasteurized frozen whole hen eggs using size-exclusion chromatography and small-angle X-ray scattering

Hen eggs are rich in proteins and are an important source of protein for humans. Pasteurized frozen whole hen eggs are widely used in cooking and confectionery and can be stored for long periods. However, processed eggs differ from raw eggs in properties such as viscosity, foaming ability, and thermal aggregation. To develop pasteurized frozen whole egg products with properties similar to those of unpasteurized whole eggs, it is necessary to establish a method that can differentiate between the two egg types with respect to the structures of their proteins. In this study, size-exclusion chromatography (SEC) and SEC coupled with small-angle X-ray scattering (SEC-SAXS) were successfully used to differentiate between the proteins in unpasteurized and pasteurized frozen whole eggs. We found that proteins in the plasma fraction of egg yolk, especially apovitellenins I and II, formed large aggregates in the pasteurized eggs, indicating that their structures are sensitive to temperature changes during pasteurization, freezing, and thawing. The results suggest that SEC and SEC-SAXS can be used to differentiate between unpasteurized and pasteurized frozen whole eggs. Additionally, they may be useful in determining molecular sizes and shapes of multiple components in various complex biological systems such as whole eggs.

Impact of Ultra-High Pressure Homogenization on the Structural Properties of Egg Yolk Granule

Ultra-high pressure homogenization (UHPH) is a promising method for destabilizing and potentially improving the techno-functionality of the egg yolk granule. This study’s objectives were to determine the impact of pressure level (50, 175 and 300 MPa) and number of passes (1 and 4) on the physico-chemical and structural properties of egg yolk granule and its subsequent fractions. UHPH induced restructuration of the granule through the formation of a large protein network, without impacting the proximate composition and protein profile in a single pass of up to 300 MPa. In addition, UHPH reduced the particle size distribution up to 175 MPa, to eventually form larger particles through enhanced protein–protein interactions at 300 MPa. Phosvitin, apovitellenin and apolipoprotein-B were specifically involved in these interactions. Overall, egg yolk granule remains highly stable during UHPH treatment. However, more investigations are needed to characterize the resulting protein network and to evaluate the techno-functional properties of UHPH-treated granule.

Phylogeny and evolutionary history of the amniote egg

We review morphological features of the amniote egg and embryos in a comparative phylogenetic framework, including all major clades of extant vertebrates. We discuss 40 characters that are relevant for an analysis of the evolutionary history of the vertebrate egg. Special attention is given to the morphology of the cellular yolk sac, the eggshell, and extraembryonic membranes. Many features that are typically assigned to amniotes, such as a large yolk sac, delayed egg deposition, and terrestrial reproduction have evolved independently and convergently in numerous clades of vertebrates. We use phylogenetic character mapping and ancestral character state reconstruction as tools to recognize sequence, order, and patterns of morphological evolution and deduce a hypothesis of the evolutionary history of the amniote egg. Besides amnion and chorioallantois, amniotes ancestrally possess copulatory organs (secondarily reduced in most birds), internal fertilization, and delayed deposition of eggs that contain an embryo in the primitive streak or early somite stage. Except for the amnion, chorioallantois, and amniote type of eggshell, these features evolved convergently in almost all major clades of aquatic vertebrates possibly in response to selective factors such as egg predation, hostile environmental conditions for egg development, or to adjust hatching of young to favorable season. A functionally important feature of the amnion membrane is its myogenic contractility that moves the (early) embryo and prevents adhering of the growing embryo to extraembryonic materials. This function of the amnion membrane and the liquid-filled amnion cavity may have evolved under the requirements of delayed deposition of eggs that contain developing embryos. The chorioallantois is a temporary embryonic exchange organ that supports embryonic development. A possible evolutionary scenario is that the amniote egg presents an exaptation that paved the evolutionary pathway for reproduction on land. As shown by numerous examples from anamniotes, reproduction on land has occurred multiple times among vertebrates-the amniote egg presenting one "solution" that enabled the conquest of land for reproduction.

Combination of High Hydrostatic Pressure and Ultrafiltration to Generate a New Emulsifying Ingredient from Egg Yolk

Egg yolk granule phosvitin (45 kDa) is a phosphoprotein known for its emulsifying properties. Recently, high hydrostatic pressure (HHP) treatment of granule induced the transfer of phosvitin to the soluble plasma fraction. This project evaluated the performance of the ultrafiltration (UF) used to concentrate phosvitin from the plasma fraction to produce a natural emulsifier. Phosvitin was characterized in plasma from a pressure-treated granule (1.73 ± 0.07% w/w) and in its UF retentate (26.00 ± 4.12% w/w). The emulsifying properties of both retentates were evaluated. The emulsion prepared with phosvitin-enriched retentate was more resistant to flocculation and creaming. Confocal laser scanning microscopy showed a network of aggregated protein similar to a gel, which encapsulated oil droplets in emulsions made with UF-retentate of plasma from pressure-treated granule. However, although sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) showed that β-phosvitin is recovered in the cream, it is difficult to attribute the improved emulsifying properties of the UF-retentate of plasma from pressure-treated granules only to phosvitin.

Formation mechanism of low-density lipoprotein gel induced by NaCl

Salted eggs, which are a traditional Chinese egg product, are favored by Chinese consumers and have become very popular in other Asian countries due to their unique features such as "fresh, fine, tender, loose, gritty and oily texture." In order to illuminate the forming process of salted egg, the gelation behavior and mechanism of low-density lipoprotein (LDL) induced by NaCl were investigated using marinated model outside the eggshell. Results showed that as the salting proceeded, the moisture content exhibited a decreasing trend. The NaCl content, oil exudation, hardness, and surface hydrophobicity showed constantly increasing trends. In the early stages of salting, the size of the LDL particles, soluble protein content, and T21 increased, whereas T21 (with D2O), T22, and the free sulfhydryl content declined. In the later stages of salting, LDL formed a multiple composite aggregate gel structure with filamentous apoproteins and non-spherical lipid particles intertwined with each other. The soluble protein content and T23 (without D2O) decreased, whereas T21 (with D2O), T22 and the free sulfhydryl content increased. Fourier transform infrared spectroscopy revealed that the fresh LDL mainly consisted of α-helix and β-sheet structures. After the gel becomes hardened, the LDL secondary structure was changed remarkably, characterized by the decrease of α-helix elements and increase of β-sheet elements. The results suggested that the oil exudation of salted LDL gels was mainly due to LDL destruction and the release of components (apoproteins, phospholipids, and neutral lipids) facilitated by increased interactions between apoproteins and lipids.

Use of Reconstitued Yolk Systems To Study the Gelation Mechanism of Frozen-Thawed Hen Egg Yolk

Yolk gelation upon 5 week freezing-thawing was studied in four recombined yolk systems containing different plasma and granule proportions. Fractionation for mass distribution, sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) for protein distribution, and rheological properties were explored. Results indicate that both plasma and granule components, including low-density lipoprotein (LDL), high-density lipoprotein (HDL), and α-livetin proteins, contributed to gelation. Protein aggregation was reflected through a large mass increase in the granule fraction and appearance of a floating LDL layer upon fractionation of gelated yolk systems. A significant increase in gel strength (elastic modulus, G') was observed with the increase of the granule content. Overall, this study provides a better understanding of yolk gelation mechanism that may consequently lead to the design of innovative methods for preventing gelation. A schematic presentation of the yolk gelation mechanism is also proposed.

Determination of the Gelation Mechanism of Freeze-Thawed Hen Egg Yolk

A study of yolks stored up to 168 d at -20 °C was conducted to determine the gelation behavior and mechanism of freeze-thawed yolk. Methods used were rheology, native and sodium dodecyl sulfate polyacrylamide gel electrophoresis (native- and SDS-PAGE), differential scanning calorimetry (DSC), transmission electron microscopy (TEM), particle size analysis, and proton nuclear magnetic resonance ((1)H NMR) spectroscopy for matrix mobility. Results indicate that both constituents of plasma and granules contributed to gelation of yolk under freezing. PAGE analyses suggest that granular proteins participated in aggregation during freeze-thaw. Increasing gel strength and particle size and decreasing water and lipid-water mobility indicate that lipoproteins or apolipoproteins aggregated. At storage times ≥84 d, increased protein and lipid mobility, the detection of smaller particles, and secondarily increased gel strength suggest the liberation of protein or lipoprotein components from previously formed aggregates and further aggregation of these constituents. Disruption of the gelled yolk matrix observed with TEM supported that ice crystal formation was required for gelation to occur. A two-stage dynamic gelation model is thus proposed.

Egg yolk: structures, functionalities and processes

Hen egg yolk is an ideal example of natural supramolecular assemblies of lipids and proteins with different organization levels. These assemblies are mainly due to interactions between proteins and phospholipids, and these interactions are essential in understanding and controlling the production of food made with yolk, and particularly emulsions. Furthermore, these assemblies can be modulated by external constraints among which thermo-mechanical and high-pressure treatments. This review focuses on multi-scale structures present in egg yolk, and their modulation by processes, in relation with their emulsifying properties. Egg yolk is mainly composed of two fractions-plasma and granules-which are natural nano- and micro-assemblies. These two fractions possess different composition, structures and functionalities and exhibit specific behaviour under treatments such as high pressure and temperature. Plasma contains a large quantity of lipids structured as lipoproteins (low-density lipoproteins), whereas granules are mainly composed of proteins aggregated in micrometric assemblies. If plasma is responsible for the important emulsifying properties of yolk, granules bring interesting emulsifying properties when assemblies are in the form of micelles in presence of salts. High-pressure or thermal treatments, applied before or after emulsion fabrication, alter their functionalities and could be used to commercially exploit these fractions.

Histological structures of native and cooked yolks from duck egg observed by SEM and cryo-SEM

A method was used to fix duck egg yolk while retaining its original sol structure to elucidate the fine structure of native yolk by using fixation with liquid nitrogen and cryo-scanning electron microscopy (cryo-SEM). Native yolk spheres showed a polyhedron shape with a diameter at approximately 50 to 100 μm and packed closely together. Furthermore, the interior microstructure of the native yolk spheres showed that a great amount of round globules ranging from 0.5 to 1.5 μm were embedded in a continuous phase with a lot of voids. After cooking, the sizes of the spheres were almost unchanged, and the continuous phase became a fibrous network structure observed by SEM with chemical fixation probably constituted of low density lipoprotein (LDL). The fine structure of the native yolk can be observed by cryo-SEM; however, the microstructure of yolk granules and plasma from cooked shell eggs can be observed by SEM with chemical fixation.

Impact of a treatment with phospholipase A2 on the physicochemical properties of hen egg yolk

Changes in physicochemical properties of egg yolk were investigated after a treatment with phospholipase A 2 (PLA 2), where phospholipids are converted in lyso-phospholipids. Protein solubility and protein denaturation before and after modification by PLA 2 was monitored as well as the functionality of egg yolk by means of interfacial tension. Enzymatic treatment showed a significant impact on the properties of egg yolk with regard to protein solubility and denaturation behavior. To gain a closer insight, egg yolk was separated in its water-soluble fraction called plasma and the insoluble granules. Both fractions were separately modified by PLA 2. The granule fraction shows a higher protein solubility, and the plasma proteins show very high heat stability after modification by PLA 2. Hypotheses regarding related changes in the low-density lipoprotein (LDL) particles are discussed. Results suggest that significant differences in the functional properties of untreated and PLA 2-modified egg yolk do not primarily result from the existence of lyso-phospholipids but from structural changes in egg yolk granules and LDL particles.

Structuring and functionalization of dispersions containing egg yolk, plasma and granules induced by mechanical treatments

In this study, the impact of mechanical treatments on the physicochemical and emulsifying properties of hen egg yolk and its fractions plasma and granules has been assessed. Yolk, plasma, and granule dispersions at pH 4.0 and 0.75 M NaCl were subjected to rotor-stator and high-pressure pretreatments at different dynamic pressure levels: 30, 100, and 200 bar at 20 degrees C. Physicochemical characteristics (protein solubility, rheological behavior, and micro- and ultra-structures) and emulsifying properties (oil/water 60:40 emulsions: droplet size and flocculation, protein adsorption) of control dispersions and dispersions subjected to mechanical pretreatments (rotor-stator or high pressure) were compared. Homogenization at high pressures (100 and 200 bar) led to a decreased protein solubility and to an increase in apparent viscosity of yolk and plasma dispersions. These pressures certainly disrupted low-density lipoproteins (LDL) particles and generated aggregates of proteins liberated from LDL and livetins in the plasma fraction, and led to a moderated reorganization of the microstructure of granules. Despite the modifications observed in the pretreated plasma and granules dispersions, the oil droplet diameter and the bridging flocculation obtained in emulsions made with these dispersions were similar to that obtained with untreated dispersions. Results concerning interfacial protein adsorption suggested that preformed or natural aggregates at least partially persist at the oil-water interface.

Structures and rheological properties of hen egg yolk low density lipoprotein layers spread at the air–water interface at pH 3 and 7

Low density lipoproteins (LDL) from egg yolk have a classical structure of lipoprotein with a core of neutral lipids surrounded by a monolayer of apoproteins and phospholipids. This structure collapses during adsorption and all constituents spread at the interface. To understand better the nature of the interactions between apoproteins and lipids at the interface, we have deposited LDL at an air-water interface and analysed the isotherms during their compression on a Langmuir trough. Then, these LDL films were studied by atomic force microscopy (AFM) imaging. To identify the protein and lipid structures, we imaged films before and after lipid solubilisation by butanol. To study the interactions in the LDL films, we have varied the pH, ionic strength and used simplified model systems. We also studied the correlation between observed structures and interfacial rheology of the film. The isotherms of interfacial LDL films were similar for pH 3 and 7, but their structures observed in AFM were different. At surface pressures below the transition corresponding to the demixion of apoprotein-neutral lipid complexes, the LDL film structure was not governed by electrostatic interactions. However, above this surface pressure transition (45mN/m), there was an effect of charge on this structure. Around the transition zone, the rheological properties of LDL films at pH 3 were different as a function of pH (viscous at pH 3 and visco-elastic at pH 7). So, the rheological properties of LDL films could be linked to the structures formed by apoproteins and observed in AFM.

Structure modification in hen egg yolk low density lipoproteins layers between 30 and 45mN/m observed by AFM

We have studied the structure of films made by low density lipoproteins (LDL) from hen egg yolk, which are composed of apoproteins, neutral lipids and phospholipids. These LDL have been deposited on air-water interface to form a monolayer which has been compressed to measure an isotherm using Langmuir balance. This isotherm presented three transitions (neutral lipid (surface pressure, pi=19 mN/m), apoprotein-lipid (pi=41 mN/m) and phospholipid (pi=51 mN/m) transitions). We have studied only the apoprotein-lipid transition. In order to observe the LDL film structure before (pi=30 mN/m) and after (pi=45 mN/m) the apoprotein-lipid transition, the formed films were transferred and visualised by atomic force microscopy (AFM). Our results have shown that the structures observed in the LDL film were different depending on the surface pressure. The apoproteins and neutral lipids appeared to be miscible up to the apoprotein-lipid transition, when demixing occurred. The structures observed after the apoprotein-lipid transition should be due to the demixing between apoproteins and neutral lipids. On the other hand, apoproteins and phospholipids seemed miscible whatever the surface pressure. Hence, the first transition (pi=19 mN/m) should be attributed to the free neutral lipid collapse; the second transition (pi=41 mN/m) should be attributed to the demixing of apoprotein-neutral lipid complexes; and the last transition (pi=51 mN/m) should be attributed to phospholipid collapse or to demixing of apoprotein-phospholipid complexes.

Properties of low-fat, low-cholesterol egg yolk prepared by supercritical CO2 extraction

A dry egg yolk ingredient called Eggcellent has 74% less fat and 90% less cholesterol than liquid egg yolks, when reconstituted on an equal protein basis. The phospholipids and proteins are retained, enabling the ingredient to have the taste and texturizing properties of fresh egg yolk. Using the new yolk, it is possible to significantly improve the acceptability of low-fat, low-cholesterol bakery products, scrambled eggs and mayonnaise dressings without losing nutritional claims. The structures and functional properties of egg yolk components and the conditions required to optimize their benefits in foods are reviewed. The lipoproteins of low-fat, low-cholesterol yolk have valuable properties as flavorants, texturizers, foaming agents, emulsifiers, antioxidants, colorants, and nutraceuticals.